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);