drm: Mention the power state requirement on side-channel operations

The drm_connector detect, drm_dp_aux transfer and mipi_dsi_host
operations typically require to access their underlying device to
perform what is expected of them.

However, there's no guarantee on the fact that the device has been
enabled through atomic_enable or similar that will usually power the
device. The access to an unpowered device is then an undefined behaviour
ranging from the access being ignored to a hard CPU hang.

Let's document that expectation to avoid as much as possible those
consequences.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210616141529.630719-3-maxime@cerno.tech

1 files changed, 5 insertions, 0 deletions

diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 9336bfbdb389..729d5d82475e 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1950,6 +1950,11 @@ struct drm_dp_aux {
 	 * The @transfer() function must only modify the reply field of
 	 * the &drm_dp_aux_msg structure. The retry logic and i2c
 	 * helpers assume this is the case.
+	 *
+	 * Also note that this callback can be called no matter the
+	 * state @dev is in. Drivers that need that device to be powered
+	 * to perform this operation will first need to make sure it's
+	 * been properly enabled.
 	 */
 	ssize_t (*transfer)(struct drm_dp_aux *aux,
 			    struct drm_dp_aux_msg *msg);