Re: [alsa-devel] [PATCH 7/8] ALSA: pcm: Add card sync_irq field

[Takashi Iwai <tiwai@suse.de>]

On Mon, 18 Nov 2019 17:38:49 +0100,
Pierre-Louis Bossart wrote:
> 
> 
> 
> On 11/17/19 2:53 AM, Takashi Iwai wrote:
> > Many PCI and other drivers performs snd_pcm_period_elapsed() simply in
> > its interrupt handler, so the sync_stop operation is just to call
> > synchronize_irq().  Instead of putting this call multiple times,
> > introduce the common card->sync_irq field.  When this field is set,
> > PCM core performs synchronize_irq() for sync-stop operation.  Each
> > driver just needs to copy its local IRQ number to card->sync_irq, and
> > that's all we need.
> 
> Maybe a red-herring or complete non-sense, but I wonder if this is
> going to get in the way of Ranjani's multi-client work, where we could
> have multiple cards created but with a single IRQ handled by the
> parent PCI device?
>
> Ranjani, you may want to double-check this and chime in, thanks!

The synchronize_irq() is fairly safe to call multiple times, and I
don't think any problem by invoking it for multi-clients sharing the
same IRQ.  For example, Digigram miXart driver creates multiple card
objects from a single PCI entry, and I already thought of that
possibility; they set the same card->sync_irq value to all card
objects, which eventually will call synchronize_irq() multiple times.
 From the performance POV, this shouldn't be a big problem, because
the place calling this is only at hw_params, prepare and hw_free,
neither are hot-path.


thanks,

Takashi
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