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SubjectRe: [PATCH 2/3 v2] ALSA: usb-audio: avoid setting of sample rate multiple times on bus
Hi Sakamoto

On 11/30/2016 02:45 AM, Takashi Sakamoto wrote:
> Hi Jiada,
>
> I don't oppose this patch. Nevertheless, your description is not
> necessarily correct.
>
> On Nov 30 2016 16:59, Jiada Wang wrote:
>> From: Daniel Girnus <dgirnus@de.adit-jv.com>
>>
>> ALSA usually calls the prepare function twice before starting the
>> playback:
>> 1. On hw_params call from userland and
>> 2. internally when starting the stream.
>
> ALSA PCM core in kernel land doesn't perform like this.
>
> In alsa-lib, 'snd_pcm_hw_params()' calls
> 'snd_pcm_hw_params_internal()' and 'snd_pcm_prepare()' sequentially.
> http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/pcm/pcm.c;h=cd87bc759ded95953e332b7e8d56b0f2d5b4185d;hb=HEAD#l853
>
>
> In system call level (e.g. see by strace(1)), this looks like two
> ioctl(2)s with 'SNDRV_PCM_IOCTL_HW_PARAMS' and 'SNDRV_PCM_IOCTL_PREPARE'.
>
> Well, when applications are written to execute 'snd_pcm_hw_params()'
> and 'snd_pcm_hw_prepare()' sequentially, additional ioctl(2) with
> 'SNDRV_PCM_IOCTL_PREPARE' appears. PulseAudio is this kind of
> application. I indicated the useless in 2014, but it still remains:
> https://lists.freedesktop.org/archives/pulseaudio-discuss/2014-January/019773.html
>
>
> You have the misunderstanding due to a nature of alsa-lib and tendency
> of major applications, from my point of view.
>
Thanks for your indication, so because some of userland applications
call 'snd_pcm_hw_params()' and
'snd_pcm_hw_prepare()' sequentially, means the second
'SNDRV_PCM_IOCTL_PREPARE' be called in 'SNDRV_PCM_STATE_PREPARED' state,
some devices are unable to manage this and stop working.

I will update Changelog in v2 Patchset.

Thanks,
Jiada

>> Some device are not able to manage this and they will stop playback
>> if the sample rate will be configured several times over USB protocol.
>>
>> Signed-off-by: Jens Lorenz <jlorenz@de.adit-jv.com>
>> Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
>> ---
>> sound/usb/pcm.c | 21 +++++++++++----------
>> 1 file changed, 11 insertions(+), 10 deletions(-)
>>
>> diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
>> index 44d178e..a522c9a 100644
>> --- a/sound/usb/pcm.c
>> +++ b/sound/usb/pcm.c
>> @@ -806,17 +806,18 @@ static int snd_usb_pcm_prepare(struct
>> snd_pcm_substream *substream)
>> if (ret < 0)
>> goto unlock;
>>
>> - iface = usb_ifnum_to_if(subs->dev, subs->cur_audiofmt->iface);
>> - alts = &iface->altsetting[subs->cur_audiofmt->altset_idx];
>> - ret = snd_usb_init_sample_rate(subs->stream->chip,
>> - subs->cur_audiofmt->iface,
>> - alts,
>> - subs->cur_audiofmt,
>> - subs->cur_rate);
>> - if (ret < 0)
>> - goto unlock;
>> -
>> if (subs->need_setup_ep) {
>> +
>> + iface = usb_ifnum_to_if(subs->dev, subs->cur_audiofmt->iface);
>> + alts = &iface->altsetting[subs->cur_audiofmt->altset_idx];
>> + ret = snd_usb_init_sample_rate(subs->stream->chip,
>> + subs->cur_audiofmt->iface,
>> + alts,
>> + subs->cur_audiofmt,
>> + subs->cur_rate);
>> + if (ret < 0)
>> + goto unlock;
>> +
>> ret = configure_endpoint(subs);
>> if (ret < 0)
>> goto unlock;
>
>
> Regards
>
> Takashi Sakamoto

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