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

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

On Dec 5 2016 16:32, Jiada Wang wrote:
> 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,

Exactly. Furthermore, ALSA PCM core has no code to call .prepare() in 
contexts unrelated to SNDRV_PCM_IOCTL_PREPARE.

> some devices are unable to manage this and stop working.
> I will update Changelog in v2 Patchset.


Regards

Takashi Sakamoto