Message-ID: <20211025141509.3bojy4fndhapv6ti@gilmour>
Date:   Mon, 25 Oct 2021 16:15:09 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     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,

On Thu, Oct 14, 2021 at 03:15:36PM +0200, Daniel Vetter wrote:
> On Wed, Oct 13, 2021 at 05:01:03PM +0200, Maxime Ripard wrote:
> > 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,
> > > > 
> > > > 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?
> > > 
> > > Replicating the irc chat here. With
> > > 
> > > commit 6c5ed5ae353cdf156f9ac4db17e15db56b4de880
> > > Author: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
> > > Date:   Thu Apr 6 20:55:20 2017 +0200
> > > 
> > >     drm/atomic: Acquire connection_mutex lock in drm_helper_probe_single_connector_modes, v4.
> > > 
> > > 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?
> 
> Other way round. There's 3 things involved here:
> 1. kms output probe code
> 2. kms atomic commit code
> 3. calls from asoc side
> 
> The above referenced commit makes sure 1&2 are synchronized. The problem
> is that 2&3 are not synchonronized, and from 3, no matter how much locking
> you have, you cannot look at kms state. I.e. not allowed to look at
> crtc->state for example, irrespective of whether you're holding
> drm_modeset_lock or not. This is because the atomic nonblocking commit is
> done without holding any locks, protection is purely down to ownership
> rules of state structures and ordering (through drm_crtc_commit) of
> in-flight nonblocking atomic commits.
> 
> That's why you need a sperate lock _and_ copy state, so taht 2&3 stay in
> sync.
> 
> In practice you only care about modeset changes from 2 vs anything from 3,
> and most userspace does modeset atomic commits as blocking commits, which
> means you won't notice that your locking has gaps.
> 
> btw same problem exists between atomic and (vblank) irq handler. There you
> need a irqsafe spinlock and you also have to copy (because the irq handler
> just cannot access ->state in any safe way, because it doesn't own that
> structure).
> 
> This is maybe a bit the confusing thing with atomic commit: ->state isn't
> protected by locks, but through ownership rules. Only for atomic check is
> ->state protected by locks, but once we're committed we switch over to
> ownership rules for protection. swap_states() is that point of no return.

Thanks for the clarifications, I just posted a series that should be
implementing this here:
https://lore.kernel.org/dri-devel/20211025141113.702757-1-maxime@cerno.tech/

Maxime

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