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

[Takashi Sakamoto]

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.

======== 8< --------

 From bfb44ca7e13d794478c0cee1a1abf96d4b664d63 Mon Sep 17 00:00:00 2001
From: Takashi Sakamoto <o-takashi at sakamocchi.jp>
Date: Fri, 2 Feb 2018 14:44:35 +0900
Subject: [PATCH] pcm: fix a bug to copy silent samples aligned to 64 
bits for
  24 bit sample cases

A function of 'snd_pcm_area_silence()' has a fast path to copy silent data
efficiently. However, the fast path works well just for a case that target
buffer consists of data samples for which unit of data alignment is
divisors of 64 bits.

At present, the fast path handles sample data aligned to 24 bit. In this
case, the buffer can includes extra 8 bits. This has no issue for 'signed'
case because silent data is zero, however it has an issue for 'unsigned'
case.

This commit fixes the bug by skipping cases of sample data of 24 bit.

Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
---
  src/pcm/pcm.c | 7 ++++++-
  1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/pcm/pcm.c b/src/pcm/pcm.c
index 1753cdac..ddd67975 100644
--- a/src/pcm/pcm.c
+++ b/src/pcm/pcm.c
@@ -2947,7 +2947,12 @@ 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) {
+
+	/*
+	 * Iterate copying silent sample for sample data aligned to 64 bit.
+	 * This is a fast path.
+	 */
+	if (dst_area->step == (unsigned int) width && 64 % width == 0) {
		unsigned int dwords = samples * width / 64;
		uint64_t *dstp = (uint64_t *)dst;
		samples -= dwords * 64 / width;
-- 
2.14.1
======== 8< --------

Anyway, your patch assists my investigation for this issue. I appreciate
your work because it helps many users and developers who utilizes the
API of alsa-lib. Thank you.

[1] 
http://mailman.alsa-project.org/pipermail/alsa-devel/2018-January/130934.html
[2] 
https://github.com/takaswie/practices/blob/topic/alsa-lib-test/alsa-lib/test-silence.c


Takashi Sakamoto