[PATCH] ALSA: hda: Reduce udelay() at SKL+ position reporting

[Takashi Iwai]

On Fri, 10 Sep 2021 19:04:37 +0200,
Pierre-Louis Bossart wrote:
> 
> 
> 
> On 9/10/21 9:10 AM, Takashi Iwai wrote:
> > The position reporting on Intel Skylake and later chips via
> > azx_get_pos_skl() contains a udelay(20) call for the capture streams.
> > A call for this alone doesn't sound too harmful.  However, as the
> > pointer PCM ops is one of the hottest path in the PCM operations --
> > especially for the timer-scheduled operations like PulseAudio -- such
> > a delay hogs CPU usage significantly in the total performance.
> > 
> > The function there was taken from the original code in ASoC SST
> > Skylake driver blindly.  The udelay() is a workaround for the case
> > where the reported position is behind the period boundary at the
> > timing triggered from interrupts; applications often expect that the
> > full data is available for the whole period when returned (and also
> > that's the definition of the ALSA PCM period).
> 
> that initial work-around from the Intel SST driver does not seem to be
> legit in the first place, when I checked with hardware folks recently no
> one understands why there are delays and special cases for capture. The
> only requirement wrt. DPIB is that the DDR update is valid across VC0
> and VC1, while the DPIB registers are only valid with VC0. For SOF we
> don't know of any VC1 use so will default to the DPIB vendor-specific
> registers.

What are those VC0 and VC1 registers?  I can't find the definitions in
the code, so I assume that none of ALSA/ASoC drivers use VC1.

> See https://github.com/thesofproject/linux/pull/3143 for my WIP fixes
> for SOF.
> 
> I don't have the time to look at this specific patch today but wanted to
> make sure you are aware of my on-going fixes.
> 
> Note that the use of DPIB works best if you don't use the IOC interrupt.
> when the interrupt is thrown, there is likely a delay for the DPIB
> information to be refreshed.

Thanks for the information!  The delay could be the reason of the
udelay(), and that's superfluous as mentioned in the commit.

So the remaining question seems to be which method is a better
approach for the capture stream: DPIB or posbuf.  I kept the posbuf
just for conservatism, but judging from your comment, we may use DPIB
for both directions safely?

In anyway, the additional mechanism to check the delayed position
report in this patch can be kept no matter which way (DPIB or posbuf)
is used.


Takashi

> 
> > 
> > OTOH, HD-audio (legacy) driver has already some workarounds for the
> > delayed position reporting due to its relatively large FIFO, such as
> > the BDL position adjustment and the delayed period-elapsed call in the
> > work.  That said, the udelay() is almost superfluous for HD-audio
> > driver unlike SST, and we can drop the udelay().
> > 
> > Though, the current code doesn't guarantee the full period readiness
> > as mentioned in the above, but rather it checks the wallclock and
> > detects the unexpected jump.  That's one missing piece, and the drop
> > of udelay() needs a bit more sanity checks for the delayed handling.
> > 
> > This patch implements those: the drop of udelay() call in
> > azx_get_pos_skl() and the more proper check of hwptr in
> > azx_position_ok().  The latter change is applied only for the case
> > where the stream is running in the normal mode without
> > no_period_wakeup flag.  When no_period_wakeup is set, it essentially
> > ignores the period handling and rather concentrates only on the
> > current position; which implies that we don't need to care about the
> > period boundary at all.
> > 
> > Fixes: f87e7f25893d ("ALSA: hda - Improved position reporting on SKL+")
> > Reported-by: Jens Axboe <axboe at kernel.dk>
> > Signed-off-by: Takashi Iwai <tiwai at suse.de>
> > ---
> >  sound/pci/hda/hda_intel.c | 28 +++++++++++++++++++++++-----
> >  1 file changed, 23 insertions(+), 5 deletions(-)
> > 
> > diff --git a/sound/pci/hda/hda_intel.c b/sound/pci/hda/hda_intel.c
> > index 3aa432d814a2..faeeeb923d5e 100644
> > --- a/sound/pci/hda/hda_intel.c
> > +++ b/sound/pci/hda/hda_intel.c
> > @@ -637,13 +637,17 @@ static int azx_position_check(struct azx *chip, struct azx_dev *azx_dev)
> >   * the update-IRQ timing.  The IRQ is issued before actually the
> >   * data is processed.  So, we need to process it afterwords in a
> >   * workqueue.
> > + *
> > + * Returns 1 if OK to proceed, 0 for delay handling, -1 for skipping update
> >   */
> >  static int azx_position_ok(struct azx *chip, struct azx_dev *azx_dev)
> >  {
> >  	struct snd_pcm_substream *substream = azx_dev->core.substream;
> > +	struct snd_pcm_runtime *runtime = substream->runtime;
> >  	int stream = substream->stream;
> >  	u32 wallclk;
> >  	unsigned int pos;
> > +	snd_pcm_uframes_t hwptr, target;
> >  
> >  	wallclk = azx_readl(chip, WALLCLK) - azx_dev->core.start_wallclk;
> >  	if (wallclk < (azx_dev->core.period_wallclk * 2) / 3)
> > @@ -680,6 +684,24 @@ static int azx_position_ok(struct azx *chip, struct azx_dev *azx_dev)
> >  		/* NG - it's below the first next period boundary */
> >  		return chip->bdl_pos_adj ? 0 : -1;
> >  	azx_dev->core.start_wallclk += wallclk;
> > +
> > +	if (azx_dev->core.no_period_wakeup)
> > +		return 1; /* OK, no need to check period boundary */
> > +
> > +	if (runtime->hw_ptr_base != runtime->hw_ptr_interrupt)
> > +		return 1; /* OK, already in hwptr updating process */
> > +
> > +	/* check whether the period gets really elapsed */
> > +	pos = bytes_to_frames(runtime, pos);
> > +	hwptr = runtime->hw_ptr_base + pos;
> > +	if (hwptr < runtime->status->hw_ptr)
> > +		hwptr += runtime->buffer_size;
> > +	target = runtime->hw_ptr_interrupt + runtime->period_size;
> > +	if (hwptr < target) {
> > +		/* too early wakeup, process it later */
> > +		return chip->bdl_pos_adj ? 0 : -1;
> > +	}
> > +
> >  	return 1; /* OK, it's fine */
> >  }
> >  
> > @@ -874,11 +896,7 @@ static unsigned int azx_get_pos_skl(struct azx *chip, struct azx_dev *azx_dev)
> >  	if (azx_dev->core.substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
> >  		return azx_skl_get_dpib_pos(chip, azx_dev);
> >  
> > -	/* For capture, we need to read posbuf, but it requires a delay
> > -	 * for the possible boundary overlap; the read of DPIB fetches the
> > -	 * actual posbuf
> > -	 */
> > -	udelay(20);
> > +	/* read of DPIB fetches the actual posbuf */
> >  	azx_skl_get_dpib_pos(chip, azx_dev);
> >  	return azx_get_pos_posbuf(chip, azx_dev);
> >  }
> > 
>