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

[Steve Lhomme]

I know that. But I'm saying that whatever alignment we put in 
picture_Allocate() will scale fine with planes up to a multiple of 256. 
And that there's not really a problem with planes for now.

On 12/11/2018 16:04, Rémi Denis-Courmont wrote:
> Maths don't work that way. If you add a multiple of 512 offset to a 
> multiple of 16 pointer, you get a multiple of GCD(512, 16) = 16 
> pointer. Your plane is aligned to 16 bytes (or 32 bytes with the patch).
>
> Le 12 novembre 2018 15:31:16 GMT+02:00, Steve Lhomme 
> <robux4 at ycbcr.xyz> a écrit :
>
>     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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