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

[Lauri Kasanen <cand@gmx.com>]

On Sat, 09 Jan 2021 09:16:08 +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.

No, not yet. But surely an inconsistent buffer size would still play
something, instead of immediately erroring out?

> > Only through blind guessing did I stumble upon periods_min.
>
> The periods_min usually defines the hardware/software limit of the
> interrupt transfer.

Why do you say periods_min=1 makes little sense? At 44.1 khz, that'd be
172 interrupts per second, which is a lot but workable? There is no hw
limit against 172 irqs/s.

> > - 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?

No, CONFIG_DYNAMIC_DEBUG, CONFIG_DEBUG_FS and CONFIG_SND_DEBUG are all
disabled because this is a memory-constrained platform. Surely "why my
app is not producing sound" is not something that needs several
different kernel debug options enabled (+ root perms b/c debugfs).

- Lauri