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Date: Wed, 06 Apr 2011 10:06:52 +0200
From: Michal Marek <mmarek@suse.cz>
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To: Valdis.Kletnieks@vt.edu
Cc: Armin Schindler <armin@melware.de>, linux-kbuild@vger.kernel.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [PATCH 12/34] isdn/diva: Drop __TIME__ usage
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On 5.4.2011 21:17, Valdis.Kletnieks@vt.edu wrote:
> On Tue, 05 Apr 2011 17:10:34 +0200, Armin Schindler said:
>> On Tue, 5 Apr 2011, Michal Marek wrote:
>>> The kernel already prints its build timestamp during boot, no need to
>>> repeat it in random drivers and produce different object files each
>>> time.
>>
>> The module can be build separately from the kernel, therefore it can have
>> an own build timestamp.
> 
> If the same code is being built as an out-of-tree module, that's a possibly
> good reason for a code version variable, but what does the build timestamp
> actually tell you?  If you already know foo_driver.c version 0.814 was buiilt
> against 2.6.41-rc2, in what cases does it matter if the compile was on Tuesday
> or Thursday - especially since an 'ls -l foo_driver.ko' will tell you?  If it's
> a matter of "the target .config changed on Wednesday", a build timestamp still
> doesn't help over 'ls -l'.

Exactly. Build timestamps are only a poor substitute for proper version
tracking. If you want to be able to reproduce the build of a binary, you
want it to embed some source revision, not the date when you built it.
For the kernel, you can use KBUILD_BUILD_TIMESTAMP=<source timestamp>,
for out-of-tree modules, you need to come up with something own.

Michal
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