On Nov 30 2016 19:45, 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@xxxxxxxxxxxxxx>
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.
So here you should mention that current USB Audio device class driver
somewhat ignores state machine of ALSA PCM runtime.
In ALSA PCM core, state of the runtime is described in 'struct
snd_pcm_runtime.status.state' with macros of 'SNDRV_PCM_STATE_XXX'.
Applications are allowed to handle the runtime according to the state.
In your issue, the driver is programmed ignoring a case that double
calls of snd_pcm_prepare(), in short, ioctl(PREPARE) is called in
'PREPARED' state. This is not only an issue for snd-usb-audio, but also
for snd-usb-hiface.
http://mailman.alsa-project.org/pipermail/alsa-devel/2016-November/115174.html
For these issue, I have no patch proposals because I have few test
devices, sorry.
Regards
Takashi Sakamoto
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