Re: [BUG] drm: zynqmp_dp: Lockup in zynqmp_dp_bridge_detect when device is unbound

From: Sean Anderson <sean.anderson@linux.dev>
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Maxime Ripard <mripard@kernel.org>
Cc: 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: Mon, 6 May 2024 10:57:17 -0400	[thread overview]
Message-ID: <97811bfe-a1fb-419c-a148-74e3d84aa0e2@linux.dev> (raw)
In-Reply-To: <20240506073531.GA10260@pendragon.ideasonboard.com>

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.

--Sean