[PATCH] ALSA: usb-audio: Apply async workaround for Scarlett 2i4 2nd gen

[Takashi Iwai]

On Thu, 23 Apr 2020 09:22:36 +0200,
Takashi Iwai wrote:
> 
> On Thu, 23 Apr 2020 01:45:01 +0200,
> Alexander Tsoy wrote:
> > 
> > В Ср, 22/04/2020 в 22:28 +0200, Takashi Iwai пишет:
> > > On Wed, 22 Apr 2020 21:26:23 +0200,
> > > Alexander Tsoy wrote:
> > > > В Ср, 22/04/2020 в 20:55 +0200, Gregor Pintar пишет:
> > > > > On Wed, 22 Apr 2020 Alexander Tsoy wrote:
> > > > > > В Вт, 21/04/2020 в 21:31 +0200, Takashi Iwai пишет:
> > > > > > > I wonder, though, whether we can correct the rounding error
> > > > > > > in
> > > > > > > the
> > > > > > > driver code, too.
> > > > > > 
> > > > > > I'm not sure if it's possible with currently used Q16.16
> > > > > > arithmetic.
> > > > > 
> > > > > Maybe calculate fixed correction shifts (like it would be
> > > > > feedback)?
> > > > > Something like leap year.
> > > > > 
> > > > > In endpoint.c:
> > > > > static inline unsigned get_usb_high_speed_rate(unsigned int rate)
> > > > > {
> > > > > 	return ((rate << 10) + 62) / 125;
> > > > > }
> > > > > I guess 62 tries to round it, but exact number is needed. So
> > > > > exact
> > > > > value for
> > > > > 44100 should be 361267.2. For 48000 it is 360448.
> > > > > If only we can deliver that 0.2 by shifting rate somehow?
> > > > > 
> > > > > At least maybe it would be better to disable sample rates that do
> > > > > not
> > > > > divide
> > > > > by 1000 on SYNC playback endpoints, if there are others sample
> > > > > rates.
> > > > > 
> > > > > But I'm not familar with the code or USB.
> > > > 
> > > > I think instead of accumulating the fractional part of fs/fps in
> > > > Q16.16
> > > > format we should accumulating remainder of division fs/fps.
> > > > 
> > > > So for 44100 Hz and High Speed USB the calculations would be:
> > > > 
> > > > fs = 44100
> > > > fps = 8000
> > > > rem = 44100 % 8000 = 4100
> > > > accum = 0
> > > > packet_size_min = 44100 / 8000 = 5
> > > > packet_size_max = 44100 + (8000 - 1) / 8000 = 6
> > > > 
> > > > 
> > > > 1. accum += rem = 4100
> > > >    accum < fps => packet_size = packet_size_min = 5
> > > > 
> > > > 2. accum += rem = 8200
> > > >    accum >= fps => {
> > > >        packet_size = packet_size_max = 6
> > > >        accum -= fps = 200
> > > >    }
> > > > 
> > > > 3. accum += rem = 4300
> > > >    accum < fps => packet_size = packet_size_min = 5
> > > > 
> > > > ...
> > > > 
> > > > 80. accum += rem = 8000
> > > >     accum >= fps => {
> > > >         packet_size = packet_size_max = 6
> > > >         accum -= fps = 0
> > > >     }
> > > > ...
> > > 
> > > Yeah, something like that is what I had in my mind.
> > > It'd be greatly appreciated if someone can experiment it.
> > > Unfortunately I have no proper USB-audio device now at hands...
> > 
> > OK, here is a quick hacky patch, that seems to work for me:
> 
> Awesome, thanks!
> 
> The patch looks good enough.  A minor fix would be reset sample_accum
> at snd_usb_pcm_prepare().

It should be rather in snd_usb_endpoint_start().

> If someone can test more and it shows the positive result, I'd happy
> take it.

... and on the second thought, I wonder whether the new method can
be simply applied to all cases.  Your code gives basically more
accurate timing. 

If we are concerned about the possible regression by this, we can keep
the old code and make it switchable via option or kconfig (if any).


Takashi