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Subject[PATCH v4 02/24] drm/bridge: Document the probe issue with MIPI-DSI bridges
Date
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 | 57 +++++++++++++++++++++++++++
2 files changed, 63 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 baff74ea4a33..7cc2d2f94ae3 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -96,6 +96,63 @@
* 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 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);
static LIST_HEAD(bridge_list);

--
2.31.1
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