Re: [PATCH v2 0/8] drm/bridge: Make panel and bridge probe order consistent

[Jagan Teki <jagan@amarulasolutions.com>]

Hi Andrzej,

On Wed, Aug 4, 2021 at 7:48 PM a.hajda <a.hajda@samsung.com> wrote:
>
> Hi Maxime,
>
> I have been busy with other tasks, and I did not follow the list last
> time, so sorry for my late response.
>
> On 28.07.2021 15:32, Maxime Ripard wrote:
> > Hi,
> >
> > We've encountered an issue with the RaspberryPi DSI panel that prevented the
> > whole display driver from probing.
> >
> > The issue is described in detail in the commit 7213246a803f ("drm/vc4: dsi:
> > Only register our component once a DSI device is attached"), but the basic idea
> > is that since the panel is probed through i2c, there's no synchronization
> > between its probe and the registration of the MIPI-DSI host it's attached to.
> >
> > We initially moved the component framework registration to the MIPI-DSI Host
> > attach hook to make sure we register our component only when we have a DSI
> > device attached to our MIPI-DSI host, and then use lookup our DSI device in our
> > bind hook.
> >
> > However, all the DSI bridges controlled through i2c are only registering their
> > associated DSI device in their bridge attach hook, meaning with our change
>
>
> I guess this is incorrect. I have promoted several times the pattern
> that device driver shouldn't expose its interfaces (for example
> component_add, drm_panel_add, drm_bridge_add) until it gathers all
> required dependencies. In this particular case bridges should defer
> probe until DSI bus becomes available. I guess this way the patch you
> reverts would work.
>
> I advised few times this pattern in case of DSI hosts, apparently I
> didn't notice the similar issue can appear in case of bridges. Or there
> is something I have missed???

Look like Maxime is correct. I2C based DSI bridge will get probe
during bridge_attach which usually called from bridge driver
bridge_attach call. Non-I2C bridges and DSI panels will get probe
during host.attach.

We do get similar situation for dw-mipi-dsi bridges, where icn6211
bridge is not I2C-based bridge and it gets probed in host_attach and
sn65dsi83 is I2C based bridge and it gets probed in bridge_attach.

Here is the simple call trace we have observed with dw-mipi-dsi bridge
when all possible DSI device are trying to probe.

1. DSI panels and bridges will invoke the host attach
   from probe in order to find the panel_or_bridge.

   chipone_probe()
       dw_mipi_dsi_host_attach().start
           dw_mipi_dsi_panel_or_bridge()
                ...found the panel_or_bridge...

   ltdc_encoder_init().start
       dw_mipi_dsi_bridge_attach().start
                   dw_mipi_dsi_host_attach().start
                       chipone_attach(). start

                       chipone_attach(). done
                   dw_mipi_dsi_host_attach().done
       dw_mipi_dsi_bridge_attach(). done
   ltdc_encoder_init().done

2. I2C based DSI bridge will invoke the drm_bridge_attach
   from bridge attach in order to find the panel_or_bridge.

   ltdc_encoder_init().start
       dw_mipi_dsi_bridge_attach().start
           dw_mipi_dsi_panel_or_bridge()
                ...found the panel_or_bridge...
                   dw_mipi_dsi_host_attach().start
                       sn65dsi83_attach(). start

                       sn65dsi83_attach(). done
                   dw_mipi_dsi_host_attach().done
       dw_mipi_dsi_bridge_attach(). done
   ltdc_encoder_init().done

It is correct that the I2C-based bridges will attach the host via
mipi_dsi_attach in driver bridge API where as it done in probe for
Non-I2C bridges and DSI panels.

>
> Anyway there are already eleven(?) bridge drivers using this pattern. I
> wonder if fixing it would be difficult, or if it expose other issues???
> The patches should be quite straightforward - move
> of_find_mipi_dsi_host_by_node and mipi_dsi_device_register_full to probe
> time.
>
> Finally I think that if we will not fix these bridge drivers we will
> encounter another set of issues with new platforms connecting "DSI host
> drivers assuming this pattern" and "i2c/dsi device drivers assuming
> pattern already present in the bridges".

Agreed, I'm trying to understand the several ways to fix this. Right
now I'm trying this on sun6i_mipi_dsi and exynos_drm_dsi. Will let you
know for any update and suggestions on the same.

Thanks,
Jagan.