[alsa-devel] [PATCH - Intervals v2 1/1] interval: Interpret (x x+1] correctly and return x+1

[twischer at de.adit-jv.com]

From: Timo Wischer <twischer at de.adit-jv.com>

Without this change an interval of (x x+1] will be interpreted as an
empty interval but the right value would be x+1.
This leads to a failing snd_pcm_hw_params() call which returns -EINVAL.

An example issue log is given in the following:
snd_pcm_hw_params failed with err -22 (Invalid argument)
ACCESS: MMAP_NONINTERLEAVED
FORMAT: S16_LE
SUBFORMAT: STD
SAMPLE_BITS: 16
FRAME_BITS: 16
CHANNELS: 1
RATE: 16000
PERIOD_TIME: (15999 16000]
PERIOD_SIZE: (255 256]
PERIOD_BYTES: (510 512]
PERIODS: [2 3)
BUFFER_TIME: 32000
BUFFER_SIZE: 512
BUFFER_BYTES: 1024

In case of (x x+1) we have to interpret it anyway as a single value of x to
compensate rounding issues.
For example the period size will result in an interval of (352 353) when
the period time is 16ms and the sample rate 22050 Hz
(16ms * 22,05 kHz = 352,8 frames). But 352 has to be chosen to allow a
buffer size of 705 (32ms * 22,05 kHz = 705,6 frames) which has to be >= 2x
period size to avoid Xruns. The buffer size will not end up with an
interval of (705 706) simular to the period size because
snd_pcm_rate_hw_refine_cchange() calls snd_interval_floor() for the buffer
size. Therefore this value will be interpreted as an integer interval
instead of a real interval further on.

This issue seems to exist since the change of 9bb985c38 ("pcm:
snd_interval_refine_first/last: exclude value only if also excluded
before")

Signed-off-by: Timo Wischer <twischer at de.adit-jv.com>
---
> On 10/18/18 12:57, Takashi Iwai wrote:
> This change looks risky.  The snd_interval_value() might be called
> even if the interval isn't reduced to a single value.  Rather check
> openmin instead.

If I would do
"if (i->openmin)"
on an interval of (x x+1) x+1 would be returned. But this would result in
the second issue which I have tried to describe in the commit message:
x has to be returned otherwise it is not guaranteed that
buffer_size >= 2x period_size. And this would result in continuously Xruns.
Therefore I would like to prefer
"if (i->openmin && !i->openmax)"

diff --git a/src/pcm/interval_inline.h b/src/pcm/interval_inline.h
index a68e292..d9a30b2 100644
--- a/src/pcm/interval_inline.h
+++ b/src/pcm/interval_inline.h
@@ -51,12 +51,14 @@ INTERVAL_INLINE int snd_interval_single(const snd_interval_t *i)
 {
 	assert(!snd_interval_empty(i));
 	return (i->min == i->max || 
-		(i->min + 1 == i->max && i->openmax));
+		(i->min + 1 == i->max && (i->openmin || i->openmax)));
 }
 
 INTERVAL_INLINE int snd_interval_value(const snd_interval_t *i)
 {
 	assert(snd_interval_single(i));
+	if (i->openmin && !i->openmax)
+		return i->max;
 	return i->min;
 }
 
-- 
2.7.4