Re: [alsa-devel] [PATCH v2 3/3] ASoC: TDA7802: Add turn-on diagnostic routine

[Thomas Preston <thomas.preston@codethink.co.uk>]

On 30/07/2019 16:50, Mark Brown wrote:
> On Tue, Jul 30, 2019 at 04:25:56PM +0100, Thomas Preston wrote:
>> On 30/07/2019 15:19, Mark Brown wrote:
> 
>>> 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.
> 
>> 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.
> 
>> 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?