[PATCH v3 2/8] drm/bridge: Document the probe issue with MIPI-DSI bridges

[Andrzej Hajda]

Hi Maxime,

On 23.08.2021 10:47, 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>
> ---
>   Documentation/gpu/drm-kms-helpers.rst |  6 +++
>   drivers/gpu/drm/drm_bridge.c          | 58 +++++++++++++++++++++++++++
>   2 files changed, 64 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..794654233cf5 100644
> --- a/drivers/gpu/drm/drm_bridge.c
> +++ b/drivers/gpu/drm/drm_bridge.c
> @@ -96,6 +96,64 @@
>    * 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.


I guess component_add is leftover from previous iteration (as you wrote 
few lines below) component_add should be called from dsi host attach 
callback.


> + *
> + * - 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 display driver
> + *   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 display driver and the
> + * bridge driver are functional and we can't have a deadlock-like
> + * situation when probing.
> + */
> +


Beside small mistake the whole patch looks OK for me. Maybe it would be 
worth to mention what is the real cause of this "special DSI case" - 
there is mutual dependency between two following entities in display chain:

1. display driver - it provides DSI bus, and requires drm_bridge or 
drm_panel provided by child device.

2. bridge or panel with DSI transport - it requires DSI bus provided by 
display driver, and provides drm_bridge or drm_panel interface required 
by display driver.

I guess similar issues can appear with other data/control bus-es, 
apparently DSI case is the most common.


And one more thing - you use "display driver" term but this is also case 
of any bridge providing DSI bus - there are already 3 such bridges in 
kernel - cdns, nwl, synopsys, tc358768, maybe "DSI host" would be better 
term.

And another thing - downstream device can be bridge or *panel*, it would 
be good to mention that panels also should follow this pattern.

Btw this is another place where word bridge can be 1:1 replaced by word 
panel - it clearly suggest that DRM subsystem waits for brave men who 
proposes patches unifying them, we would save lot of words, and lines of 
code if we could use drm_sink instead of "if (sink is bridge) do sth 
else do sth-similar-but-with-drm_panel-interface".


Regards

Andrzej


>   static DEFINE_MUTEX(bridge_lock);
>   static LIST_HEAD(bridge_list);
>