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

[Takashi Iwai <tiwai@suse.de>]

On Sun, 10 Jan 2021 18:03:32 +0100,
Lauri Kasanen wrote:
> 
> On Sun, 10 Jan 2021 11:24:22 +0100
> Takashi Iwai <tiwai@suse.de> wrote:
> 
> > > At first there was no nextpos, and _pointer() always reported pos. This
> > > didn't work, the core played through the audio at chipmunk speed. So
> > > there must be more that I don't understand here.
> >
> > Try to set the periods_min=2 and the integer periods hw constraint at
> > first, and change the pointer callback to return nextpos.  Also, at
> > the push function, set runtime->delay = period_size as well.
> 
> When I do all this, it still causes the chipmunk speed. Several seconds
> of audio gets played in 0.3s or so. Sorry if this is taking too much of
> your time, I'm a bit lost here at what the alsa core is expecting.
> 
> Printks show the following repeats:
> start, period size 1024
> push, bool irq=0
> irq fired
> push, bool irq=1
> pointer at 8192
> stop

Hm, is the above about the result with the pointer callback returning
pos, not nextpos?  If so,

> start, period size 1024
> push, bool irq=0

At this moment, nextpos is 1024, and it should take some time until

> irq fired

... this IRQ is triggered; it must be the period time.
Was the reported timing as expected?

> push, bool irq=1
> pointer at 8192

If it's the first IRQ, isn't the nextpos 2048?

> stop

It's stopped likely because the position reported the driver is beyond
the data size the application filled (XRUN).

> It stops and starts again for some reason. This does not happen in the
> current pos/nextpos implementation.
> 
> > > - the DMA unit has errata if (start + len) & 0x3fff == 0x2000, this
> > > must never happen
> >
> > Ditto.
> 
> Can it really handle a constraint this complex? That'd be impressive.
> It'd remove the memcpy need if so.

The hw constraint can be just a function that filters the min/max
value the driver may accept, so such a rule is possible.

> > > - the audio IRQ is not a timer, it fires when the card's internal
> > > buffers are empty and need immediate refill
> >
> > It's the current implementation, right?
> 
> Yes, but I was wondering if a different setup would work better. The
> alsa setup is a bit confusing to me still.

The implementation became a bit complex due to the hardware
restriction, yeah.


thanks,

Takashi