Re: [PATCH v4 02/24] drm/bridge: Document the probe issue with MIPI-DSI bridges

[Andrzej Hajda <a.hajda@samsung.com>]

W dniu 10.09.2021 o 12:11, Maxime Ripard pisze:
> 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);
>   


Nice work with documenting this initialization dance. It clearly shows 
that bridge API lacks better mechanism - usage of mipi dsi callbacks to 
get notifications about bridge appearance is ugly. It remains me my 
resource tracking patches which I have posted long time ago [1] - they 
would solve the issue in much more elegant way, described here [2]. 
Apparently I was not stubborn enough in promoting this solution.

Anyway:

Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>


[1]: https://lkml.org/lkml/2014/12/10/342

[2]: 
https://events19.linuxfoundation.org/wp-content/uploads/2017/12/Deferred-Problem-Issues-With-Complex-Dependencies-Between-Devices-in-Linux-Kernel-Andrzej-Hajda-Samsung.pdf


Regards
Andrzej