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

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Date:   Thu, 20 Jul 2017 15:40:45 +0100
From:   Liviu Dudau <Liviu.Dudau@....com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     David Airlie <airlied@...ux.ie>,
        DRI devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [PATCH] drm/i2c: tda998x: Fix lockdep warning about possible
 circular dependency

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.

Best regards,
Liviu

[1] https://lists.freedesktop.org/archives/dri-devel/2017-July/147940.html

> 
> -- 
> RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
> FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
> according to speedtest.net.

-- 
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