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From:   Philippe Cornu <philippe.cornu@st.com>
To:     Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Gustavo Padovan <gustavo@padovan.org>,
        "Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>,
        Sean Paul <seanpaul@chromium.org>,
        David Airlie <airlied@linux.ie>,
        <dri-devel@lists.freedesktop.org>, <linux-kernel@vger.kernel.org>
CC:     Benjamin Gaignard <benjamin.gaignard@linaro.org>,
        Yannick Fertre <yannick.fertre@st.com>,
        Philippe Cornu <philippe.cornu@st.com>,
        "Vincent Abriou" <vincent.abriou@st.com>
Subject: [PATCH] drm: clarify adjusted_mode documentation for bridges
Date:   Mon, 9 Apr 2018 17:24:27 +0200
Message-ID: <20180409152427.12449-1-philippe.cornu@st.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

This patch clarifies the adjusted_mode documentation
for bridges.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
---
This patch follows discussions in:
- "drm: clarify adjusted_mode for a bridge connected to a crtc"
  https://patchwork.freedesktop.org/patch/206801/
- "drm: bridge: Constify mode arguments to bridge .mode_set() operation"
  https://patchwork.freedesktop.org/patch/215449/

 include/drm/drm_bridge.h | 16 ++++++++++++++++
 include/drm/drm_crtc.h   | 11 +++++++----
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index 3270fec46979..7d632c6a9214 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -178,6 +178,22 @@ struct drm_bridge_funcs {
 	 * then this would be &drm_encoder_helper_funcs.mode_set. The display
 	 * pipe (i.e.  clocks and timing signals) is off when this function is
 	 * called.
+	 *
+	 * The adjusted_mode parameter corresponds to the mode output by the CRTC
+	 * for the first bridge in the chain. It can be different from the mode
+	 * parameter that contains the desired mode for the connector at the end
+	 * of the bridges chain, for instance when the first bridge in the chain
+	 * performs scaling. The adjusted mode is mostly useful for the first
+	 * bridge in the chain and is likely irrelevant for the other bridges.
+	 *
+	 * For atomic drivers the adjusted_mode is the mode stored in
+	 * &drm_crtc_state.adjusted_mode.
+	 *
+	 * NOTE:
+	 *
+	 * If a need arises to store and access modes adjusted for other locations
+	 * than the connection between the CRTC and the first bridge, the DRM
+	 * framework will have to be extended with DRM bridge states.
 	 */
 	void (*mode_set)(struct drm_bridge *bridge,
 			 struct drm_display_mode *mode,
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index a2d81d2907a9..65f749a9e9d3 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -134,10 +134,13 @@ struct drm_crtc_state {
 	 *
 	 * Internal display timings which can be used by the driver to handle
 	 * differences between the mode requested by userspace in @mode and what
-	 * is actually programmed into the hardware. It is purely driver
-	 * implementation defined what exactly this adjusted mode means. Usually
-	 * it is used to store the hardware display timings used between the
-	 * CRTC and encoder blocks.
+	 * is actually programmed into the hardware.
+	 *
+	 * For drivers using drm_bridge, this stores the hardware display timings
+	 * used between the CRTC and the first bridge. For other drivers, the
+	 * meaning of the adjusted_mode field is purely driver implementation
+	 * defined information, and will usually be used to store the hardware
+	 * display timings used between the CRTC and encoder blocks.
 	 */
 	struct drm_display_mode adjusted_mode;
 
-- 
2.15.1