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

[Takashi Iwai <tiwai@suse.de>]

On Sat, 09 Jan 2021 18:46:01 +0100,
Lauri Kasanen wrote:
> 
> 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?

In some cases, it's possible.  PCM OSS translation has some special
way depending on the period ("fragment" in OSS) setup...

> > > 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.

Well, it's not about the sample rate or the process speed.  You need
to know what periods=1 means.  periods=1 is a VERY special usage.  No
double buffering and the driver has to report the precise accurate
position without period updates; i.e. it's almost for free-wheeling
DMA transfer.  Hence periods_min=1 makes sense if the driver may
behave like that.

> > > - 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).

But you are debugging the *kernel* problem, not the application.
I agree that debugfs isn't always needed for hunting application bugs,
yeah.


Takashi