[alsa-devel] [PATCH] aplay: support no period wakeup option in argument

[Takashi Sakamoto]

On Thu, Dec 27, 2018 at 02:13:28AM +0000, S.j. Wang wrote:
> > -----Original Message-----
> > From: Takashi Sakamoto <o-takashi at sakamocchi.jp>
> > Sent: Wednesday, December 26, 2018 9:35 PM
> > 
> > Hi,
> > 
> > On Wed, Dec 26, 2018 at 11:28:11AM +0000, S.j. Wang wrote:
> > > In the case that alsa driver can't support period wakeup, we need to
> > > set the no period wakeup flag
> > >
> > > Signed-off-by: Shengjiu Wang <shengjiu.wang at nxp.com>
> > > ---
> > >  aplay/aplay.c | 13 +++++++++++++
> > >  1 file changed, 13 insertions(+)
> > >
> > > diff --git a/aplay/aplay.c b/aplay/aplay.c index
> > > efc1eb4cae3a..4f562bfe2884 100644
> > > --- a/aplay/aplay.c
> > > +++ b/aplay/aplay.c
> > > @@ -137,6 +137,7 @@ static int use_strftime = 0;  volatile static int
> > > recycle_capture_file = 0;  static long term_c_lflag = -1;  static int
> > > dump_hw_params = 0;
> > > +static int no_period_wakeup = 0;
> > >
> > >  static int fd = -1;
> > >  static off64_t pbrec_count = LLONG_MAX, fdcount; @@ -243,6 +244,7
> > @@
> > > _("Usage: %s [OPTION]... [FILE]...\n"
> > >  "    --use-strftime      apply the strftime facility to the output file name\n"
> > >  "    --dump-hw-params    dump hw_params of the device\n"
> > >  "    --fatal-errors      treat all errors as fatal\n"
> > > +"    --no-period-wakeup  set no period wakeup flag if necessary\n"
> > >    )
> > >  		, command);
> > >  	printf(_("Recognized sample formats are:")); @@ -429,6 +431,7 @@
> > > enum {
> > >  	OPT_USE_STRFTIME,
> > >  	OPT_DUMP_HWPARAMS,
> > >  	OPT_FATAL_ERRORS,
> > > +	OPT_NO_PERIOD_WAKEUP,
> > >  };
> > >
> > >  /*
> > > @@ -516,6 +519,7 @@ int main(int argc, char *argv[])
> > >  		{"interactive", 0, 0, 'i'},
> > >  		{"dump-hw-params", 0, 0, OPT_DUMP_HWPARAMS},
> > >  		{"fatal-errors", 0, 0, OPT_FATAL_ERRORS},
> > > +		{"no-period-wakeup", 0, 0, OPT_NO_PERIOD_WAKEUP},
> > >  #ifdef CONFIG_SUPPORT_CHMAP
> > >  		{"chmap", 1, 0, 'm'},
> > >  #endif
> > > @@ -799,6 +803,9 @@ int main(int argc, char *argv[])
> > >  		case OPT_FATAL_ERRORS:
> > >  			fatal_errors = 1;
> > >  			break;
> > > +		case OPT_NO_PERIOD_WAKEUP:
> > > +			no_period_wakeup = 1;
> > > +			break;
> > >  #ifdef CONFIG_SUPPORT_CHMAP
> > >  		case 'm':
> > >  			channel_map =
> > snd_pcm_chmap_parse_string(optarg);
> > > @@ -1396,6 +1403,12 @@ static void set_params(void)
> > >  							     &buffer_frames);
> > >  	}
> > >  	assert(err >= 0);
> > > +
> > > +	if (no_period_wakeup) {
> > > +		err = snd_pcm_hw_params_set_period_wakeup(handle,
> > params, 0);
> > > +		assert(err >= 0);
> > > +	}
> > > +
> > >  	monotonic = snd_pcm_hw_params_is_monotonic(params);
> > >  	can_pause = snd_pcm_hw_params_can_pause(params);
> > >  	err = snd_pcm_hw_params(handle, params);
> > > --
> > > 1.9.1
> > 
> > As of v4.21-rc1, runtime of PCM substream can't run no_period_wakeup
> > mode unless two conditions are satisfied[1]:
> >  - driver supports SNDRV_PCM_INFO_NO_PERIOD_WAKEUP
> >  - PCM applications set SNDRV_PCM_HW_PARAMS_NO_PERIOD_WAKEUP
> > flag to
> >    hardware parameter structure.
> > 
> > Neither alsa-lib nor the most of existent userspace applications set
> > SNDRV_PCM_HW_PARAMS_NO_PERIOD_WAKEUP without explicit call of
> > helper API. In this point, I can get your intention for this patch. If a driver
> > just supports no_period_wakeup mode, such driver can't work well.
> > 
> > However, such driver is problematic because apparently it can not run with
> > many existent userspace applications and alsa-lib. Before applying this
> > patch, we have enough discussion to prevent problems to introduce such
> > problematic drivers into Linux sound subsystem, in my opinion.
> 
> What is helper API?  I found that you have a similar function in axfer/xfer-libasound.c,
> Which is disable_period_wakeup().  Do you think I need to porting this function to
> Aplay? 

I focus on your intention itself instead of change of aplay. Here, I
refer your commit description:

"In the case that alsa driver can't support period wakeup, we need to
set the no period wakeup flag"

In my understanding, your intention is a kind of driver added yet in
mainline. The driver doesn't adopt period wakeup. It just support
no-period-wakeup. Thus the driver works without any periodical
hardware/software IRQs.

At present, the feature, no-period-wakeup is an option of PCM substream.
This means that all of in-kernel drivers _should_ adopt period
wakeup at first. Then, if possible, it can implement no-period-wakeup
with the optional flag.

An addition of driver which support no-period-wakeup only brings
problems to existing applications. For example, execution of ioctl(2)
with SNDRV_PCM_IOCTL_[READ|WRITE][I|N] can bring below message:

"%s write error (DMA or IRQ trouble?)"

This is output in 'wait_for_avail()' of 'sound/core/pcm_lib.c'[1].
This is because nothing awaken up a task of the execution. With normal
drivers, the task is awakened by handlers on any IRQ context.

ALSA drivers should work with such applications, thus it's not good
idea to develop for such drivers. In a case that hardware has no feature
to generate periodical IRQs, kernel timer or hrtimer can be available to
achieve 'pseudo' period wakeup for such drivers.

If you wish for drivers which don't achieve period wakeup, it's better
to have enough discussion to change behaviour of ALSA PCM core at first,
then work for aplay next.

Please inform me if I misread in your description.


[1] https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/tree/sound/core/pcm_lib.c#n1901


Thanks

Takashi Sakamoto