Re: [PATCH] ALSA: usb-audio: Increase max buffer size

[Geraldo Nascimento <geraldogabriel@gmail.com>]

On Sat, Apr 09, 2022 at 10:43:13PM +0200, Jaroslav Kysela wrote:
> On 09. 04. 22 22:36, Geraldo Nascimento wrote:
> > On Thu, Apr 07, 2022 at 11:27:40PM +0200, Takashi Iwai wrote:
> >> The current limit of max buffer size 1MB seems too small for modern
> >> devices with lots of channels and high sample rates.
> >> Let's make bigger, 4MB.
> > 
> > Hi Takashi,
> > 
> > I did some math and 4MB is still too little for some extreme hardware
> > like the Behringer Wing, which is USB 2.0 by the way. According to my
> > calculations, at 192 KHz and 48 channels, even with this 4MB patch, we
> > would still have only 151.7 ms of buffer for the Wing.
> > 
> > Therefore my suggestion is to expose the MAX_BUFFER_BYTES as Kconf.
> > 
> > Do you think this is plausible?
> 
> I think that much better behaviour may be to calculate and limit the max 
> buffer size at runtime depending on the max channels / rate / sample bits 
> obtained from the USB descriptors by default.

Hi Jaroslav,

Yes, I agree, your approach seems to be the most cost-effective way of
dealing with extreme USB sound hardware, plus if it's calculated at
runtime the users won't need to recompile their kernels for new devices
and the whole thing is painless from the perspective of the end-user.

> For standard hardware, those big 
> buffers do not make much sense and there's usually a mix of the USB sound 
> hardware in the system.

That is true, thanks for pointing it out.

Thank you,
Geraldo Nascimento

> 
> 					Jaroslav
> 
> > 
> > Thanks,
> > Geraldo Nascimento
> > 
> >>
> >> Signed-off-by: Takashi Iwai <tiwai@suse.de>
> >> ---
> >>   sound/usb/pcm.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
> >> index 6a460225f2e3..37ee6df8b15a 100644
> >> --- a/sound/usb/pcm.c
> >> +++ b/sound/usb/pcm.c
> >> @@ -659,7 +659,7 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
> >>   #define hwc_debug(fmt, args...) do { } while(0)
> >>   #endif
> >>   
> >> -#define MAX_BUFFER_BYTES	(1024 * 1024)
> >> +#define MAX_BUFFER_BYTES	(4 * 1024 * 1024)
> >>   #define MAX_PERIOD_BYTES	(512 * 1024)
> >>   
> >>   static const struct snd_pcm_hardware snd_usb_hardware =
> >> -- 
> >> 2.31.1
> >>
> 
> 
> -- 
> Jaroslav Kysela <perex@perex.cz>
> Linux Sound Maintainer; ALSA Project; Red Hat, Inc.