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

[Maxime Ripard <maxime@cerno.tech>]

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Hi,

On Tue, Sep 14, 2021 at 09:00:28PM +0200, Andrzej Hajda wrote:
> 
> 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,

Done

> 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.

Rob was nice enough to give it a try last week for msm and do the needed
changes. He tested it with the sn65dsi86 bridge. John was also saying it
was on their todo list (for kirin I assume?). So hopefully it can be
fairly smooth for everyone.

I tested sn65dsi83 and ps8640 with the vc4 driver. I don't have the
hardware so it was just making sure that everything was probing
properly, but it's what we're interested in anyway.

> 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.

Ack, I'll merge them, thanks!
Maxime

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