Re: [PATCH] drm/i2c: tda998x: Fix lockdep warning about possible circular dependency

[Liviu Dudau <Liviu.Dudau@arm.com>]

On Thu, Jul 20, 2017 at 04:57:12PM +0200, Daniel Vetter wrote:
> On Thu, Jul 20, 2017 at 4:40 PM, Liviu Dudau <Liviu.Dudau@arm.com> wrote:
> > On Thu, Jul 20, 2017 at 03:24:13PM +0100, Russell King - ARM Linux wrote:
> >> On Thu, Jul 20, 2017 at 03:19:10PM +0100, Liviu Dudau wrote:
> >> > On Thu, Jul 20, 2017 at 02:08:29PM +0100, Russell King - ARM Linux wrote:
> >> > > On Thu, Jul 20, 2017 at 01:54:04PM +0100, Liviu Dudau wrote:
> >> > > > On Thu, Jul 20, 2017 at 12:44:49PM +0100, Russell King - ARM Linux wrote:
> >> > > > > Actually, scrub that idea - drm_helper_probe_single_connector_modes()
> >> > > > > calls drm_edid_to_eld() for these cases anyway, so we must call
> >> > > > > drm_helper_probe_single_connector_modes() with the audio_mutex held.
> >> > > >
> >> > > > OK, so the lockdep warning is spurious?
> >> > >
> >> > > I don't think so.  I think there's two ways to solve this:
> >> > >
> >> > > 1. replace the audio_mutex in tda998x_audio_get_eld() and
> >> > >    tda998x_connector_fill_modes() with a new mutex (eld_mutex) to
> >> > >    protect just the ELD.
> >> > >
> >> > > 2. remove the mutex from these two functions, and take the connection_mutex
> >> > >    modeset lock in tda998x_audio_get_eld().
> >> > >
> >> > > However, I don't have a view on which would be best.
> >> >
> >> > If you don't mind, I took the liberty of picking option 2, just because
> >> > I don't like adding new locks when existing ones might do the job.
> >>
> >> I don't mind - but one question for the DRM people in connection with
> >> your patch is whether we need the acquire context for this relatively
> >> simple lock/copy/unlock sequence.  This path for getting the ELD
> >> shouldn't be holding any other DRM locks.
> >
> > Cc-ing Daniel Vetter in hope of clarifications / nod of approval.
> > However, I can only see my emails in the online dri-devel archive, not
> > yours, so I can't point him to the whole discussion.
> >
> > danvet: a while ago while I was debugging the delayed fb setup I found
> > a lockdep warning with the tda998x driver. Now I've had some more time
> > to investigate so I have created a patch trying to fix the issue, which
> > was on v1 just a re-ordering of places where tda998x's audio_mutex lock
> > was taken. Russell suggested a different approach, which I have
> > implemented in [1], but we wonder if we really have to go through the
> > whole dance.
> 
> If all you do is take only one ww mutex (wrapped up in
> drm_modeset_lock for kms) then you can pass a NULL acquire context.
> The context is only needed when you want to take multiple locks at the
> same time (to be able to resolve deadlocks). Taking a single lock
> within the modeset lock class can't deadlock.

Hi Daniel,

Thanks for clarification. I'll post a v3 with a NULL acquire context.

Best regards,
Liviu

> 
> Reading the kerneldoc that's not explained at all :-( Can you pls type
> a patch to improve the docs for drm_modeset_lock?
> 
> Thanks, Daniel
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch

-- 
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