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

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Message-ID: <20240506-charcoal-griffin-of-tact-174dde@houat>
Date: Mon, 6 May 2024 09:29:36 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
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

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.

Maxime

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