[PATCH 04/10] drm/bridge: Document the probe issue with MIPI-DSI bridges

Maxime Ripard
2021-07-20T13:53:17Z

Interactions between bridges, panels, MIPI-DSI host and the component
framework are not trivial and can lead to probing issues when
implementing a display driver. Let's document the various cases we need
too consider, and the solution to support all the cases.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
---
Documentation/gpu/drm-kms-helpers.rst | 6 +++
drivers/gpu/drm/drm_bridge.c | 60 +++++++++++++++++++++++++++
2 files changed, 66 insertions(+)

diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst
index 10f8df7aecc0..ec2f65b31930 100644
--- a/Documentation/gpu/drm-kms-helpers.rst
+++ b/Documentation/gpu/drm-kms-helpers.rst
@@ -157,6 +157,12 @@ Display Driver Integration
.. kernel-doc:: drivers/gpu/drm/drm_bridge.c
:doc: display driver integration

+Special Care with MIPI-DSI bridges
+----------------------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_bridge.c
+ :doc: special care dsi
+
Bridge Operations
-----------------

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index c9a950bfdfe5..81f8dac12367 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -95,6 +95,66 @@
* documentation of bridge operations for more details).
*/

+/**
+ * DOC: special care dsi
+ *
+ * The interaction between the bridges and other frameworks involved in
+ * the probing of the display driver and the bridge driver can be
+ * challenging. Indeed, there's multiple cases that needs to be
+ * considered:
+ *
+ * - The display 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 display driver should try to probe again by returning
+ * EPROBE_DEFER as long as the bridge driver hasn't probed.
+ *
+ * - The display 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 display 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 display 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 display 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 display 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
+ * display driver case) is to split the operations like this:
+ *
+ * - In the display driver must run mipi_dsi_host_register() and
+ * component_add 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 not try to find its
+ * MIPI-DSI host or register as a MIPI-DSI device. As far as the
+ * framework is concerned, it must only call drm_bridge_add().
+ *
+ * - In its bind hook, the display driver must try to find the bridge
+ * and return -EPROBE_DEFER if it doesn't find it. If it's there, it
+ * must call drm_bridge_attach(). The MIPI-DSI host is now functional.
+ *
+ * - In its &drm_bridge_funcs.attach hook, the bridge driver can now try
+ * to find its MIPI-DSI host and can register as a MIPI-DSI device.
+ *
+ * At this point, we're now certain that both the display driver and the
+ * bridge driver are functional and we can't have a deadlock-like
+ * situation when probing.
+ */
+
static DEFINE_MUTEX(bridge_lock);
static LIST_HEAD(bridge_list);

--
2.31.1