[alsa-devel] Questions on locking, snd-usb-caiaq
tiwai at suse.de
Mon Sep 21 15:35:06 CEST 2009
At Sun, 20 Sep 2009 13:07:05 +0100 (BST),
Mark Hills wrote:
> I fixed some race issues in snd-usb-caiaq, but have some questions on the
> interface between the driver and ALSA.
> First fix is a lock in pointer(), which caused snd_pcm_update_hw_ptr_pos()
> to report an out of range buffer position (even though it was sucessfully
> corrected to zero).
> Second is a lock as part of trigger(). On testing, this appears to fix an
> issue where white noise could sometimes be transferred by the driver.
Both look fine to me.
> According to the documentation, trigger() and pointer() are called with
> local interrupts disabled. This means they should really be non-irqsave
They can be non-irqsave one, but not necessarily to be.
> Even for callbacks documented as atomic, if read_completed() and
> write_completed() can be called at any time interrupts are enabled (eg. on
> another CPU) it seems that additional locking my be required. Candidates
> Are there any guarantees on these functions which mean the lock is not
They are all protected via PCM substream lock, thus they aren't racy.
> To lock in prepare() would (I think) require demoting a
> spin_lock_irqsave() to a spin_lock() to send initialisation commands. If
> so, what is the best design for this?
The best is to sync the deactivation. Right now, activation /
deactivation are done in asynchronous way in caiaq driver.
At the beginning of prepare callback, you should be sure that all URBs
are killed, then proceed the rest preparation job.
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