[Qemu-devel] Re: [PATCH] alsaaudio: increase default buffer sizes

[malc <av1474@comtv.ru>]

On Fri, 2 May 2008, Jan Kiszka wrote:

> malc wrote:
>> On Fri, 2 May 2008, Jan Kiszka wrote:
>>
>>> Sound though the ALSA driver is skipping here unless I increase the
>>> buffer size. OSS seems to use 16K as well, and 1K was obviously to small
>>> for recording anyway.
>>>
>>> [ PS: Can someone explain to me why I also have to override the
>>> DAC/ADC_FIXED_FREQ to 48000 to make ALSA work? Suboptimal... ]
>>
>> How exactly it doesn't work if you don't override it? Do you get any
>> messages prefixed with "alsa:"? As for the defaults they were set to
>> 1024/256 because that's what was needed to make it sound on par with OSS
>> on the two machines i had at the time.
>
> alsa: Could not initialize ADC
> alsa: Failed to set period size 256
> alsa: Reason: Invalid argument

The code first asks for minimal period size than adjusts period_size to be
no less than the obtained value then calls set_period which fails, there's
little that can be done which ammounts to(never tested):

diff --git a/audio/alsaaudio.c b/audio/alsaaudio.c
index 43cfa25..ca17bc0 100644
--- a/audio/alsaaudio.c
+++ b/audio/alsaaudio.c
@@ -428,15 +428,15 @@ static int alsa_open (int in, struct alsa_params_req 
*req,
                      }
                  }

-                err = snd_pcm_hw_params_set_period_size (
+                err = snd_pcm_hw_params_set_period_size_near (
                      handle,
                      hw_params,
-                    period_size,
+                    &period_size,
                      0
                      );
                  if (err < 0) {
                      alsa_logerr2 (err, typ, "Failed to set period size 
%d\n",
-                                  req->period_size);
+                                  period_size);
                      goto err;
                  }
              }


Could you give it a whirl please?

[..snip..]

>
> BTW, I picked up those ALSA settings from user reports on kvm-devel,
> dating back to mid of last year. So I'm not alone, and it shouldn't be a
> wm8750-specific thing.
>
>>
>> By the by: OSSs fragment size is specified in bytes, ALSAs on the other
>> hand in frames. So 16*1024 in the patch bellow means(considering
>> S16/stereo) 65536 bytes large buffer and 1024 bytes large period. For
>> OSS it's 16K bytes buffer and 4096 bytes period.
>>
>
> Ah, OK. However, 4*1024 is not enough for the DAC here, and there were
> similar reports by kvm users.

It's apparently plenty enough for ALSA when it emulates OSS and not when
it does things "native" way, oh boy.

-- 
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