1 files changed, 69 insertions, 9 deletions

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index a8ed66751c2d..c96847fc0ebc 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -28,6 +28,7 @@
 #include <drm/drm_atomic_state_helper.h>
 #include <drm/drm_bridge.h>
 #include <drm/drm_encoder.h>
+#include <drm/drm_of.h>
 #include <drm/drm_print.h>
 
 #include "drm_crtc_internal.h"
@@ -49,12 +50,19 @@
  * Chaining multiple bridges to the output of a bridge, or the same bridge to
  * the output of different bridges, is not supported.
  *
+ * &drm_bridge, like &drm_panel, aren't &drm_mode_object entities like planes,
+ * CRTCs, encoders or connectors and hence are not visible to userspace. They
+ * just provide additional hooks to get the desired output at the end of the
+ * encoder chain.
+ */
+
+/**
+ * DOC:	display driver integration
+ *
  * Display drivers are responsible for linking encoders with the first bridge
  * in the chains. This is done by acquiring the appropriate bridge with
- * of_drm_find_bridge() or drm_of_find_panel_or_bridge(), or creating it for a
- * panel with drm_panel_bridge_add_typed() (or the managed version
- * devm_drm_panel_bridge_add_typed()). Once acquired, the bridge shall be
- * attached to the encoder with a call to drm_bridge_attach().
+ * devm_drm_of_get_bridge(). Once acquired, the bridge shall be attached to the
+ * encoder with a call to drm_bridge_attach().
  *
  * Bridges are responsible for linking themselves with the next bridge in the
  * chain, if any. This is done the same way as for encoders, with the call to
@@ -85,11 +93,63 @@
  * helper to create the &drm_connector, or implement it manually on top of the
  * connector-related operations exposed by the bridge (see the overview
  * documentation of bridge operations for more details).
- *
- * &drm_bridge, like &drm_panel, aren't &drm_mode_object entities like planes,
- * CRTCs, encoders or connectors and hence are not visible to userspace. They
- * just provide additional hooks to get the desired output at the end of the
- * encoder chain.
+ */
+
+/**
+ * DOC: special care dsi
+ *
+ * The interaction between the bridges and other frameworks involved in
+ * the probing of the upstream driver and the bridge driver can be
+ * challenging. Indeed, there's multiple cases that needs to be
+ * considered:
+ *
+ * - The upstream driver doesn't use the component framework and isn't a
+ *   MIPI-DSI host. In this case, the bridge driver will probe at some
+ *   point and the upstream driver should try to probe again by returning
+ *   EPROBE_DEFER as long as the bridge driver hasn't probed.
+ *
+ * - The upstream driver doesn't use the component framework, but is a
+ *   MIPI-DSI host. The bridge device uses the MIPI-DCS commands to be
+ *   controlled. In this case, the bridge device is a child of the
+ *   display device and when it will probe it's assured that the display
+ *   device (and MIPI-DSI host) is present. The upstream driver will be
+ *   assured that the bridge driver is connected between the
+ *   &mipi_dsi_host_ops.attach and &mipi_dsi_host_ops.detach operations.
+ *   Therefore, it must run mipi_dsi_host_register() in its probe
+ *   function, and then run drm_bridge_attach() in its
+ *   &mipi_dsi_host_ops.attach hook.
+ *
+ * - The upstream driver uses the component framework and is a MIPI-DSI
+ *   host. The bridge device uses the MIPI-DCS commands to be
+ *   controlled. This is the same situation than above, and can run
+ *   mipi_dsi_host_register() in either its probe or bind hooks.
+ *
+ * - The upstream driver uses the component framework and is a MIPI-DSI
+ *   host. The bridge device uses a separate bus (such as I2C) to be
+ *   controlled. In this case, there's no correlation between the probe
+ *   of the bridge and upstream drivers, so care must be taken to avoid
+ *   an endless EPROBE_DEFER loop, with each driver waiting for the
+ *   other to probe.
+ *
+ * The ideal pattern to cover the last item (and all the others in the
+ * MIPI-DSI host driver case) is to split the operations like this:
+ *
+ * - The MIPI-DSI host driver must run mipi_dsi_host_register() in its
+ *   probe hook. It will make sure that the MIPI-DSI host sticks around,
+ *   and that the driver's bind can be called.
+ *
+ * - In its probe hook, the bridge driver must try to find its MIPI-DSI
+ *   host, register as a MIPI-DSI device and attach the MIPI-DSI device
+ *   to its host. The bridge driver is now functional.
+ *
+ * - In its &struct mipi_dsi_host_ops.attach hook, the MIPI-DSI host can
+ *   now add its component. Its bind hook will now be called and since
+ *   the bridge driver is attached and registered, we can now look for
+ *   and attach it.
+ *
+ * At this point, we're now certain that both the upstream driver and
+ * the bridge driver are functional and we can't have a deadlock-like
+ * situation when probing.
  */
 
 static DEFINE_MUTEX(bridge_lock);