[RFC][PATCH 0/3] ALSA: pcm/firewire: allow to queue period elapse event in process context

[Takashi Iwai]

On Sun, 06 Jun 2021 11:18:35 +0200,
Takashi Sakamoto wrote:
> 
> Hi,
> 
> All of drivers in ALSA firewire stack processes two chances to process
> isochronous packets in any isochronous context; in software IRQ context
> for 1394 OHCI, and in process context of ALSA PCM application.
> 
> In the process context, callbacks of .pointer and .ack are utilized. The
> callbacks are done by ALSA PCM core under acquiring lock of PCM substream,
> 
> In design of ALSA PCM core, call of snd_pcm_period_elapsed() is used for
> drivers to awaken user processes from waiting for available frames. The
> function voluntarily acquires lock of PCM substream, therefore it is not
> called in the process context since it causes dead lock. As a workaround
> to avoid the dead lock, all of drivers in ALSA firewire stack uses
> workqueue to delegate the call.
> 
> This patchset is my attempt for the issue. A variant of 
> 'snd_pcm_period_elapsed()' without lock acquisition is going to be added,
> named 'snd_pcm_period_elapsed_without_lock()'. This is used in callbacks
> of .pointer and .ack of snd_pcm_ops structure.
> 
> The patchset is still under my test, but it looks to work well in my
> easy and rough test. Before posting for merge, I'd like to get your
> comment to the idea. When evaluating, please merge below two histories:
>  * 64584f329352 (for-next)
>  * 9981b20a5e36 (for-linus)
> 
> Takashi Sakamoto (3):
>   ALSA: pcm: add snd_pcm_period_elapsed() variant without acquiring lock
>     of PCM substream
>   ALSA: firewire-lib: queue event of period elapse in process context
>   ALSA: firewire-lib: obsolete workqueue for period update

The idea is fine, but moving snd_pcm_period_elapsed() as inline static
doesn't give much benefit, IMO.  Although it can avoid an exported
symbol, its cost is much higher, since it'd expand the code at each
place of snd_pcm_period_elapsed(), i.e. almost in all driver code.
Just provide two exported functions instead in a more straightforward
way.


thanks,

Takashi