drm/dp: Add backpointer to drm_device in drm_dp_aux

This is something that we've wanted for a while now: the ability to
actually look up the respective drm_device for a given drm_dp_aux struct.
This will also allow us to transition over to using the drm_dbg_*() helpers
for debug message printing, as we'll finally have a drm_device to reference
for doing so.

Note that there is one limitation with this - because some DP AUX adapters
exist as platform devices which are initialized independently of their
respective DRM devices, one cannot rely on drm_dp_aux->drm_dev to always be
non-NULL until drm_dp_aux_register() has been called. We make sure to point
this out in the documentation for struct drm_dp_aux.

v3:
* Add WARN_ON_ONCE() to drm_dp_aux_register() if drm_dev isn't filled out

Signed-off-by: Lyude Paul <lyude@redhat.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210423184309.207645-4-lyude@redhat.com
Reviewed-by: Dave Airlie <airlied@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_dp_aux_dev.c b/drivers/gpu/drm/drm_dp_aux_dev.c
index e25181bf2c48..06b374cae956 100644
--- a/drivers/gpu/drm/drm_dp_aux_dev.c
+++ b/drivers/gpu/drm/drm_dp_aux_dev.c
@@ -278,6 +278,12 @@ void drm_dp_aux_unregister_devnode(struct drm_dp_aux *aux)
 	if (!aux_dev) /* attach must have failed */
 		return;
 
+	/*
+	 * As some AUX adapters may exist as platform devices which outlive their respective DRM
+	 * devices, we clear drm_dev to ensure that we never accidentally reference a stale pointer
+	 */
+	aux->drm_dev = NULL;
+
 	mutex_lock(&aux_idr_mutex);
 	idr_remove(&aux_idr, aux_dev->index);
 	mutex_unlock(&aux_idr_mutex);