[vlc-devel] [PATCH v3] picture: align pictures on 64 bytes

[Steve Lhomme]

On 12/11/2018 14:09, Rémi Denis-Courmont wrote:
> No. The plane offset from the start of the buffer / the first plane 
> will be a multiple of 512 maybe. But the plane alignment cannot be 
> larger than the containing buffer alignment.

I don't see a problem here.

If you allocate a 4:4:4 1x1 pixels picture it's adjusted to 32x16 pixels 
internally. Your lines will be 32 bytes and you'll have 16 of them per 
plane. Each plane will be 512 bytes after the previous one.
If you allocate a 4:2:0 1x1 pixels picture it's adjusted to 32x32 pixels 
internally. The first plane is 1024 bytes big, the other two are 256 
bytes big.

(this wasn't true when we arbitrarily added the 2 extra lines)

So with the current picture_Setup() was cannot go lower than 256 bytes 
aligned for any planes. That's far more than the 64 bytes alignment lavc 
requires on all planes.

> So we should probably align the allocation to at least 512 for 
> consistency. IMO we should align to page size and forget about the 
> problem going forward, as we can probably never align beyond that.
>
> Le 12 novembre 2018 13:44:08 GMT+02:00, Steve Lhomme 
> <robux4 at ycbcr.xyz> a écrit :
>
>     On 12/11/2018 11:59, Rémi Denis-Courmont wrote:
>
>         As far as I am concerned, the picture buffer could and maybe
>         should be page-aligned anyway. The real question is what the
>         plane and scan line alignment should be, which this patch
>         leaves entirely open. 
>
>
>     picture_Allocate() is called after picture_Setup() which aligns lines to
>     at least 32 bytes. Then the amount of lines is also made a multiple of
>     16. So all the planes are always aligned to 512 bytes.
>
>     >
>
>         Le 12 novembre 2018 11:48:25 GMT+02:00, Francois Cartegnie
>         <fcvlcdev at free.fr> a écrit : Le 12/11/2018 à 09:39, Steve
>         Lhomme a écrit : Code relying on AVX-2 (like dav1d) cannot
>         work without an alignment of 32 bytes at least. Even avcodec
>         has a requirement of 64 (likely to get at least 32 on all
>         planes). So it's probably time to upgrade. picture_Setup
>         ensures a horizontal alignment to 32 bytes and a vertical to
>         16 bytes. So for all buffers allocated by the core, we have a
>         size multiple of 512 bytes. Didn't we had an issue with some
>         codec*filter*idontremember and anything 16 ? -- Envoyé de mon
>         appareil Android avec Courriel K-9 Mail. Veuillez excuser ma
>         brièveté.
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> -- 
> Envoyé de mon appareil Android avec Courriel K-9 Mail. Veuillez 
> excuser ma brièveté.
>
>
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