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+
+        Applying Patches To The Linux Kernel
+        ------------------------------------
+
+        (Written by Jesper Juhl, August 2005)
+
+
+
+A frequently asked question on the Linux Kernel Mailing List is how to apply
+a patch to the kernel or, more specifically, what base kernel a patch for
+one of the many trees/branches should be applied to. Hopefully this document
+will explain this to you.
+
+In addition to explaining how to apply and revert patches, a brief
+description of the different kernel trees (and examples of how to apply
+their specific patches) is also provided.
+
+
+What is a patch?
+---
+ A patch is a small text document containing a delta of changes between two
+different versions of a source tree. Patches are created with the `diff'
+program.
+To correctly apply a patch you need to know what base it was generated from
+and what new version the patch will change the source tree into. These
+should both be present in the patch file metadata or be possible to deduce
+from the filename.
+
+
+How do I apply or revert a patch?
+---
+ You apply a patch with the `patch' program. The patch program reads a diff
+(or patch) file and makes the changes to the source tree described in it.
+
+Patches for the Linux kernel are generated relative to the parent directory
+holding the kernel source dir.
+
+This means that paths to files inside the patch file contain the name of the
+kernel source directories it was generated against (or some other directory
+names like "a/" and "b/").
+Since this is unlikely to match the name of the kernel source dir on your
+local machine (but is often useful info to see what version an otherwise
+unlabeled patch was generated against) you should change into your kernel
+source directory and then strip the first element of the path from filenames
+in the patch file when applying it (the -p1 argument to `patch' does this).
+
+To revert a previously applied patch, use the -R argument to patch.
+So, if you applied a patch like this:
+        patch -p1 < ../patch-x.y.z
+
+You can revert (undo) it like this:
+        patch -R -p1 < ../patch-x.y.z
+
+
+How do I feed a patch/diff file to `patch'?
+---
+ This (as usual with Linux and other UNIX like operating systems) can be
+done in several different ways.
+In all the examples below I feed the file (in uncompressed form) to patch
+via stdin using the following syntax:
+        patch -p1 < path/to/patch-x.y.z
+
+If you just want to be able to follow the examples below and don't want to
+know of more than one way to use patch, then you can stop reading this
+section here.
+
+Patch can also get the name of the file to use via the -i argument, like
+this:
+        patch -p1 -i path/to/patch-x.y.z
+
+If your patch file is compressed with gzip or bzip2 and you don't want to
+uncompress it before applying it, then you can feed it to patch like this
+instead:
+        zcat path/to/patch-x.y.z.gz | patch -p1
+        bzcat path/to/patch-x.y.z.bz2 | patch -p1
+
+If you wish to uncompress the patch file by hand first before applying it
+(what I assume you've done in the examples below), then you simply run
+gunzip or bunzip2 on the file - like this:
+        gunzip patch-x.y.z.gz
+        bunzip2 patch-x.y.z.bz2
+
+Which will leave you with a plain text patch-x.y.z file that you can feed to
+patch via stdin or the -i argument, as you prefer.
+
+A few other nice arguments for patch are -s which causes patch to be silent
+except for errors which is nice to prevent errors from scrolling out of the
+screen too fast, and --dry-run which causes patch to just print a listing of
+what would happen, but doesn't actually make any changes. Finally --verbose
+tells patch to print more information about the work being done.
+
+
+Common errors when patching
+---
+ When patch applies a patch file it attempts to verify the sanity of the
+file in different ways.
+Checking that the file looks like a valid patch file, checking the code
+around the bits being modified matches the context provided in the patch are
+just two of the basic sanity checks patch does.
+
+If patch encounters something that doesn't look quite right it has two
+options. It can either refuse to apply the changes and abort or it can try
+to find a way to make the patch apply with a few minor changes.
+
+One example of something that's not 'quite right' that patch will attempt to
+fix up is if all the context matches, the lines being changed match, but the
+line numbers are different. This can happen, for example, if the patch makes
+a change in the middle of the file but for some reasons a few lines have
+been added or removed near the beginning of the file. In that case
+everything looks good it has just moved up or down a bit, and patch will
+usually adjust the line numbers and apply the patch.
+
+Whenever patch applies a patch that it had to modify a bit to make it fit
+it'll tell you about it by saying the patch applied with 'fuzz'.
+You should be wary of such changes since even though patch probably got it
+right it doesn't /always/ get it right, and the result will sometimes be
+wrong.
+
+When patch encounters a change that it can't fix up with fuzz it rejects it
+outright and leaves a file with a .rej extension (a reject file). You can
+read this file to see exactely what change couldn't be applied, so you can
+go fix it up by hand if you wish.
+
+If you don't have any third party patches applied to your kernel source, but
+only patches from kernel.org and you apply the patches in the correct order,
+and have made no modifications yourself to the source files, then you should
+never see a fuzz or reject message from patch. If you do see such messages
+anyway, then there's a high risk that either your local source tree or the
+patch file is corrupted in some way. In that case you should probably try
+redownloading the patch and if things are still not OK then you'd be advised
+to start with a fresh tree downloaded in full from kernel.org.
+
+Let's look a bit more at some of the messages patch can produce.
+
+If patch stops and presents a "File to patch:" prompt, then patch could not
+find a file to be patched. Most likely you forgot to specify -p1 or you are
+in the wrong directory. Less often, you'll find patches that need to be
+applied with -p0 instead of -p1 (reading the patch file should reveal if
+this is the case - if so, then this is an error by the person who created
+the patch but is not fatal).
+
+If you get "Hunk #2 succeeded at 1887 with fuzz 2 (offset 7 lines)." or a
+message similar to that, then it means that patch had to adjust the location
+of the change (in this example it needed to move 7 lines from where it
+expected to make the change to make it fit).
+The resulting file may or may not be OK, depending on the reason the file
+was different than expected.
+This often happens if you try to apply a patch that was generated against a
+different kernel version than the one you are trying to patch.
+
+If you get a message like "Hunk #3 FAILED at 2387.", then it means that the
+patch could not be applied correctly and the patch program was unable to
+fuzz its way through. This will generate a .rej file with the change that
+caused the patch to fail and also a .orig file showing you the original
+content that couldn't be changed.
+
+If you get "Reversed (or previously applied) patch detected!  Assume -R? [n]"
+then patch detected that the change contained in the patch seems to have
+already been made.
+If you actually did apply this patch previously and you just re-applied it
+in error, then just say [n]o and abort this patch. If you applied this patch
+previously and actually intended to revert it, but forgot to specify -R,
+then you can say [y]es here to make patch revert it for you.
+This can also happen if the creator of the patch reversed the source and
+destination directories when creating the patch, and in that case reverting
+the patch will in fact apply it.
+
+A message similar to "patch: **** unexpected end of file in patch" or "patch
+unexpectedly ends in middle of line" means that patch could make no sense of
+the file you fed to it. Either your download is broken or you tried to feed
+patch a compressed patch file without uncompressing it first.
+
+As I already mentioned above, these errors should never happen if you apply
+a patch from kernel.org to the correct version of an unmodified source tree.
+So if you get these errors with kernel.org patches then you should probably
+assume that either your patch file or your tree is broken and I'd advice you
+to start over with a fresh download of a full kernel tree and the patch you
+wish to apply.
+
+
+Are there any alternatives to `patch'?
+---
+ Yes there are alternatives. You can use the `interdiff' program
+(http://cyberelk.net/tim/patchutils/) to generate a patch representing the
+differences between two patches and then apply the result.
+This will let you move from something like 2.6.12.2 to 2.6.12.3 in a single
+step. The -z flag to interdiff will even let you feed it patches in gzip or
+bzip2 compressed form directly without the use of zcat or bzcat or manual
+decompression.
+
+Here's how you'd go from 2.6.12.2 to 2.6.12.3 in a single step:
+        interdiff -z ../patch-2.6.12.2.bz2 ../patch-2.6.12.3.gz | patch -p1
+
+Although interdiff may save you a step or two you are generally advised to
+do the additional steps since interdiff can get things wrong in some cases.
+
+ Another alternative is `ketchup', which is a python script for automatic
+downloading and applying of patches (http://www.selenic.com/ketchup/).
+
+Other nice tools are diffstat which shows a summary of changes made by a
+patch, lsdiff which displays a short listing of affected files in a patch
+file, along with (optionally) the line numbers of the start of each patch
+and grepdiff which displays a list of the files modified by a patch where
+the patch contains a given regular expression.
+
+
+Where can I download the patches?
+---
+ The patches are available at http://kernel.org/
+Most recent patches are linked from the front page, but they also have
+specific homes.
+
+The 2.6.x.y (-stable) and 2.6.x patches live at
+ ftp://ftp.kernel.org/pub/linux/kernel/v2.6/
+
+The -rc patches live at
+ ftp://ftp.kernel.org/pub/linux/kernel/v2.6/testing/
+
+The -git patches live at
+ ftp://ftp.kernel.org/pub/linux/kernel/v2.6/snapshots/
+
+The -mm kernels live at
+ ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/
+
+In place of ftp.kernel.org you can use ftp.cc.kernel.org, where cc is a
+country code. This way you'll be downloading from a mirror site that's most
+likely geographically closer to you, resulting in faster downloads for you,
+less bandwidth used globally and less load on the main kernel.org servers -
+these are good things, do use mirrors when possible.
+
+
+The 2.6.x kernels
+---
+ These are the base stable releases released by Linus. The highest numbered
+release is the most recent.
+
+If regressions or other serious flaws are found then a -stable fix patch
+will be released (see below) on top of this base. Once a new 2.6.x base
+kernel is released, a patch is made available that is a delta between the
+previous 2.6.x kernel and the new one.
+
+To apply a patch moving from 2.6.11 to 2.6.12 you'd do the following (note
+that such patches do *NOT* apply on top of 2.6.x.y kernels but on top of the
+base 2.6.x kernel - if you need to move from 2.6.x.y to 2.6.x+1 you need to
+first revert the 2.6.x.y patch).
+
+Here are some examples:
+
+# moving from 2.6.11 to 2.6.12
+$ cd ~/linux-2.6.11                     # change to kernel source dir
+$ patch -p1 < ../patch-2.6.12           # apply the 2.6.12 patch
+$ cd ..
+$ mv linux-2.6.11 linux-2.6.12          # rename source dir
+
+# moving from 2.6.11.1 to 2.6.12
+$ cd ~/linux-2.6.11.1                   # change to kernel source dir
+$ patch -p1 -R < ../patch-2.6.11.1      # revert the 2.6.11.1 patch
+                                        # source dir is now 2.6.11
+$ patch -p1 < ../patch-2.6.12           # apply new 2.6.12 patch
+$ cd ..
+$ mv linux-2.6.11.1 inux-2.6.12         # rename source dir
+
+
+The 2.6.x.y kernels
+---
+ Kernels with 4 digit versions are -stable kernels. They contain small(ish)
+critical fixes for security problems or significant regressions discovered
+in a given 2.6.x kernel.
+
+This is the recommended branch for users who want the most recent stable
+kernel and are not interested in helping test development/experimental
+versions.
+
+If no 2.6.x.y kernel is available, then the highest numbered 2.6.x kernel is
+the current stable kernel.
+
+These patches are not incremental, meaning that for example the 2.6.12.3
+patch does not apply on top of the 2.6.12.2 kernel source, but rather on top
+of the base 2.6.12 kernel source.
+So, in order to apply the 2.6.12.3 patch to your existing 2.6.12.2 kernel
+source you have to first back out the 2.6.12.2 patch (so you are left with a
+base 2.6.12 kernel source) and then apply the new 2.6.12.3 patch.
+
+Here's a small example:
+
+$ cd ~/linux-2.6.12.2                   # change into the kernel source dir
+$ patch -p1 -R < ../patch-2.6.12.2      # revert the 2.6.12.2 patch
+$ patch -p1 < ../patch-2.6.12.3         # apply the new 2.6.12.3 patch
+$ cd ..
+$ mv linux-2.6.12.2 linux-2.6.12.3      # rename the kernel source dir
+
+
+The -rc kernels
+---
+ These are release-candidate kernels. These are development kernels released
+by Linus whenever he deems the current git (the kernel's source management
+tool) tree to be in a reasonably sane state adequate for testing.
+
+These kernels are not stable and you should expect occasional breakage if
+you intend to run them. This is however the most stable of the main
+development branches and is also what will eventually turn into the next
+stable kernel, so it is important that it be tested by as many people as
+possible.
+
+This is a good branch to run for people who want to help out testing
+development kernels but do not want to run some of the really experimental
+stuff (such people should see the sections about -git and -mm kernels below).
+
+The -rc patches are not incremental, they apply to a base 2.6.x kernel, just
+like the 2.6.x.y patches described above. The kernel version before the -rcN
+suffix denotes the version of the kernel that this -rc kernel will eventually
+turn into.
+So, 2.6.13-rc5 means that this is the fifth release candidate for the 2.6.13
+kernel and the patch should be applied on top of the 2.6.12 kernel source.
+
+Here are 3 examples of how to apply these patches:
+
+# first an example of moving from 2.6.12 to 2.6.13-rc3
+$ cd ~/linux-2.6.12                     # change into the 2.6.12 source dir
+$ patch -p1 < ../patch-2.6.13-rc3       # apply the 2.6.13-rc3 patch
+$ cd ..
+$ mv linux-2.6.12 linux-2.6.13-rc3      # rename the source dir
+
+# now let's move from 2.6.13-rc3 to 2.6.13-rc5
+$ cd ~/linux-2.6.13-rc3                 # change into the 2.6.13-rc3 dir
+$ patch -p1 -R < ../patch-2.6.13-rc3    # revert the 2.6.13-rc3 patch
+$ patch -p1 < ../patch-2.6.13-rc5       # apply the new 2.6.13-rc5 patch
+$ cd ..
+$ mv linux-2.6.13-rc3 linux-2.6.13-rc5  # rename the source dir
+
+# finally let's try and move from 2.6.12.3 to 2.6.13-rc5
+$ cd ~/linux-2.6.12.3                   # change to the kernel source dir
+$ patch -p1 -R < ../patch-2.6.12.3      # revert the 2.6.12.3 patch
+$ patch -p1 < ../patch-2.6.13-rc5       # apply new 2.6.13-rc5 patch
+$ cd ..
+$ mv linux-2.6.12.3 linux-2.6.13-rc5    # rename the kernel source dir
+
+
+The -git kernels
+---
+ These are daily snapshots of Linus' kernel tree (managed in a git
+repository, hence the name).
+
+These patches are usually released daily and represent the current state of
+Linus' tree. They are more experimental than -rc kernels since they are
+generated automatically without even a cursory glance to see if they are
+sane.
+
+-git patches are not incremental and apply either to a base 2.6.x kernel or
+a base 2.6.x-rc kernel - you can see which from their name.
+A patch named 2.6.12-git1 applies to the 2.6.12 kernel source and a patch
+named 2.6.13-rc3-git2 applies to the source of the 2.6.13-rc3 kernel.
+
+Here are some examples of how to apply these patches:
+
+# moving from 2.6.12 to 2.6.12-git1
+$ cd ~/linux-2.6.12                     # change to the kernel source dir
+$ patch -p1 < ../patch-2.6.12-git1      # apply the 2.6.12-git1 patch
+$ cd ..
+$ mv linux-2.6.12 linux-2.6.12-git1     # rename the kernel source dir
+
+# moving from 2.6.12-git1 to 2.6.13-rc2-git3
+$ cd ~/linux-2.6.12-git1                # change to the kernel source dir
+$ patch -p1 -R < ../patch-2.6.12-git1   # revert the 2.6.12-git1 patch
+                                        # we now have a 2.6.12 kernel
+$ patch -p1 < ../patch-2.6.13-rc2       # apply the 2.6.13-rc2 patch
+                                        # the kernel is now 2.6.13-rc2
+$ patch -p1 < ../patch-2.6.13-rc2-git3  # apply the 2.6.13-rc2-git3 patch
+                                        # the kernel is now 2.6.13-rc2-git3
+$ cd ..
+$ mv linux-2.6.12-git1 linux-2.6.13-rc2-git3    # rename source dir
+
+
+The -mm kernels
+---
+ These are experimental kernels released by Andrew Morton.
+
+The -mm tree serves as a sort of proving ground for new features and other
+experimental patches.
+Once a patch has proved its worth in -mm for a while Andrew pushes it on to
+Linus for inclusion in mainline.
+
+Although it's encouraged that patches flow to Linus via the -mm tree, this
+is not always enforced.
+Subsystem maintainers (or individuals) sometimes push their patches directly
+to Linus, even though (or after) they have been merged and tested in -mm (or
+sometimes even without prior testing in -mm).
+
+You should generally strive to get your patches into mainline via -mm to
+ensure maximum testing.
+
+This branch is in constant flux and contains many experimental features, a
+lot of debugging patches not appropriate for mainline etc and is the most
+experimental of the branches described in this document.
+
+These kernels are not appropriate for use on systems that are supposed to be
+stable and they are more risky to run than any of the other branches (make
+sure you have up-to-date backups - that goes for any experimental kernel but
+even more so for -mm kernels).
+
+These kernels in addition to all the other experimental patches they contain
+usually also contain any changes in the mainline -git kernels available at
+the time of release.
+
+Testing of -mm kernels is greatly appreciated since the whole point of the
+tree is to weed out regressions, crashes, data corruption bugs, build
+breakage (and any other bug in general) before changes are merged into the
+more stable mainline Linus tree.
+But testers of -mm should be aware that breakage in this tree is more common
+than in any other tree.
+
+The -mm kernels are not released on a fixed schedule, but usually a few -mm
+kernels are released in between each -rc kernel (1 to 3 is common).
+The -mm kernels apply to either a base 2.6.x kernel (when no -rc kernels
+have been released yet) or to a Linus -rc kernel.
+
+Here are some examples of applying the -mm patches:
+
+# moving from 2.6.12 to 2.6.12-mm1
+$ cd ~/linux-2.6.12                     # change to the 2.6.12 source dir
+$ patch -p1 < ../2.6.12-mm1             # apply the 2.6.12-mm1 patch
+$ cd ..
+$ mv linux-2.6.12 linux-2.6.12-mm1      # rename the source appropriately
+
+# moving from 2.6.12-mm1 to 2.6.13-rc3-mm3
+$ cd ~/linux-2.6.12-mm1
+$ patch -p1 -R < ../2.6.12-mm1          # revert the 2.6.12-mm1 patch
+                                        # we now have a 2.6.12 source
+$ patch -p1 < ../patch-2.6.13-rc3       # apply the 2.6.13-rc3 patch
+                                        # we now have a 2.6.13-rc3 source
+$ patch -p1 < ../2.6.13-rc3-mm3         # apply the 2.6.13-rc3-mm3 patch
+$ cd ..
+$ mv linux-2.6.12-mm1 linux-2.6.13-rc3-mm3      # rename the source dir
+
+
+This concludes this list of explanations of the various kernel trees and I
+hope you are now crystal clear on how to apply the various patches and help
+testing the kernel.
+