Re: [PATCH 5/6 v2] sound: Add n64 driver

[Takashi Iwai <tiwai@suse.de>]

On Sat, 09 Jan 2021 08:23:03 +0100,
Lauri Kasanen wrote:
> 
> Hi,
> 
> On Fri, 08 Jan 2021 10:06:48 +0100
> Takashi Iwai <tiwai@suse.de> wrote:
> 
> > > +static const struct snd_pcm_hardware n64audio_pcm_hw = {
> > > +	.info = (SNDRV_PCM_INFO_MMAP |
> > > +		 SNDRV_PCM_INFO_MMAP_VALID |
> > > +		 SNDRV_PCM_INFO_INTERLEAVED |
> > > +		 SNDRV_PCM_INFO_BLOCK_TRANSFER),
> > > +	.formats =          SNDRV_PCM_FMTBIT_S16_BE,
> > > +	.rates =            SNDRV_PCM_RATE_8000_48000,
> > > +	.rate_min =         8000,
> > > +	.rate_max =         48000,
> > > +	.channels_min =     2,
> > > +	.channels_max =     2,
> > > +	.buffer_bytes_max = 32768,
> > > +	.period_bytes_min = 1024,
> > > +	.period_bytes_max = 32768,
> > > +	.periods_min =      1,
> >
> > periods_min=1 makes little sense for this driver.
> 
> I have some questions about this.
> 
> When I had periods_min = 128, OSS apps were broken. I mean simple ones,
> open /dev/dsp, ioctl the format/rate/stereo, write data. They got an IO
> error errno IIRC, and no clarifying error in dmesg.
> 
> I tried following the error with printks, several levels deep. I gave
> up when it got to the constraint resolving function, and there was no
> good way to print and track which constraint failed, why, and who set
> the constraint.

Did you try to set up the hw constraint for the integer PERIODS like
below at open?
  snd_pcm_hw_constraint_integer(runtime, SNDRV_PCM_HW_PARAM_PERIODS)

Without this, it'd allow inconsistent buffer/period set up in your
case.

> Only through blind guessing did I stumble upon periods_min.

The periods_min usually defines the hardware/software limit of the
interrupt transfer.

> - why did it break OSS apps?
> - how does the OSS layer interact with periods? I didn't find any "set
> period" ioctl

OSS layers do the same as the native API via OSS emulation in
sound/core/oss/pcm*.c.

> - why was there no clarifying error in dmesg? Just an errno that means
> a million things makes it impossible for the userspace app writer to
> know why it's not working

Did you check the debug messages with dyndbg enabled?


Takashi