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

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

W dniu 14.09.2021 o 16:35, Maxime Ripard pisze:
> Hi,
>
> On Mon, Sep 13, 2021 at 08:29:37AM +0200, Andrzej Hajda wrote:
>> 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.
> Yeah, there's so many moving parts it's definitely not great.
>
>> 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.
> Wow, that sounds like a massive change indeed :/
>
>> Anyway:
>>
>> Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
> I assume you'll want me to hold off that patch before someone reviews
> the rest?

The last exynos patch should be dropped, kirin patch should be 
tested/reviewed/acked by kirin maintaner. I am not sure about bridge 
patches, which ones have been tested by you, and which one have other users.

If yes it would be good to test them as well - changes in initialization 
flow can beat sometimes :)

I think patches 1-4 can be merged earlier, if you like, as they are on 
the list for long time.


Regards

Andrzej


>
> Thanks!
> Maxime