From: Maxime Ripard <mripard@kernel.org>
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Sean Anderson <sean.anderson@linux.dev>,
Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>,
Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
Thomas Zimmermann <tzimmermann@suse.de>,
David Airlie <airlied@gmail.com>,
Daniel Vetter <daniel@ffwll.ch>,
Michal Simek <michal.simek@amd.com>,
dri-devel@lists.freedesktop.org,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [BUG] drm: zynqmp_dp: Lockup in zynqmp_dp_bridge_detect when device is unbound
Date: Tue, 7 May 2024 09:58:25 +0200 [thread overview]
Message-ID: <20240507-shiny-sophisticated-mustang-bf2d2e@houat> (raw)
In-Reply-To: <20240506165057.GD29108@pendragon.ideasonboard.com>
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On Mon, May 06, 2024 at 07:50:57PM GMT, Laurent Pinchart wrote:
> On Mon, May 06, 2024 at 10:57:17AM -0400, Sean Anderson wrote:
> > On 5/6/24 03:35, Laurent Pinchart wrote:
> > > On Mon, May 06, 2024 at 09:29:36AM +0200, Maxime Ripard wrote:
> > >> Hi Laurent, Sean,
> > >>
> > >> On Sat, May 04, 2024 at 03:21:18PM GMT, Laurent Pinchart wrote:
> > >> > On Fri, May 03, 2024 at 05:54:32PM -0400, Sean Anderson wrote:
> > >> > > I have discovered a bug in the displayport driver on drm-misc-next. To
> > >> > > trigger it, run
> > >> > >
> > >> > > echo fd4a0000.display > /sys/bus/platform/drivers/zynqmp-dpsub/unbind
> > >> > >
> > >> > > The system will become unresponsive and (after a bit) splat with a hard
> > >> > > LOCKUP. One core will be unresponsive at the first zynqmp_dp_read in
> > >> > > zynqmp_dp_bridge_detect.
> > >> > >
> > >> > > I believe the issue is due the registers being unmapped and the block
> > >> > > put into reset in zynqmp_dp_remove instead of zynqmp_dpsub_release.
> > >> >
> > >> > That is on purpose. Drivers are not allowed to access the device at all
> > >> > after .remove() returns.
> > >>
> > >> It's not "on purpose" no. Drivers indeed are not allowed to access the
> > >> device after remove, but the kernel shouldn't crash. This is exactly
> > >> why we have drm_dev_enter / drm_dev_exit.
> > >
> > > I didn't mean the crash was on purpose :-) It's the registers being
> > > unmapped that is, as nothing should touch those registers after
> > > .remove() returns.
> >
> > OK, so then we need to have some kind of flag in the driver or in the drm
> > subsystem so we know not to access those registers.
>
> To avoid race conditions, the .remove() function should mark the device
> as removed, wait for all ongoing access from userspace to be complete,
> and then proceed to unmapping registers and doing other cleanups.
> Userspace may still have open file descriptors to the device at that
> point. Any new userspace access should be disallowed (by checking the
> removed flag), with the only userspace-initiated operations that still
> need to run being the release-related operations (unmapping memory,
> closing file descriptors, ...).
And for the record, this is exactly what drm_dev_unplug and
drm_dev_enter/drm_dev_exit does.
Maxime
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next prev parent reply other threads:[~2024-05-07 7:58 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-03 21:54 [BUG] drm: zynqmp_dp: Lockup in zynqmp_dp_bridge_detect when device is unbound Sean Anderson
2024-05-04 12:21 ` Laurent Pinchart
2024-05-06 7:29 ` Maxime Ripard
2024-05-06 7:35 ` Laurent Pinchart
2024-05-06 14:57 ` Sean Anderson
2024-05-06 16:50 ` Laurent Pinchart
2024-05-07 7:58 ` Maxime Ripard [this message]
2024-05-06 11:16 ` Tomi Valkeinen
2024-05-06 14:46 ` Sean Anderson
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