RFC: DSI/DRM multiplexer bridge

[Alexander Stein]

Hi Jagan,

thanks for your reply.

Am Mittwoch, 5. April 2023, 16:39:07 CEST schrieb 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).

Looks quite similar. This platform can be used using imx8mq, imx8mm or im8xmn. 
That mentioned GPIO is also not the HDMI HPD, but connected to a DIP switch on 
the mainboard to be changed manually.

> > 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.

My setup looks like this, but the switch is a different one than yours.

                              | => SN54DSI86 => DP Connector
DSI Host => display-switch => |
                              | => SN65DSI83 => Panel-Simple

> > 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]

Thanks for the links. From what I read in those discussions dynamically 
(re)building the bridge chains it not correct/acceptable. Instead two bridges 
shall be created, but only one connector at the end shall be enabled. This 
would look like this:

   switched-bridge
    +------------+                 +-------------+
    | drm_bridge-|- next_bridge -- | LVDS bridge |- connector
    |            |                 +-------------+
in -|            |
    |            |                 +-------------+
    | drm_bridge-|- next_bridge -- | eDP bridge  |- connector
    +------------+                 +-------------+
          ^
          |
      DIP switch

But here I'm not so sure how an (hotplug) event (e.g. IRQ) on the switched-
bridge can be forwarded to the connectors. But this hotplug event seems to be 
essential so that DRM master can reconfigure it's output.

Best regards,
Alexander

> [1]
> https://lore.kernel.org/all/CAMty3ZD7eFi4o7ZXNtjShoLd5yj3wn85Fm6ZNL89=QpWj4
> 4KPw at mail.gmail.com/T/ [2]
> https://patchwork.kernel.org/project/dri-devel/patch/20230218111712.2380225
> -6-treapking at chromium.org/ [3]
> https://indico.freedesktop.org/event/2/contributions/76/
> [4] https://www.youtube.com/watch?v=PoYdP9fPn-4&t=624s
> 
> Thanks,
> Jagan.


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