CONFIG_LOCALVERSION_AUTO和git describe

最近在編譯某家大廠的BSP時，發現kernel version在正常的tag之後被加了一大串東西：

include/config/kernel.release:1:2.6.32.11+drm33.2-16040-g98cebdd

2.6.32.11+drm33.2是Ubuntu Lucid kernel的version沒錯，但是後面的-16040-g98cebdd並不是Ubuntu kernel版號的一部份，看起來是像`git describe`的結果。

執行一下，果然沒錯：

$ git describe
v2.6.31-rc4-16040-g98cebdd

v2.6.31-rc4是某個tag，16040表示目前的HEAD是從tag v2.6.31-rc4之後第16040個commit，g98cebdd表示目前commit的git sha1前幾個值是98cebdd...。

而這個版號是因為 CONFIG_LOCALVERSION_AUTO=y 產生出來的。從LOCALVERSION_AUTO的文義有點難聯想到它的行為。

以下是完整的文件說明：

config LOCALVERSION_AUTO
bool "Automatically append version information to the version string"
default y
help
This will try to automatically determine if the current tree is a
release tree by looking for git tags that belong to the current
top of tree revision.

A string of the format -gxxxxxxxx will be added to the localversion
if a git-based tree is found. The string generated by this will be
appended after any matching localversion* files, and after the value
set in CONFIG_LOCALVERSION.

(The actual string used here is the first eight characters produced
by running the command:

$ git rev-parse --verify HEAD

which is done within the script "scripts/setlocalversion".)