On 30/07/2019 16:50, Mark Brown wrote:

On Tue, Jul 30, 2019 at 04:25:56PM +0100, Thomas Preston wrote:

quoted

On 30/07/2019 15:19, Mark Brown wrote:

quoted

quoted

It is unclear what this mutex usefully protects, it only gets taken when
writing to the debugfs file to trigger this diagnostic mode but doesn't
do anything to control interactions with any other code path in the
driver.

quoted

If another process reads the debugfs node "diagnostic" while the turn-on 
diagnostic mode is running, this mutex prevents the second process
restarting the diagnostics.

quoted

This is redundant if debugfs reads are atomic, but I don't think they are.

Like I say it's not just debugfs though, there's the standard driver
interface too.

Ah right, I understand. So if we run the turn-on diagnostics routine, there's
nothing stopping anyone from interacting with the device in other ways.

I guess there's no way to share that mutex with ALSA? In that case, it doesn't
matter if this mutex is there or not - this feature is incompatible. How
compatible do debugfs interfaces have to be? I was under the impression anything
goes. I would argue that the debugfs is better off for having the mutex so
that no one re-reads "diagnostic" within the 5s poll timeout.

Alternatively, this diagnostic feature could be handled with an external-handler
kcontrol SOC_SINGLE_EXT? I'm not sure if this is an atomic interface either.

What would be acceptable?