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drm/scdc: Document hotplug gotchas

Message ID 20210927161055.529050-1-maxime@cerno.tech (mailing list archive)
State New, archived
Headers show
Series drm/scdc: Document hotplug gotchas | expand

Commit Message

Maxime Ripard Sept. 27, 2021, 4:10 p.m. UTC 
There's some interactions between the SCDC setup and the disconnection /
reconnection of displays. Let's document it and a solution.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
---
 drivers/gpu/drm/drm_scdc_helper.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)
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Patch

diff --git a/drivers/gpu/drm/drm_scdc_helper.c b/drivers/gpu/drm/drm_scdc_helper.c
index 48a382464d54..12e09fe8c8df 100644
--- a/drivers/gpu/drm/drm_scdc_helper.c
+++ b/drivers/gpu/drm/drm_scdc_helper.c
@@ -34,6 +34,19 @@ 
  * HDMI 2.0 specification. It is a point-to-point protocol that allows the
  * HDMI source and HDMI sink to exchange data. The same I2C interface that
  * is used to access EDID serves as the transport mechanism for SCDC.
+ *
+ * Note: The SCDC status is going to be lost when the display is
+ * disconnected. This can happen physically when the user disconnects
+ * the cable, but also when a display is switched on (such as waking up
+ * a TV).
+ *
+ * This is further complicated by the fact that, upon a disconnection /
+ * reconnection, KMS won't change the mode on its own. This means that
+ * one can't just rely on setting the SCDC status on enable, but also
+ * has to track the connector status changes using interrupts and
+ * restore the SCDC status. The typical place to restore that state is
+ * in drm_encoder_helper_funcs.detect() or
+ * drm_connector_helper_funcs.detect_ctx().
  */
 
 #define SCDC_I2C_SLAVE_ADDRESS 0x54