Resolving Conflicts when Rebuilding drm-tip

When you push patches with dim drm-tip always gets rebuilt and this can sometimes fail, for example like this:

Updating rerere cache and nightly.conf... Done.
Fetching drm-upstream... Done.
Fetching origin... Done.
Fetching sound-upstream... Done.
Merging origin/drm-intel-fixes... Reset. Done.
Merging drm-upstream/drm-fixes... Fast-forward. Done.
Merging origin/drm-intel-next-fixes... Done.
Merging origin/drm-intel-next... ++<<<<<<< HEAD
++=======
++>>>>>>> origin/drm-intel-next
Fail: conflict merging origin/drm-intel-next

Often it’s very easy to resolve such conflicts, but maintainers can take over when it’s tricky or something fails in the below procedure.

1. First check that drm-intel-next was indeed pushed correctly and that your local and remote branches match.

2. Then rebuild the integration branch just to confirm:

   $ dim rebuild-tip
   

   It’s handy to keep the log output for context so that you know which branch caused the conflicts, and which branches are already included.

3. Switch to $DIM_PREFIX/drm-tip and analyze the conflict:

   $ cd $DIM_PREFIX/drm-tip
   $ git diff # shows three-way diff of conflict
   $ gitk --merge # lists all commits git believes to be relevant
   

   If the conflict is simple and created by one of the patches you pushed fix things up and compile/test the resulting kernel. In case of doubt just ping authors of other patches or maintainers on IRC.

4. When you’re happy with the resolution commit it with:

   $ git commit -a
   

   git will then store the conflict resolution internally (see git help rerere for how this is implemented). Then re-run drm-tip generation to confirm the resolution has been captured correctly by git (sometimes git rerere can’t match up your resolution with the conflict for odd reasons) and to make sure there’s no other conflict in later merges:

   $ dim rebuild-tip
   

   This will also push the stored conflict resolution to the drm-intel-rerere branch and therefore publishes your resolution. Everything before this step has just local effects.

And if any step fails or the conflict is tricky just ping maintainers.

Removing a Wrong Conflict Resolution

Occasionally someone screws up and pushes a broken merge conflict resolution to drm-tip. Getting rid of that requires a few steps.

1. First identify which commit in the drm-rerere branch contains the bad merge resolution. If you know who pushed the bad merge then searching by author is easiest, otherwise figure out which stored resolution is the wrong one.

2. Revert that commit and make sure there’s no other cache resolutions around:

   $ cd $DIM_PREFIX/drm-rerere
   $ git pull # to avoid conflicts with others
   $ git revert $broken_resolution_sha1
   $ git clean -dfx
   

3. Re-resolve the conflict and try to get it right this time around:

   $ dim rebuild-tip
   

If the Conflict Reappears

In some odd cases git rerere fails to recognize the conflict, and doesn’t reapply the stored conflict resolution. This needs to be handled with a manual fixup patch, and the best way to go about this is:

1. Try to resolve the conflict normally, but then running:

   $ dim rebuild-tip
   

   fails. First, store the current state, including the conflict markers and with no other changes applied:

   $ cd $DIM_PREFIX/drm-tip
   $ git add -u
   $ git commit
   

2. Resolve the conflict normally, but don’t stage it or commit it in any fashion. Check that the resolution looks correct and removes all the conflict markers you’ve just committed:

   $ git diff
   

   Then store it as a manual fixup patch:

   $ git diff | dim cat-to-fixup
   

3. Finally rebuild the integration tree, which should now go through smoothly, at least for this merge:

   $ dim rebuild-tip
   

Anytime you add or change a manual merge fixup please inform the maintainers of both involved trees so that they are aware of the situation and can consider resolving the conflict permantly with a backmerge or pull.

Fixing Silent Conflicts

A really annoying case is when a merge has a silent conflict, i.e. git merge succeeds, but the resulting source fails to compile or run. Often this happens when one branch changes a function or structure, while a 2nd branch adds a new user. The important part is to make sure we supply the fixup patch for the right merge commit.

1. Identify the merge that breaks the build.

2. Rebuild drm-tip in interactive mode, and stop after the broken merge has been done:

   $ dim -i rebuild-tip
   

   Interrupt the rebuilding of drm-tip by hitting ^C when it asks you to hit a key to proceed.

3. Resolve the conflict normally, but don’t stage it or commit it in any fashion. Check that the resolution looks correct:

   $ git diff
   

   Of course also make sure it actually builds/works. Then store it as a manual fixup patch:

   $ git diff | dim cat-to-fixup
   

4. Finally rebuild the integration tree, which should now result in a working tree with no broken merges:

   $ dim rebuild-tip
   

Anytime you add or change a manual merge fixup please inform the maintainers of both involved trees so that they are aware of the situation and can consider resolving the conflict permantly with a backmerge or pull.