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

[Maxime Ripard]

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 at 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 at samsung.com>

I assume you'll want me to hold off that patch before someone reviews
the rest?

Thanks!
Maxime
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