[alsa-devel] [PATCH - alsa-lib 1/1] Patch for another bug in snd_pcm_area_silence().

[Jaroslav Kysela]

Dne 2.2.2018 v 07:41 Takashi Sakamoto napsal(a):
> Hi,
> 
> On Feb 2 2018 03:12, furrywolf wrote:
>  > Only silence areas 64 bits at a time if it's possible to do so, which is
>  > when either the silence values are all zero, or when the format width
>  > divides evenly into 64 bits.  For formats that are neither of these, let
>  > the width-specific code handle the entire silencing.  Silencing formats
>  > that are not evenly divisible into 64 bits in 64-bit chunks, when the
>  > data isn't just zeroes, results in values being written to shifting
>  > positions in the sample, giving garbage.
>  >
>  > Makes Takashi Sakamoto's tester happy for all tests.  Yay!  (And Thanks!)
>  >
>  > Signed-off-by: furrywolf <alsa2 at bushytails.net>
>  >
>  > diff --git a/src/pcm/pcm.c b/src/pcm/pcm.c
>  > index 1753cda..5f9bd9f 100644
>  > --- a/src/pcm/pcm.c
>  > +++ b/src/pcm/pcm.c
>  > @@ -2947,7 +2947,7 @@ int snd_pcm_area_silence(const 
> snd_pcm_channel_area_t *dst_area, snd_pcm_uframes
>  >       dst = snd_pcm_channel_area_addr(dst_area, dst_offset);
>  >       width = snd_pcm_format_physical_width(format);
>  >       silence = snd_pcm_format_silence_64(format);
>  > -     if (dst_area->step == (unsigned int) width) {
>  > +     if (dst_area->step == (unsigned int) width && (!silence || !(64 
> % width))) {
>  >               unsigned int dwords = samples * width / 64;
>  >               uint64_t *dstp = (uint64_t *)dst;
>  >               samples -= dwords * 64 / width;
> 
> A condition of '!silence' seems to skip cases of 'signed' format of PCM
> samples; e.g. S16, because 'snd_pcm_format_silence_64()' returns 0 as
> silent data for these samples. However, the above block of code is a
> fast path for data samples which are 'divisors of 64 bit'. In the code
> block, silent data is handled as 64 bit variable to be copied to given
> buffer iteratively. In my understanding, the condition is not
> appropriate to the code block.
> 
> On the other hand, a condition of '!(64 % width)' is appropriate. If any
> 24 bit data sample is handled in the block, copied silent data aligned
> to 64 bit include invalid 8 bits. For example of 'U24_3LE':
> 
> Given buffer, aligned 64 bit:
>    0x'xxxx'xxxx'xxxx'xxxx'yyyy'yyyy'yyyy'yyyy'zzzz'zzzz'zzzz'zzzz
> 
> Silent data for 'U24_3LE' returned from 'snd_pcm_format_silence_64():
>    0x'0000'8000'0080'0000ull (little endian)
> 
> As a result of the fast path:
>    0x'xxxx'xxxx'xxxx'xxxx'yyyy'yyyy'yyyy'yyyy'zzzz'zzzz'zzzz'zzzz
>    v                                                            v
>    0x'0000'8000'0080'0000'0000'8000'0080'0000'0000'8000'0080'0000
>    (sample data with 'x-z' represent data aligned to 64 bits)
> 
> However, expected:
>    0x'8000'0080'0000'8000'0080'0000'8000'0080'0000'8000'0080'0000
>    0x'gggg'gghh'hhhh'iiii'iijj'jjjj'kkkk'kkll'llll'mmmm'mmnn'nnnn
>    (sample data with 'g-n' represent data aligned to 24 bit)
> 
> Practically, for any 24 bit sample data, we didn't see any issue caused
> by the above mechanism because in most case 'signed' sample data is
> handles by usual use cases, in my opinion.
> 
> Of course, a condition of '!silence' is a good optimization for
> cases of 'signed' PCM samples. But it's not good for the other kind of
> data format (e.g. DSD). At present, there's no sample format which has
> 24 bit data for non-PCM format, however it's better to avoid future
> problem, unless we were astrologers.
> 
> Overall, the fast path is not designed to handle any 24 bit sample data.
> We _should_ skip the fast path for any 24 bit samples, regardless of
> sign. Thus, below patch is enough for an issue to which I addressed in
> my previous message[1]. I can pass my test[2] with this patch.\

I applied both patches (furrywolf's and yours). I think that there
should be more work on the fast path, because 'dst' might not be aligned
and some CPUs needs more time (and even show warnings) when the word is
stored to an unaligned destination. Also, we may use memset() for the
special case when all bytes are identical.

				Thanks,
					Jaroslav

-- 
Jaroslav Kysela <perex at perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.