linux-kernel - [PATCH 1/3] drm: Mention the power state requirement on side-channel operations

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Message-Id: <20210525091059.234116-2-maxime@cerno.tech>
Date:   Tue, 25 May 2021 11:10:57 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Maxime Ripard <maxime@...no.tech>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Boris Brezillon <bbrezillon@...nel.org>,
        linux-kernel@...r.kernel.org, Eric Anholt <eric@...olt.net>,
        Emma Anholt <emma@...olt.net>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Phil Elwell <phil@...pberrypi.com>,
        Tim Gover <tim.gover@...pberrypi.com>,
        Dom Cobley <dom@...pberrypi.com>,
        Maxime Ripard <mripard@...nel.org>
Subject: [PATCH 1/3] 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@...no.tech>
---
 include/drm/drm_connector.h | 5 +++++
 include/drm/drm_dp_helper.h | 4 ++++
 include/drm/drm_mipi_dsi.h  | 5 +++++
 3 files changed, 14 insertions(+)

diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
index 714d1a01c065..0a1d9a0fcbb2 100644
--- a/include/drm/drm_connector.h
+++ b/include/drm/drm_connector.h
@@ -848,6 +848,11 @@ struct drm_connector_funcs {
 	 * locks to avoid races with concurrent modeset changes need to use
 	 * &drm_connector_helper_funcs.detect_ctx instead.
 	 *
+	 * Also note that this callback can be called no matter the
+	 * state the connector is in. Drivers that need the underlying
+	 * device to be powered to perform the detection will first need
+	 * to make sure it's been properly enabled.
+	 *
 	 * RETURNS:
 	 *
 	 * drm_connector_status indicating the connector's status.
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 06681bf46d81..4fcb027c8821 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1884,6 +1884,10 @@ struct drm_dp_aux_cec {
  * Note that the aux helper code assumes that the @transfer() function only
  * modifies the reply field of the &drm_dp_aux_msg structure. The retry logic
  * and i2c helpers assume this is the case.
+ *
+ * Also note that @transfer() 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.
  */
 struct drm_dp_aux {
 	const char *name;
diff --git a/include/drm/drm_mipi_dsi.h b/include/drm/drm_mipi_dsi.h
index 360e6377e84b..849d3029e303 100644
--- a/include/drm/drm_mipi_dsi.h
+++ b/include/drm/drm_mipi_dsi.h
@@ -80,6 +80,11 @@ int mipi_dsi_create_packet(struct mipi_dsi_packet *packet,
  * Note that typically DSI packet transmission is atomic, so the .transfer()
  * function will seldomly return anything other than the number of bytes
  * contained in the transmit buffer on success.
+ *
+ * Also note that those callbacks can be called no matter the state the
+ * host is in. Drivers that need the underlying device to be powered to
+ * perform these operations will first need to make sure it's been
+ * properly enabled.
  */
 struct mipi_dsi_host_ops {
 	int (*attach)(struct mipi_dsi_host *host,
-- 
2.31.1