[FFmpeg-devel] Yellow fever

[Måns Rullgård]

Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:

> On Mon, Aug 23, 2010 at 08:29:59PM +0200, Michael Niedermayer wrote:
>> On Mon, Aug 23, 2010 at 06:46:23PM +0100, M?ns Rullg?rd wrote:
>> > Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:
>> > 
>> > > On Sat, Aug 21, 2010 at 10:12:35AM +0100, M?ns Rullg?rd wrote:
>> > >> As you all have no doubt completely ignored, FATE turned much more
>> > >> yellow yesterday.  The reason for this is that I disabled unaligned
>> > >> memory access fixup on those machines which do not support it in
>> > >> hardware (Alpha, ARMv5, MIPS).  Most of these are failing a single
>> > >> test: lavfi-pixfmts_scale_le.
>> > >> 
>> > >> The problem here is btr32ToY() and related functions in libswscale:
>> > >> 
>> > >> #define BGR2Y(type, name, shr, shg, shb, maskr, maskg, maskb, RY, GY, BY, S)\
>> > >> static inline void name(uint8_t *dst, const uint8_t *src, long width, uint32_t *unused)\
>> > >> {\
>> > >>     int i;\
>> > >>     for (i=0; i<width; i++) {\
>> > >>         int b= (((const type*)src)[i]>>shb)&maskb;\
>> > >>         int g= (((const type*)src)[i]>>shg)&maskg;\
>> > >>         int r= (((const type*)src)[i]>>shr)&maskr;\
>> > >> \
>> > >>         dst[i]= (((RY)*r + (GY)*g + (BY)*b + (33<<((S)-1)))>>(S));\
>> > >>     }\
>> > >> }
>> > >> 
>> > >> These functions are called from hyscale(), like this:
>> > >> 
>> > >> static inline void RENAME(hyscale)(SwsContext *c, uint16_t *dst, long dstWidth, const uint8_t *src, int srcW, int xInc,
>> > >>                                    const int16_t *hLumFilter,
>> > >>                                    const int16_t *hLumFilterPos, int hLumFilterSize,
>> > >>                                    uint8_t *formatConvBuffer,
>> > >>                                    uint32_t *pal, int isAlpha)
>> > >> {
>> > >>     void (*toYV12)(uint8_t *, const uint8_t *, long, uint32_t *) = isAlpha ? c->alpToYV12 : c->lumToYV12;
>> > >>     void (*convertRange)(uint16_t *, int) = isAlpha ? NULL : c->lumConvertRange;
>> > >> 
>> > >>     src += isAlpha ? c->alpSrcOffset : c->lumSrcOffset;
>> > >> 
>> > >>     if (toYV12) {
>> > >>         toYV12(formatConvBuffer, src, srcW, pal);
>> > >>         src= formatConvBuffer;
>> > >>     }
>> > >> [...]
>> > >> }
>> > >> 
>> > >> c->lumSrcOffset is either 1 or -1, meaning src will _always_ be
>> > >> unaligned.
>> > >
>> > > Not that it makes much of a difference overall, but I think you
>> > > are mistaken there (or I misunderstood you).
>> > > c->lumSrcOffset is one for PIX_FMT_RGB48LE, and 1 or -1 for the
>> > > 32_1 formats, but 0 for the other cases.
>> > > So only these (rather uncommon formats) need a fix, and IMO
>> > > for those even a solution with bad performance would be ok.
>> > 
>> > I didn't dig very deeply, but it is 1 or -1 in the test that's
>> > failing, and that's bad enough.  How would you propose fixing it?
>> 
>> quite simply, if the hardware needs alignment then seperate 32_1 functions
>> are needed instead of reusing the existing 32 ones with +-1
>
> Considering that which 555/565 formats are supported already depends
> on the hardware the quick one would be to disable support and testing
> for those formats if the hardware does not have unaligned reads.

That is the stupidest idea I've heard in a long time.

-- 
M?ns Rullg?rd
mans at mansr.com