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

[Daniel Vetter]

On Tue, Jul 20, 2021 at 03:45:19PM +0200, Maxime Ripard wrote:
> 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 at cerno.tech>

I still have this dream that eventually we resurrect a patch to add
device_link to bridges/panels (ideally automatically), to help with some
of the suspend/resume issues around here.

Will this make things worse?

I think it'd be really good to figure that out with some coding, since if
we have incompatible solution to handle probe issues vs suspend/resume
issues, we're screwed.

Atm the duct-tape is to carefully move things around between suspend and
suspend_early hooks (and resume and resume_late) and hope it all works ...
-Daniel

> ---
>  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
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch