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

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

Hi,

On Sun, Jun 06, 2021 at 12:20:57PM +0200, Takashi Iwai wrote:
> 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 for your positive comment.

I agree with it. When adding parameters for function internal, we will
discuss about the inlining for variations of kernel API again, I guess.

After merging for-linus branch into for-next branch, I'm going to post
it again. At the time, I may finish enough test.


Thanks

Takashi Sakamoto