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

[Alexander Tsoy]

В Ср, 22/04/2020 в 22:26 +0300, Alexander Tsoy пишет:
> 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
>     }
> ...

And step 79 (if I get steps counting right) also produce 6 samples per
frame:

79. accum += rem = 11900
    accum >= fps => {
        packet_size = packet_size_max = 6
        accum -= fps = 3900
    }