linux-kernel - [PATCH 1/2] drm/bridge: parade-ps8640: avoid race condition on driver unbinding

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Date:   Fri, 25 Feb 2022 14:45:03 +0100
From:   Ricardo Cañuelo <ricardo.canuelo@...labora.com>
To:     dri-devel@...ts.freedesktop.org
Cc:     Ricardo Cañuelo <ricardo.canuelo@...labora.com>,
        linux-kernel@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        kernel@...labora.com, andrzej.hajda@...el.com,
        narmstrong@...libre.com, maarten.lankhorst@...ux.intel.com,
        mripard@...nel.org
Subject: [PATCH 1/2] drm/bridge: parade-ps8640: avoid race condition on driver unbinding

When unbinding a DRM master driver there's a race condition that
sometimes results in an invalid vm access when userspace (gnome-shell)
issues a DRM_IOCTL_MODE_GETCONNECTOR ioctl right after a bridge has been
removed from an encoder's bridge chain.

This means that once a bridge has been disabled and gone through
ps8640_post_disable(), if ps8640_bridge_get_edid() runs afterwards as a
result of that ioctl call it will call drm_bridge_chain_pre_enable()
and drm_bridge_chain_post_disable() again in a bridge that has been
already detached.

Setting `ps_bridge->pre_enabled = false` at a later stage of the
bringdown path and calling drm_bridge_chain_pre_enable() inside
ps8640_bridge_get_edid() only if needed avoid this, although a proper
subsystem-wide fix would be the proper solution, since the same type of
race conditions might happen somewhere else.

Tested on an Acer Chromebook R13 (Elm, MT8173) with Debian Sid.

Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@...labora.com>
---
 drivers/gpu/drm/bridge/parade-ps8640.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
index 3f17337ee389..a927787a89bf 100644
--- a/drivers/gpu/drm/bridge/parade-ps8640.c
+++ b/drivers/gpu/drm/bridge/parade-ps8640.c
@@ -434,8 +434,6 @@ static void ps8640_post_disable(struct drm_bridge *bridge)
 {
 	struct ps8640 *ps_bridge = bridge_to_ps8640(bridge);

-	ps_bridge->pre_enabled = false;
-
 	ps8640_bridge_vdo_control(ps_bridge, DISABLE);
 	pm_runtime_put_sync_suspend(&ps_bridge->page[PAGE0_DP_CNTL]->dev);
 }
@@ -487,6 +485,7 @@ static void ps8640_bridge_detach(struct drm_bridge *bridge)
 	drm_dp_aux_unregister(&ps_bridge->aux);
 	if (ps_bridge->link)
 		device_link_del(ps_bridge->link);
+	ps_bridge->pre_enabled = false;
 }

 static struct edid *ps8640_bridge_get_edid(struct drm_bridge *bridge,
@@ -508,7 +507,8 @@ static struct edid *ps8640_bridge_get_edid(struct drm_bridge *bridge,
 	 * EDID, for this chip, we need to do a full poweron, otherwise it will
 	 * fail.
 	 */
-	drm_bridge_chain_pre_enable(bridge);
+	if (poweroff)
+		drm_bridge_chain_pre_enable(bridge);

 	edid = drm_get_edid(connector,
 			    ps_bridge->page[PAGE0_DP_CNTL]->adapter);
-- 
2.25.1