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

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Date: Mon, 6 May 2024 10:35:31 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Maxime Ripard <mripard@...nel.org>
Cc: Sean Anderson <sean.anderson@...ux.dev>,
	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 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.

-- 
Regards,

Laurent Pinchart