linux-kernel - Re: [BUG] drm: zynqmp_dp: Lockup in zynqmp_dp_bridge

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Date: Mon, 6 May 2024 10:57:17 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Maxime Ripard <mripard@...nel.org>
Cc: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Daniel Vetter <daniel@...ll.ch>, Michal Simek <michal.simek@....com>,
 dri-devel@...ts.freedesktop.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [BUG] drm: zynqmp_dp: Lockup in zynqmp_dp_bridge_detect when
 device is unbound

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