Bug#738760: libav: Add proper raspberry pi CPU detection

[Reinhard Tartler]

On January 6, 2015 8:16:51 AM EST, Sebastian Ramacher <sramacher at debian.org> wrote:
>Control: tags -1 + patch
>
>Hi Peter
>
>On 2015-01-03 22:36:13, peter green wrote:
>> Sebastian Ramacher wrote:
>> >there was a request to change the handling of the Raspberry Pi in
>the
>> >libav package. Could you please explain the changes applied to the
>> >Raspbian version?
>> In raspbian we have a checker that runs after all our autobuilds (and
>I
>> manually run a similar check when I do manual builds) that looks for
>files
>> tagged as armv7 (note: it seems using neon compiler options causes
>files to
>> be tagged as armv7 even if the CPU options aren't changed) and
>prevents the
>> autobuilder from uploading them.
>> 
>> In general i've been adopting the principle that having armv7 code in
>> raspbian is at best a pointless waste of space (if the code in
>question is
>> correctly guarded behind runtime CPU detection) and at worst a
>serious
>> problem (if it isn't). Therefore I have been responding to cases like
>this
>> by simply disabling the armv7/neon code. The raspbian libav package
>got this
>> treatment.
>> 
>> It's possible that I dodn't selct the best combination of flags, I'm
>no
>> libav expert.
>> 
>> I have not tested whether an unodified debian libav package built in
>a
>> raspbian chroot actually works on the Pi or not.
>
>Thank you for the explanation. What about the attached patch? [1]
>
>I've looked through the code and the only place where
>--enable-runtime-cpudetect
>makes a difference is on powerpc (libavutil/ppc/cpu.c). Special
>handling for
>Raspbian should not make a difference. Reinhard, is my reading of the
>code
>correct?

I don't know if it is correct, but skimmed over it and did not spot obvious mistakes. Nevertheless, I personally do not have a rpi, nor the infrastructure to test it. Peter, if that patch works for raspian, and if you would be willing to report breakage as it occurs, then I'd be happy to have this change in the Debian package. Otherwise I'd fear it would just bitrot.

Many thanks to Sebastian for working in this patch!

Reinhard



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