[PATCH 04/10] drm/bridge: Document the probe issue with MIPI-DSI bridges

[Jagan Teki]

Hi Maxime,

On Wed, Jul 28, 2021 at 7:05 PM Maxime Ripard <maxime at cerno.tech> wrote:
>
> Hi Jagan,
>
> On Tue, Jul 27, 2021 at 03:12:09PM +0530, Jagan Teki wrote:
> > On Tue, Jul 20, 2021 at 7:15 PM Maxime Ripard <maxime at cerno.tech> 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          | 60 +++++++++++++++++++++++++++
> > >  2 files changed, 66 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 c9a950bfdfe5..81f8dac12367 100644
> > > --- a/drivers/gpu/drm/drm_bridge.c
> > > +++ b/drivers/gpu/drm/drm_bridge.c
> > > @@ -95,6 +95,66 @@
> > >   * 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.
> > > + *
> > > + * - In its probe hook, the bridge driver must not try to find its
> > > + *   MIPI-DSI host or register as a MIPI-DSI device. As far as the
> > > + *   framework is concerned, it must only call drm_bridge_add().
> > > + *
> > > + * - In its bind hook, the display driver must try to find the bridge
> > > + *   and return -EPROBE_DEFER if it doesn't find it. If it's there, it
> > > + *   must call drm_bridge_attach(). The MIPI-DSI host is now functional.
> >
> > There is an another problem occur for this scenario in the case of kms
> > hotplug driver, sun6i_mipi_dsi.c. When host attach wait till drm
> > device pointer found and drm device pointer would found only when bind
> > done, and bind would complete only when &drm_bridge_funcs.attach hooks
> > are complete. But, If DSI driver is fully bridge driven then this
> > attach in bind will trigger panel_bridge hook attach and at this point
> > we cannot get panel_bridge at all which indeed second attach would
> > would failed.
> >
> > This is one of the reason I'm trying to use drm_bridge_attach host
> > attach itself instead of component bind, not yet succeeded.
>
> I'm not really sure what you mean, but if you mention the code we have
> in the DSI driver to make sure we can probe without our panel, then it's
> not something that we really can support. Bridges cannot be hotplugged
> in DRM and having some inconsistencies between drivers (since none of
> them behave the same way there) and between what's plugged on the other
> side of the DSI bus feels weird.

Yes, but for associated bridges to attach on component base DSI
drivers the panel_bridge or bridge pointer look necessary.

Here is the pseudo code for sun6i_mipi_dsi which support the DSI probe
without panel, by waiting for drm pointer to found, but the same seems
not possible for bridge cases.

static int sun6i_dsi_bridge_attach(struct drm_bridge *bridge,
                                   enum drm_bridge_attach_flags flags)
{
        return drm_bridge_attach(bridge->encoder, dsi->bridge, bridge, flags);
}

static int sun6i_dsi_attach(struct mipi_dsi_host *host,
                            struct mipi_dsi_device *device)
{
   ...
   if (!dsi->drm || !dsi->drm->registered)
                return -EPROBE_DEFER;

   panel = of_drm_find_panel(device->dev.of_node);
        if (IS_ERR(panel)) {
                panel = NULL;

                bridge = of_drm_find_bridge(device->dev.of_node);
                if (IS_ERR(bridge)) {
                        dev_err(dsi->dev, "failed to find bridge\n");
                        return PTR_ERR(bridge);
                }
        } else {
                bridge = NULL;
    }

    dsi->panel = panel;
    dsi->bridge = bridge;
    ....
}

static int sun6i_dsi_bind(struct device *dev, struct device *master,
                         void *data)
{
   ...
   ret = drm_bridge_attach(&dsi->encoder, dsi->bridge, NULL, 0);
   ..
   dsi->drm = drm;
}

I believe some-sort bridge handling in hotpulg would necessary to keep
the hotplug probing happens for bridge pointers as well.

Thanks,
Jagan.