Git rebase 的應用經驗

為了玩 Android，學會了 的初步使用。不過僅止於基本的 git checkout、git log、git diff、git status、git commit 等。

這幾天為了協助同事將 Android 移植到廠商的板子上，必須將廠商提供的 kernel 與 Android kernel 做結合。為此請教一些 git 高手。經過一番練習，總算對 git rebase 的使用有了初步的掌握。也漸漸了解了 git 的強大威力，為什麼高手都愛用… 特別將這段經驗記載在這邊，給有興趣的人參考。

有兩個 git repositories 要做合併。一個是廠商提供的 kernel：

git://git.kernel.org/pub/scm/linux/kernel/git/kki_ap/samsung-ap-2.6.git

另一個是 Android 的 kernel：

git://android.git.kernel.org/kernel/common.git

首先，先建立一個工作目錄

$ mkdir kernel-git
$ cd kernel-git
$ git init
Initialized empty Git repository in /home/cwhuang/git/kernel-git/.git/

然後加入兩個遠端的 repositories：

$ git remote add samsung git://git.kernel.org/pub/scm/linux/kernel/git/kki_ap/samsung-ap-2.6.git
$ git remote add android git://android.git.kernel.org/kernel/common.git

將遠端 repositories 的資料取回：

$ git fetch samsung
remote: Counting objects: 11677, done.
remote: Compressing objects: 100% (2462/2462), done.
remote: Total 10383 (delta 7976), reused 10138 (delta 7743)
Receiving objects: 100% (10383/10383), 5.04 MiB, done.
Resolving deltas: 100% (7976/7976), completed with 324 local objects.
From git://git.kernel.org/pub/scm/linux/kernel/git/kki_ap/samsung-ap-2.6.git
 * [new branch]      master     -> samsung/master
 * [new tag]         v2.6.28.6-samsung-v1 -> v2.6.28.6-samsung-v1
From git://git.kernel.org/pub/scm/linux/kernel/git/kki_ap/samsung-ap-2.6.git
 * [new tag]         v2.6.28-rc4-s3c64xx -> v2.6.28-rc4-s3c64xx
 * [new tag]         v2.6.28-rc4-s5pc1xx -> v2.6.28-rc4-s5pc1xx
 * [new tag]         v2.6.28-rc5-s3c64xx -> v2.6.28-rc5-s3c64xx
 * [new tag]         v2.6.28-rc5-s5pc1xx -> v2.6.28-rc5-s5pc1xx
 * [new tag]         v2.6.28-rc6-s3c64xx -> v2.6.28-rc6-s3c64xx
 * [new tag]         v2.6.28-rc6-s5pc1xx -> v2.6.28-rc6-s5pc1xx
 * [new tag]         v2.6.28-rc7-s3c64xx -> v2.6.28-rc7-s3c64xx
 * [new tag]         v2.6.28-rc7-s5pc1xx -> v2.6.28-rc7-s5pc1xx
 * [new tag]         v2.6.28-rc8-s3c64xx -> v2.6.28-rc8-s3c64xx
 * [new tag]         v2.6.28-rc8-s5pc1xx -> v2.6.28-rc8-s5pc1xx
 * [new tag]         v2.6.28-rc8-s5pc1xx-v1 -> v2.6.28-rc8-s5pc1xx-v1
 * [new tag]         v2.6.28.6-samsung -> v2.6.28.6-samsung

$ git fetch android
remote: Counting objects: 1077412, done.
remote: Compressing objects: 100% (176782/176782), done.
Receiving objects: 100% (1077412/1077412), 263.30 MiB | 28369 KiB/s, done.
remote: Total 1077412 (delta 896526), reused 1076403 (delta 895639)
Resolving deltas: 100% (896526/896526), done.
From git://android.git.kernel.org/kernel/common.git
 * [new branch]      2.6.27     -> android/2.6.27
 * [new branch]      2.6.29     -> android/2.6.29
From git://android.git.kernel.org/kernel/common.git
 * [new tag]         v2.6.12    -> v2.6.12
 * [new tag]         v2.6.12-rc2 -> v2.6.12-rc2
(skipped...)
 * [new tag]         v2.6.29-rc8 -> v2.6.29-rc8

看看有哪些 branches?

$ git branch -a
  android/2.6.27
  android/2.6.29
  samsung/master

在合併之前，先了解一下已存在的 branches。我們可以先看看這些 branches 的共同祖先(common ancestor)：

$ git merge-base samsung/master android/2.6.27
3fa8749e584b55f1180411ab1b51117190bac1e5

$ git show 3fa8749e584b55f1180411ab1b51117190bac1e5
commit 3fa8749e584b55f1180411ab1b51117190bac1e5
Author: Linus Torvalds 
Date:   Thu Oct 9 15:13:53 2008 -0700

    Linux 2.6.27

diff --git a/Makefile b/Makefile
index ce9eceb..16e3fbb 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 VERSION = 2
 PATCHLEVEL = 6
 SUBLEVEL = 27
-EXTRAVERSION = -rc9
+EXTRAVERSION =
 NAME = Rotary Wombat

 # *DOCUMENTATION*

$ git merge-base samsung/master android/2.6.29
4a6908a3a050aacc9c3a2f36b276b46c0629ad91

$ git show 4a6908a3a050aacc9c3a2f36b276b46c0629ad91
commit 4a6908a3a050aacc9c3a2f36b276b46c0629ad91
Author: Linus Torvalds 
Date:   Wed Dec 24 15:26:37 2008 -0800

    Linux 2.6.28

    Happy holidays..

diff --git a/Makefile b/Makefile
index 4c8d797..71e98e9 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 VERSION = 2
 PATCHLEVEL = 6
 SUBLEVEL = 28
-EXTRAVERSION = -rc9
+EXTRAVERSION =
 NAME = Erotic Pickled Herring

 # *DOCUMENTATION*

因此 branch samsung/master 和 android/2.6.27 都是從 v2.6.27 來的，而 samsung/master 和 android/2.6.29 則從 v2.6.28 而來。事實上，不難驗證 samsung/master 是從 v2.6.28.6 改來的。

可以用 gitk 這個圖形化工具來看看從 v2.6.27 到 android/2.6.27 的變化：

$ gitk v2.6.27..android/2.6.27

同樣的，samsung/master 的變化是：

$ gitk v2.6.28..samsung/master

我們要決定如何進行合併。可以將 samsung 的修改合併到 Android kernel，也可以將 Android 的修改合併到 samsung kernel。我決定選擇後者，因為從 gitk 可以看出 Android 的變化相當簡單而清楚，就是單線發展而已。相對的，samsung kernel 就併來併去，線條亂七八糟，看得眼花瞭亂。因此將 Android 的修改併到 samsung kernel，應該比較簡單而容易成功。

可以用 git rebase 來進行合併。首先，建立一個新的本地 branch 來保存合併結果：

$ git branch --track samsung-android samsung/master

然後用 git rebase 指令，將 v2.6.27 到 android/2.6.27 的修改，一個一個的 apply 到 samsung-android 這個 branch 中：

$ git rebase --whitespace=fix --onto samsung-android v2.6.27 android/2.6.27
First, rewinding head to replay your work on top of it...
Applying: PM: Add wake lock api.
Applying: PM: Add early suspend api.
Applying: PM: Implement wakelock api.
Applying: PM: Implement early suspend api
Applying: PM: Enable early suspend through /sys/power/state
Applying: PM: Add user-space wake lock api.
Applying: PM: wakelock: Abort task freezing if a wake lock is held.
Applying: PM: earlysuspend: Add console switch when user requested sleep state changes.
Applying: PM: earlysuspend: Removing dependence on console.
Applying: rtc: Add android alarm driver.
Applying: rtc: Try to prevent RTC errors from accumulating.
Applying: ledtrig-sleep: Add led trigger for sleep debugging.
error: patch failed: drivers/leds/Makefile:29
error: drivers/leds/Makefile: patch does not apply
Using index info to reconstruct a base tree...
Falling back to patching base and 3-way merge...
Auto-merged drivers/leds/Kconfig
Auto-merged drivers/leds/Makefile
Applying: PM: Wait for console in resume.
Applying: PM: Fix suspend_console/resume_console use only one semaphore.
Applying: switch: switch class and GPIO drivers.
Applying: lowmemorykiller: Initial support for 2.6.27
error: patch failed: drivers/misc/Kconfig:475
error: drivers/misc/Kconfig: patch does not apply
error: patch failed: drivers/misc/Makefile:30
error: drivers/misc/Makefile: patch does not apply
Using index info to reconstruct a base tree...
Falling back to patching base and 3-way merge...
Auto-merged drivers/misc/Kconfig
CONFLICT (content): Merge conflict in drivers/misc/Kconfig
Auto-merged drivers/misc/Makefile
CONFLICT (content): Merge conflict in drivers/misc/Makefile
Failed to merge in the changes.
Patch failed at 0016.

由這些訊息可看出，前 15 個修改被成功的 apply 上去，但第 16 個產生 conflicts。先用 git status 看看問題出在哪裡：

$ git status
drivers/misc/Kconfig: needs merge
drivers/misc/Makefile: needs merge
# Not currently on any branch.
# Changes to be committed:
#   (use "git reset HEAD ..." to unstage)
#
#	new file:   drivers/misc/lowmemorykiller.c
#
# Changed but not updated:
#   (use "git add ..." to update what will be committed)
#
#	unmerged:   drivers/misc/Kconfig
#	unmerged:   drivers/misc/Makefile
#

可以看出有兩個檔案有 conflicts，必須手動修改。例如，在 drivers/misc/Makefile 裡 conflicts 大約在 35 至 39 行：

<<<<<<< HEAD:drivers/misc/Makefile
obj-$(CONFIG_C2PORT)            += c2port/
=======
obj-$(CONFIG_LOW_MEMORY_KILLER) += lowmemorykiller.o
>>>>>>> lowmemorykiller: Initial support for 2.6.27:drivers/misc/Makefile

不難看出，這兩行都要加到 Makefile 中。因此我改成：

obj-$(CONFIG_C2PORT)            += c2port/
obj-$(CONFIG_LOW_MEMORY_KILLER) += lowmemorykiller.o

接著

$ git add drivers/misc/Makefile

用同樣的方法修改 drivers/misc/Kconfig，然後

$ git rebase --continue

如此會繼續合併的過程。當然可能產生其它的 conflicts，都用類似的方法修正，直到 rebase 完成。

如果覺得某個產生 conflicts 的修改是不必要的，可以跳過：

$ git rebase --skip

如果覺得 conflicts 太複雜了，不知要麼修改，想要放棄，可以

$ git rebase --abort

如此會放棄所有的合併結果，而將 source tree 回到合併前的狀態。