[v7,11/12] drm/bridge: Document bridge init order with enable_next_first

Commit Message

Jagan Teki March 29, 2023, 1:19 p.m. UTC

In order to switch HS mode properly by DSI host, the DSI sink has to
send the MIPI-DCS commands first before the DSI host switches to HS
mode.

This behavior requires a bridge init alter in @enable and @disable
function calls with the help of @enable_next_first.

Document the affected bridge init order with a proper explanation.

Signed-off-by: Jagan Teki <[e-mail]>
---
Changes for v7:
- new patch

 drivers/gpu/drm/drm_bridge.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

Patch

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index cdc2669b3512..3c6c9937537a 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -190,6 +190,21 @@ 
  * Ultra Low Power State (ULPS) is not explicitly supported by DRM. If
  * implemented, it therefore needs to be handled entirely within the DSI Host
  * driver.
+ *
+ * DSI sink devices typically send the MIPI-DCS commands to the DSI host via
+ * general MIPI_DSI_DCS read and write API. The classical DSI sequence
+ * mentioned that the DSI host receives MIPI-DCS commands from the DSI sink
+ * first in order to switch HS mode properly. Once the DSI host switches to
+ * HS mode any MIPI-DCS commands from the DSI sink are unfunctional.
+ *
+ * DSI sink uses the @enable function to send the MIPI-DCS commands. In a
+ * typical DSI host, sink pipeline the @enable call chain start with the
+ * DSI host, and then the DSI sink which is the "wrong" order as DSI host
+ * @enable is called and switched to HS mode before DSI sink @enable. If
+ * the DSI host enables with the @enable_next_first flag then the @enable
+ * for the DSI sink will be called first before the @enable of the DSI host.
+ * This alter bridge init order makes sure that the MIPI-DCS commands send
+ * first and then switch to the HS mode properly by the DSI host.
  */
 
 static DEFINE_MUTEX(bridge_lock);