On 9/10/21 8:10 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). > > 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. I tested this on top of 5.15-rc1, and it seems like a massive improvement. No longer have stalled or slowed down nestopia, seems smooth all the time. I'll report back in a few days, just in case. -- Jens Axboe
