Unverified Commit 209264a8 authored by Maxime Ripard's avatar Maxime Ripard
Browse files

drm/bridge: Document the probe issue with MIPI-DSI bridges 



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: default avatarMaxime Ripard <maxime@cerno.tech>
Reviewed-by: default avatarAndrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210910101218.1632297-3-maxime@cerno.tech
parent 8886815f

Documentation/gpu/drm-kms-helpers.rst

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@@ -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

-----------------

drivers/gpu/drm/drm_bridge.c

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@@ -95,6 +95,63 @@ 
 * 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);