linux-kernel - [PATCH 5.15 195/201] drm/amd/display: Dont lock connection

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Date:   Wed, 27 Jul 2022 18:11:39 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Jude Shih <shenshih@....com>,
        Anson Jacob <Anson.Jacob@....com>,
        Daniel Wheeler <daniel.wheeler@....com>,
        Nicholas Kazlauskas <nicholas.kazlauskas@....com>,
        Alex Deucher <alexander.deucher@....com>
Subject: [PATCH 5.15 195/201] drm/amd/display: Dont lock connection_mutex for DMUB HPD

From: Nicholas Kazlauskas <nicholas.kazlauskas@....com>

commit d82b3266ef88dc10fe0e7031b2bd8ba7eedb7e59 upstream.

[Why]
Per DRM spec we only need to hold that lock when touching
connector->state - which we do not do in that handler.

Taking this locking introduces unnecessary dependencies with other
threads which is bad for performance and opens up the potential for
a deadlock since there are multiple locks being held at once.

[How]
Remove the connection_mutex lock/unlock routine and just iterate over
the drm connectors normally. The iter helpers implicitly lock the
connection list so this is safe to do.

DC link access also does not need to be guarded since the link
table is static at creation - we don't dynamically add or remove links,
just streams.

Fixes: e27c41d5b068 ("drm/amd/display: Support for DMUB HPD interrupt handling")

Reviewed-by: Jude Shih <shenshih@....com>
Acked-by: Anson Jacob <Anson.Jacob@....com>
Tested-by: Daniel Wheeler <daniel.wheeler@....com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@....com>
Signed-off-by: Alex Deucher <alexander.deucher@....com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c |    4 ----
 1 file changed, 4 deletions(-)

--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -668,10 +668,7 @@ void dmub_hpd_callback(struct amdgpu_dev
 		return;
 	}
 
-	drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
-
 	link_index = notify->link_index;
-
 	link = adev->dm.dc->links[link_index];
 
 	drm_connector_list_iter_begin(dev, &iter);
@@ -684,7 +681,6 @@ void dmub_hpd_callback(struct amdgpu_dev
 		}
 	}
 	drm_connector_list_iter_end(&iter);
-	drm_modeset_unlock(&dev->mode_config.connection_mutex);
 
 }