[FFmpeg-devel] [PATCH] avcodec: add AVCODEC_REQUIRED_INPUT_BUFFER_PADDING_SIZE, split FF_INPUT_BUFFER_PADDING_SIZE

[Reimar Döffinger]

On 13.06.2014, at 12:41, wm4 <nfxjfg at googlemail.com> wrote:

> On Fri, 13 Jun 2014 03:32:59 +0200
> Michael Niedermayer <michaelni at gmx.at> wrote:
> 
>> On Fri, Jun 13, 2014 at 02:49:01AM +0200, wm4 wrote:
>> 
>> AVCODEC_REQUIRED_INPUT_BUFFER_PADDING_SIZE represents the amount of
>> padding that is required for input to libavcodec
>> 
>> FF_INPUT_BUFFER_PADDING_SIZE represents the amount that we pad.
>> Using this we can pass the output from libavcodec to other libs like
>> libavfilter in the case where (due to rawvideo) the input has been
>> passed through
>> 
>> I hope above is understandable
> 
> Yes, but still confusing. At least it could be a global constant, maybe
> defined in libavutil/mem.h? Actually, why does not just av_malloc add
> the padding? It also ensures alignment, so why not. (It will waste some
> memory, but you do that for alignment too, with every allocation, even
> strings.)
> 
> Declaring that input buffers must be allocated with av_malloc because
> ffmpeg is so damn "special" is probably easier to understand for API
> users. For advanced users, enumerating the memory buffer requirements
> somewhere else might be useful too.

That has serious implications for zero-copy implementations, which at the 4k resolution level at least are critical.
However it also means that increasing alignment requirements would mean doing a major version bump of avutil, instead of just e.g. updating the existing avcodec functions that specify the required alignment to an application.