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1 <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
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2
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3 <chapter id="chap:mq-collab">
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4 <?dbhtml filename="advanced-uses-of-mercurial-queues.html"?>
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5 <title>Advanced uses of Mercurial Queues</title>
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6
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7 <para>While it's easy to pick up straightforward uses of Mercurial
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8 Queues, use of a little discipline and some of MQ's less
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9 frequently used capabilities makes it possible to work in
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10 complicated development environments.</para>
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11
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12 <para>In this chapter, I will use as an example a technique I have
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13 used to manage the development of an Infiniband device driver for
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14 the Linux kernel. The driver in question is large (at least as
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15 drivers go), with 25,000 lines of code spread across 35 source
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16 files. It is maintained by a small team of developers.</para>
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17
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18 <para>While much of the material in this chapter is specific to
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19 Linux, the same principles apply to any code base for which you're
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20 not the primary owner, and upon which you need to do a lot of
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21 development.</para>
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22
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23 <sect1>
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24 <title>The problem of many targets</title>
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25
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26 <para>The Linux kernel changes rapidly, and has never been
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27 internally stable; developers frequently make drastic changes
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28 between releases. This means that a version of the driver that
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29 works well with a particular released version of the kernel will
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30 not even <emphasis>compile</emphasis> correctly against,
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31 typically, any other version.</para>
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32
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33 <para>To maintain a driver, we have to keep a number of distinct
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34 versions of Linux in mind.</para>
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35 <itemizedlist>
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36 <listitem><para>One target is the main Linux kernel development
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37 tree. Maintenance of the code is in this case partly shared
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38 by other developers in the kernel community, who make
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39 <quote>drive-by</quote> modifications to the driver as they
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40 develop and refine kernel subsystems.</para>
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41 </listitem>
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42 <listitem><para>We also maintain a number of
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43 <quote>backports</quote> to older versions of the Linux
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44 kernel, to support the needs of customers who are running
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45 older Linux distributions that do not incorporate our
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46 drivers. (To <emphasis>backport</emphasis> a piece of code
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47 is to modify it to work in an older version of its target
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48 environment than the version it was developed for.)</para>
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49 </listitem>
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50 <listitem><para>Finally, we make software releases on a schedule
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51 that is necessarily not aligned with those used by Linux
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52 distributors and kernel developers, so that we can deliver
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53 new features to customers without forcing them to upgrade
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54 their entire kernels or distributions.</para>
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55 </listitem></itemizedlist>
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56
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57 <sect2>
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58 <title>Tempting approaches that don't work well</title>
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59
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60 <para>There are two <quote>standard</quote> ways to maintain a
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61 piece of software that has to target many different
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62 environments.</para>
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63
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64 <para>The first is to maintain a number of branches, each
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65 intended for a single target. The trouble with this approach
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66 is that you must maintain iron discipline in the flow of
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67 changes between repositories. A new feature or bug fix must
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68 start life in a <quote>pristine</quote> repository, then
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69 percolate out to every backport repository. Backport changes
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70 are more limited in the branches they should propagate to; a
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71 backport change that is applied to a branch where it doesn't
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72 belong will probably stop the driver from compiling.</para>
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73
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74 <para>The second is to maintain a single source tree filled with
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75 conditional statements that turn chunks of code on or off
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76 depending on the intended target. Because these
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77 <quote>ifdefs</quote> are not allowed in the Linux kernel
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78 tree, a manual or automatic process must be followed to strip
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79 them out and yield a clean tree. A code base maintained in
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80 this fashion rapidly becomes a rat's nest of conditional
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81 blocks that are difficult to understand and maintain.</para>
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82
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83 <para>Neither of these approaches is well suited to a situation
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84 where you don't <quote>own</quote> the canonical copy of a
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85 source tree. In the case of a Linux driver that is
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86 distributed with the standard kernel, Linus's tree contains
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87 the copy of the code that will be treated by the world as
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88 canonical. The upstream version of <quote>my</quote> driver
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89 can be modified by people I don't know, without me even
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90 finding out about it until after the changes show up in
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91 Linus's tree.</para>
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92
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93 <para>These approaches have the added weakness of making it
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94 difficult to generate well-formed patches to submit
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95 upstream.</para>
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96
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97 <para>In principle, Mercurial Queues seems like a good candidate
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98 to manage a development scenario such as the above. While
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99 this is indeed the case, MQ contains a few added features that
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100 make the job more pleasant.</para>
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101
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102 </sect2>
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103 </sect1>
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104 <sect1>
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105 <title>Conditionally applying patches with guards</title>
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106
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107 <para>Perhaps the best way to maintain sanity with so many targets
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108 is to be able to choose specific patches to apply for a given
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109 situation. MQ provides a feature called <quote>guards</quote>
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110 (which originates with quilt's <literal>guards</literal>
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111 command) that does just this. To start off, let's create a
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112 simple repository for experimenting in.</para>
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113
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114 &interaction.mq.guards.init;
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115
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116 <para>This gives us a tiny repository that contains two patches
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117 that don't have any dependencies on each other, because they
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118 touch different files.</para>
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119
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120 <para>The idea behind conditional application is that you can
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121 <quote>tag</quote> a patch with a <emphasis>guard</emphasis>,
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122 which is simply a text string of your choosing, then tell MQ to
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123 select specific guards to use when applying patches. MQ will
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124 then either apply, or skip over, a guarded patch, depending on
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125 the guards that you have selected.</para>
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126
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127 <para>A patch can have an arbitrary number of guards; each one is
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128 <emphasis>positive</emphasis> (<quote>apply this patch if this
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129 guard is selected</quote>) or <emphasis>negative</emphasis>
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130 (<quote>skip this patch if this guard is selected</quote>). A
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131 patch with no guards is always applied.</para>
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132
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133 </sect1>
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134 <sect1>
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135 <title>Controlling the guards on a patch</title>
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136
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137 <para>The <command role="hg-ext-mq">qguard</command> command lets
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138 you determine which guards should apply to a patch, or display
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139 the guards that are already in effect. Without any arguments, it
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140 displays the guards on the current topmost patch.</para>
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141
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142 &interaction.mq.guards.qguard;
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143
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144 <para>To set a positive guard on a patch, prefix the name of the
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145 guard with a <quote><literal>+</literal></quote>.</para>
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146
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147 &interaction.mq.guards.qguard.pos;
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148
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149 <para>To set a negative guard
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150 on a patch, prefix the name of the guard with a
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151 <quote><literal>-</literal></quote>.</para>
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152
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153 &interaction.mq.guards.qguard.neg;
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154
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155 <note>
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156 <para> The <command role="hg-ext-mq">qguard</command> command
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157 <emphasis>sets</emphasis> the guards on a patch; it doesn't
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158 <emphasis>modify</emphasis> them. What this means is that if
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159 you run <command role="hg-cmd">hg qguard +a +b</command> on a
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160 patch, then <command role="hg-cmd">hg qguard +c</command> on
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161 the same patch, the <emphasis>only</emphasis> guard that will
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162 be set on it afterwards is <literal>+c</literal>.</para>
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163 </note>
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164
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165 <para>Mercurial stores guards in the <filename
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166 role="special">series</filename> file; the form in which they
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167 are stored is easy both to understand and to edit by hand. (In
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168 other words, you don't have to use the <command
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169 role="hg-ext-mq">qguard</command> command if you don't want
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170 to; it's okay to simply edit the <filename
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171 role="special">series</filename> file.)</para>
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172
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173 &interaction.mq.guards.series;
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174
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175 </sect1>
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176 <sect1>
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177 <title>Selecting the guards to use</title>
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178
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179 <para>The <command role="hg-ext-mq">qselect</command> command
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180 determines which guards are active at a given time. The effect
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181 of this is to determine which patches MQ will apply the next
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182 time you run <command role="hg-ext-mq">qpush</command>. It has
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183 no other effect; in particular, it doesn't do anything to
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184 patches that are already applied.</para>
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185
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186 <para>With no arguments, the <command
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187 role="hg-ext-mq">qselect</command> command lists the guards
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188 currently in effect, one per line of output. Each argument is
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189 treated as the name of a guard to apply.</para>
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190
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191 &interaction.mq.guards.qselect.foo;
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192
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193 <para>In case you're interested, the currently selected guards are
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194 stored in the <filename role="special">guards</filename> file.</para>
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195
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196 &interaction.mq.guards.qselect.cat;
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197
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198 <para>We can see the effect the selected guards have when we run
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199 <command role="hg-ext-mq">qpush</command>.</para>
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200
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201 &interaction.mq.guards.qselect.qpush;
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202
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203 <para>A guard cannot start with a
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204 <quote><literal>+</literal></quote> or
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205 <quote><literal>-</literal></quote> character. The name of a
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206 guard must not contain white space, but most other characters
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207 are acceptable. If you try to use a guard with an invalid name,
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208 MQ will complain:</para>
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209
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210 &interaction.mq.guards.qselect.error;
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211
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212 <para>Changing the selected guards changes the patches that are
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213 applied.</para>
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214
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215 &interaction.mq.guards.qselect.quux;
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216
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217 <para>You can see in the example below that negative guards take
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218 precedence over positive guards.</para>
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219
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220 &interaction.mq.guards.qselect.foobar;
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221
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222 </sect1>
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223 <sect1>
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224 <title>MQ's rules for applying patches</title>
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225
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226 <para>The rules that MQ uses when deciding whether to apply a
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227 patch are as follows.</para>
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228 <itemizedlist>
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229 <listitem><para>A patch that has no guards is always
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230 applied.</para>
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231 </listitem>
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232 <listitem><para>If the patch has any negative guard that matches
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233 any currently selected guard, the patch is skipped.</para>
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234 </listitem>
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235 <listitem><para>If the patch has any positive guard that matches
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236 any currently selected guard, the patch is applied.</para>
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237 </listitem>
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238 <listitem><para>If the patch has positive or negative guards,
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239 but none matches any currently selected guard, the patch is
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240 skipped.</para>
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241 </listitem></itemizedlist>
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242
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243 </sect1>
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244 <sect1>
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245 <title>Trimming the work environment</title>
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246
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247 <para>In working on the device driver I mentioned earlier, I don't
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248 apply the patches to a normal Linux kernel tree. Instead, I use
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249 a repository that contains only a snapshot of the source files
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250 and headers that are relevant to Infiniband development. This
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251 repository is 1% the size of a kernel repository, so it's easier
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252 to work with.</para>
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253
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254 <para>I then choose a <quote>base</quote> version on top of which
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255 the patches are applied. This is a snapshot of the Linux kernel
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256 tree as of a revision of my choosing. When I take the snapshot,
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257 I record the changeset ID from the kernel repository in the
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258 commit message. Since the snapshot preserves the
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259 <quote>shape</quote> and content of the relevant parts of the
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260 kernel tree, I can apply my patches on top of either my tiny
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261 repository or a normal kernel tree.</para>
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262
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263 <para>Normally, the base tree atop which the patches apply should
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264 be a snapshot of a very recent upstream tree. This best
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265 facilitates the development of patches that can easily be
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266 submitted upstream with few or no modifications.</para>
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267
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268 </sect1>
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269 <sect1>
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270 <title>Dividing up the <filename role="special">series</filename>
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271 file</title>
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272
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273 <para>I categorise the patches in the <filename
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274 role="special">series</filename> file into a number of logical
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275 groups. Each section of like patches begins with a block of
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276 comments that describes the purpose of the patches that
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277 follow.</para>
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278
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279 <para>The sequence of patch groups that I maintain follows. The
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280 ordering of these groups is important; I'll describe why after I
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281 introduce the groups.</para>
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282 <itemizedlist>
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283 <listitem><para>The <quote>accepted</quote> group. Patches that
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284 the development team has submitted to the maintainer of the
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285 Infiniband subsystem, and which he has accepted, but which
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286 are not present in the snapshot that the tiny repository is
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287 based on. These are <quote>read only</quote> patches,
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288 present only to transform the tree into a similar state as
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289 it is in the upstream maintainer's repository.</para>
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290 </listitem>
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291 <listitem><para>The <quote>rework</quote> group. Patches that I
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292 have submitted, but that the upstream maintainer has
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293 requested modifications to before he will accept
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294 them.</para>
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295 </listitem>
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296 <listitem><para>The <quote>pending</quote> group. Patches that
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297 I have not yet submitted to the upstream maintainer, but
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298 which we have finished working on. These will be <quote>read
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299 only</quote> for a while. If the upstream maintainer
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300 accepts them upon submission, I'll move them to the end of
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301 the <quote>accepted</quote> group. If he requests that I
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302 modify any, I'll move them to the beginning of the
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303 <quote>rework</quote> group.</para>
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304 </listitem>
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305 <listitem><para>The <quote>in progress</quote> group. Patches
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306 that are actively being developed, and should not be
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307 submitted anywhere yet.</para>
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308 </listitem>
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309 <listitem><para>The <quote>backport</quote> group. Patches that
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310 adapt the source tree to older versions of the kernel
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311 tree.</para>
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312 </listitem>
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313 <listitem><para>The <quote>do not ship</quote> group. Patches
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314 that for some reason should never be submitted upstream.
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315 For example, one such patch might change embedded driver
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316 identification strings to make it easier to distinguish, in
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317 the field, between an out-of-tree version of the driver and
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318 a version shipped by a distribution vendor.</para>
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319 </listitem></itemizedlist>
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320
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321 <para>Now to return to the reasons for ordering groups of patches
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322 in this way. We would like the lowest patches in the stack to
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323 be as stable as possible, so that we will not need to rework
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324 higher patches due to changes in context. Putting patches that
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325 will never be changed first in the <filename
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326 role="special">series</filename> file serves this
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327 purpose.</para>
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328
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329 <para>We would also like the patches that we know we'll need to
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330 modify to be applied on top of a source tree that resembles the
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331 upstream tree as closely as possible. This is why we keep
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332 accepted patches around for a while.</para>
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333
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334 <para>The <quote>backport</quote> and <quote>do not ship</quote>
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335 patches float at the end of the <filename
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336 role="special">series</filename> file. The backport patches
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337 must be applied on top of all other patches, and the <quote>do
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338 not ship</quote> patches might as well stay out of harm's
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339 way.</para>
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340
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341 </sect1>
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342 <sect1>
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343 <title>Maintaining the patch series</title>
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344
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345 <para>In my work, I use a number of guards to control which
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346 patches are to be applied.</para>
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347
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348 <itemizedlist>
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349 <listitem><para><quote>Accepted</quote> patches are guarded with
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350 <literal>accepted</literal>. I enable this guard most of
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351 the time. When I'm applying the patches on top of a tree
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352 where the patches are already present, I can turn this patch
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353 off, and the patches that follow it will apply
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354 cleanly.</para>
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355 </listitem>
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356 <listitem><para>Patches that are <quote>finished</quote>, but
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357 not yet submitted, have no guards. If I'm applying the
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358 patch stack to a copy of the upstream tree, I don't need to
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359 enable any guards in order to get a reasonably safe source
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360 tree.</para>
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361 </listitem>
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362 <listitem><para>Those patches that need reworking before being
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363 resubmitted are guarded with
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364 <literal>rework</literal>.</para>
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365 </listitem>
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366 <listitem><para>For those patches that are still under
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367 development, I use <literal>devel</literal>.</para>
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368 </listitem>
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369 <listitem><para>A backport patch may have several guards, one
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370 for each version of the kernel to which it applies. For
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371 example, a patch that backports a piece of code to 2.6.9
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372 will have a <literal>2.6.9</literal> guard.</para>
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373 </listitem></itemizedlist>
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374 <para>This variety of guards gives me considerable flexibility in
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375 determining what kind of source tree I want to end up with. For
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376 most situations, the selection of appropriate guards is
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377 automated during the build process, but I can manually tune the
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378 guards to use for less common circumstances.</para>
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379
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380 <sect2>
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381 <title>The art of writing backport patches</title>
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382
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383 <para>Using MQ, writing a backport patch is a simple process.
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384 All such a patch has to do is modify a piece of code that uses
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385 a kernel feature not present in the older version of the
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386 kernel, so that the driver continues to work correctly under
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387 that older version.</para>
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388
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389 <para>A useful goal when writing a good backport patch is to
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390 make your code look as if it was written for the older version
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391 of the kernel you're targeting. The less obtrusive the patch,
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392 the easier it will be to understand and maintain. If you're
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393 writing a collection of backport patches to avoid the
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394 <quote>rat's nest</quote> effect of lots of
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395 <literal>#ifdef</literal>s (hunks of source code that are only
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396 used conditionally) in your code, don't introduce
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397 version-dependent <literal>#ifdef</literal>s into the patches.
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398 Instead, write several patches, each of which makes
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399 unconditional changes, and control their application using
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400 guards.</para>
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401
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402 <para>There are two reasons to divide backport patches into a
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403 distinct group, away from the <quote>regular</quote> patches
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404 whose effects they modify. The first is that intermingling the
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405 two makes it more difficult to use a tool like the <literal
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406 role="hg-ext">patchbomb</literal> extension to automate the
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407 process of submitting the patches to an upstream maintainer.
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408 The second is that a backport patch could perturb the context
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409 in which a subsequent regular patch is applied, making it
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410 impossible to apply the regular patch cleanly
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411 <emphasis>without</emphasis> the earlier backport patch
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412 already being applied.</para>
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413
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414 </sect2>
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415 </sect1>
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416 <sect1>
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417 <title>Useful tips for developing with MQ</title>
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418
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419 <sect2>
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420 <title>Organising patches in directories</title>
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421
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422 <para>If you're working on a substantial project with MQ, it's
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423 not difficult to accumulate a large number of patches. For
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424 example, I have one patch repository that contains over 250
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425 patches.</para>
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426
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427 <para>If you can group these patches into separate logical
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428 categories, you can if you like store them in different
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429 directories; MQ has no problems with patch names that contain
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430 path separators.</para>
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431
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432 </sect2>
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433 <sect2 id="mq-collab:tips:interdiff">
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434 <title>Viewing the history of a patch</title>
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435
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436 <para>If you're developing a set of patches over a long time,
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437 it's a good idea to maintain them in a repository, as
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438 discussed in section <xref linkend="sec:mq:repo"/>. If you do
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439 so, you'll quickly
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440 discover that using the <command role="hg-cmd">hg
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441 diff</command> command to look at the history of changes to
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442 a patch is unworkable. This is in part because you're looking
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443 at the second derivative of the real code (a diff of a diff),
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444 but also because MQ adds noise to the process by modifying
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445 time stamps and directory names when it updates a
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446 patch.</para>
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447
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448 <para>However, you can use the <literal
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449 role="hg-ext">extdiff</literal> extension, which is bundled
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450 with Mercurial, to turn a diff of two versions of a patch into
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451 something readable. To do this, you will need a third-party
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452 package called <literal role="package">patchutils</literal>
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453 <citation>web:patchutils</citation>. This provides a command
|
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454 named <command>interdiff</command>, which shows the
|
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455 differences between two diffs as a diff. Used on two versions
|
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456 of the same diff, it generates a diff that represents the diff
|
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457 from the first to the second version.</para>
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458
|
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459 <para>You can enable the <literal
|
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460 role="hg-ext">extdiff</literal> extension in the usual way,
|
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461 by adding a line to the <literal
|
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462 role="rc-extensions">extensions</literal> section of your
|
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463 <filename role="special"> /.hgrc</filename>.</para>
|
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464 <programlisting>[extensions] extdiff =</programlisting>
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465 <para>The <command>interdiff</command> command expects to be
|
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466 passed the names of two files, but the <literal
|
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467 role="hg-ext">extdiff</literal> extension passes the program
|
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468 it runs a pair of directories, each of which can contain an
|
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469 arbitrary number of files. We thus need a small program that
|
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470 will run <command>interdiff</command> on each pair of files in
|
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471 these two directories. This program is available as <filename
|
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472 role="special">hg-interdiff</filename> in the <filename
|
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473 class="directory">examples</filename> directory of the
|
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474 source code repository that accompanies this book. <!--
|
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475 &example.hg-interdiff; --></para>
|
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476
|
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477 <para>With the <filename role="special">hg-interdiff</filename>
|
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478 program in your shell's search path, you can run it as
|
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479 follows, from inside an MQ patch directory:</para>
|
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480 <programlisting>hg extdiff -p hg-interdiff -r A:B
|
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481 my-change.patch</programlisting>
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bos@559
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482 <para>Since you'll probably want to use this long-winded command
|
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483 a lot, you can get <literal role="hg-ext">hgext</literal> to
|
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484 make it available as a normal Mercurial command, again by
|
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485 editing your <filename role="special">
|
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486 /.hgrc</filename>.</para>
|
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487 <programlisting>[extdiff] cmd.interdiff =
|
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488 hg-interdiff</programlisting>
|
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489 <para>This directs <literal role="hg-ext">hgext</literal> to
|
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490 make an <literal>interdiff</literal> command available, so you
|
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491 can now shorten the previous invocation of <command
|
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492 role="hg-ext-extdiff">extdiff</command> to something a
|
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493 little more wieldy.</para>
|
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494 <programlisting>hg interdiff -r A:B
|
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|
495 my-change.patch</programlisting>
|
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496
|
bos@559
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497 <note>
|
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498 <para> The <command>interdiff</command> command works well
|
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499 only if the underlying files against which versions of a
|
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500 patch are generated remain the same. If you create a patch,
|
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501 modify the underlying files, and then regenerate the patch,
|
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502 <command>interdiff</command> may not produce useful
|
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503 output.</para>
|
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|
504 </note>
|
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505
|
bos@559
|
506 <para>The <literal role="hg-ext">extdiff</literal> extension is
|
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507 useful for more than merely improving the presentation of MQ
|
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|
508 patches. To read more about it, go to section <xref
|
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|
509 linkend="sec:hgext:extdiff"/>.</para>
|
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510
|
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|
511 </sect2>
|
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|
512 </sect1>
|
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|
513 </chapter>
|
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514
|
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515 <!--
|
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516 local variables:
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517 sgml-parent-document: ("00book.xml" "book" "chapter")
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518 end:
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519 -->
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