RFC: DSI/DRM multiplexer bridge

[Jagan Teki]

On Wed, Apr 5, 2023 at 7:39 PM Alexander Stein
<alexander.stein at ew.tq-group.com> wrote:
>
> Hi,
>
> my platform has a DIP switch controlled multiplexer for MIPI DSI output
> signals. One output has a TI SN65DSI84 (LVDS) and the other output has a TI
> SN65DSI86 (eDP) attached to it. The Multiplexer status can be read back from a
> GPIO. The GPIO is also IRQ capable, so it would be possible to support hotplug
> additionally later on.

I have this requirement a year back [1] but my design has i.mx8mq DSI
outputs to SN65DSI84(LVDS) and ADV7533 (HDMI) and GPIO has muxed as
IRQ in order to find the bridge selection. (not confused with HDMI
HPD).

>
> My initial idea was to create a DRM multiplexer bridge driver which (depending
> on the input GPIO) allows only one output to be enabled. Unfortunately ti-
> sn65dsi86.c driver expects a DSI host on remote node 0 (see ti_sn_attach_host
> and ti_sn_bridge_parse_dsi_host), so it does not work. ti-sn65dsi83.c just
> requires a drm_bridge.

Yes, we need to have a finite amount of pipeline changes. assuming
that your mux bridge sits between DSI to Output interfaces the
proposed mux bridge selects which pipeline.

>
> What is the best approach for this? I currently see two approaches:
> * Create an explicit DSI/DRM multiplexer bridge driver which registers itself
> as DSI host
> * Create a DRM multiplexer bridge (only). But this needs to remove the DSI
> device registration from ti-sn65dsi86.c

Based on my experience, having a muxed bridge between in and out would
be proper in order to satisfy the pipeline as well as the design. That
mux bridge has to be a normal bridge doesn't aware of DSI or any other
interface like one of the submissions has done in the recent mailing
list. [2] Things would be complicated when we switch the outputs but
we still use normal static switching of outputs in a proper way.

>
> I am aware that DSI support is suboptimal, so I'm not sure which approach on
> the TI bridge drivers is the correct one and needs to be considered as given.
> Any ideas?

I did implement some complicated things of switching outputs at
runtime but the idea of the switching pipelines would be similar if
you want to implement it in a normal static way. Here are some details
and a demo of how those been worked. [3] [4]

[1] https://lore.kernel.org/all/CAMty3ZD7eFi4o7ZXNtjShoLd5yj3wn85Fm6ZNL89=QpWj44KPw@mail.gmail.com/T/
[2] https://patchwork.kernel.org/project/dri-devel/patch/20230218111712.2380225-6-treapking@chromium.org/
[3] https://indico.freedesktop.org/event/2/contributions/76/
[4] https://www.youtube.com/watch?v=PoYdP9fPn-4&t=624s

Thanks,
Jagan.