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Date:   Wed, 13 Oct 2021 17:01:03 +0200
From:   Maxime Ripard <maxime@cerno.tech>
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        Emma Anholt <emma@anholt.net>, David Airlie <airlied@linux.ie>,
        Philipp Zabel <p.zabel@pengutronix.de>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: Regression with mainline kernel on rpi4
Message-ID: <20211013150103.p3rea3ny3rjmohtd@gilmour>
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On Thu, Sep 30, 2021 at 11:19:59AM +0200, Daniel Vetter wrote:
> On Tue, Sep 28, 2021 at 10:34:46AM +0200, Maxime Ripard wrote:
> > Hi Daniel,
> >=20
> > On Sat, Sep 25, 2021 at 12:50:17AM +0200, Daniel Vetter wrote:
> > > On Fri, Sep 24, 2021 at 3:30 PM Maxime Ripard <maxime@cerno.tech> wro=
te:
> > > >
> > > > On Wed, Sep 22, 2021 at 01:25:21PM -0700, Linus Torvalds wrote:
> > > > > On Wed, Sep 22, 2021 at 1:19 PM Sudip Mukherjee
> > > > > <sudipm.mukherjee@gmail.com> wrote:
> > > > > >
> > > > > > I added some debugs to print the addresses, and I am getting:
> > > > > > [   38.813809] sudip crtc 0000000000000000
> > > > > >
> > > > > > This is from struct drm_crtc *crtc =3D connector->state->crtc;
> > > > >
> > > > > Yeah, that was my personal suspicion, because while the line numb=
er
> > > > > implied "crtc->state" being NULL, the drm data structure document=
ation
> > > > > and other drivers both imply that "crtc" was the more likely one.
> > > > >
> > > > > I suspect a simple
> > > > >
> > > > >         if (!crtc)
> > > > >                 return;
> > > > >
> > > > > in vc4_hdmi_set_n_cts() is at least part of the fix for this all,=
 but
> > > > > I didn't check if there is possibly something else that needs to =
be
> > > > > done too.
> > > >
> > > > Thanks for the decode_stacktrace.sh and the follow-up
> > > >
> > > > Yeah, it looks like we have several things wrong here:
> > > >
> > > >   * we only check that connector->state is set, and not
> > > >     connector->state->crtc indeed.
> > > >
> > > >   * We also check only in startup(), so at open() and not later on =
when
> > > >     the sound streaming actually start. This has been there for a w=
hile,
> > > >     so I guess it's never really been causing a practical issue bef=
ore.
> > >=20
> > > You also have no locking
> >=20
> > Indeed. Do we just need locking to prevent a concurrent audio setup and
> > modeset, or do you have another corner case in mind?
> >=20
> > Also, generally, what locks should we make sure we have locked when
> > accessing the connector and CRTC state? drm_mode_config.connection_mutex
> > and drm_mode_config.mutex, respectively?
> >=20
> > > plus looking at ->state objects outside of atomic commit machinery
> > > makes no sense because you're not actually in sync with the hw state.
> > > Relevant bits need to be copied over at commit time, protected by some
> > > spinlock (and that spinlock also needs to be held over whatever other
> > > stuff you're setting to make sure we don't get a funny out-of-sync
> > > state anywhere).
> >=20
> > If we already have a lock protecting against having both an ASoC and KMS
> > function running, it's not clear to me what the spinlock would prevent
> > here?
>=20
> Replicating the irc chat here. With
>=20
> commit 6c5ed5ae353cdf156f9ac4db17e15db56b4de880
> Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Date:   Thu Apr 6 20:55:20 2017 +0200
>=20
>     drm/atomic: Acquire connection_mutex lock in drm_helper_probe_single_=
connector_modes, v4.
>=20
> this is already taken care of for drivers and should be all good from a
> locking pov.

So, if I understand this properly, this superseeds your comment on the
spinlock for the hw state, but not the comment that we need some locking
to synchronize between the audio and KMS path (and CEC?). Right?

Maxime

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