linux-kernel - Re: Regression with mainline kernel on rpi4

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Date:   Tue, 28 Sep 2021 10:34:46 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Emma Anholt <emma@...olt.net>, David Airlie <airlied@...ux.ie>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Regression with mainline kernel on rpi4

Hi Daniel,

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@...no.tech> wrote:
> >
> > 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@...il.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 = connector->state->crtc;
> > >
> > > Yeah, that was my personal suspicion, because while the line number
> > > implied "crtc->state" being NULL, the drm data structure documentation
> > > 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 while,
> >     so I guess it's never really been causing a practical issue before.
> 
> You also have no locking

Indeed. Do we just need locking to prevent a concurrent audio setup and
modeset, or do you have another corner case in mind?

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?

> 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).

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?

Maxime