Re: [RFC PATCH 0/4] drm: bridge: anx7688 and mux drivers

[Archit Taneja]

On 06/27/2016 01:18 PM, Nicolas Boichat wrote:
> Hi all,
>
> This is a follow up to the 2 patches to add support for ANX7688 sent here:
> https://patchwork.kernel.org/patch/9187809/, thanks Archit and Philipp for
> the comments.
>
> I also added 2 patches to add support for a simple display MUX, as I'm facing
> similar issues while trying to implement it, i.e. the current DRM core does not
> seem to support this kind of simple pass-thru bridge very well: it is not very
> clear where connectors should be defined and attached. In this case, not
> defining any connectors in the 2 bridges (and reusing the connector in MTK
> HDMI driver) seem to work best, but makes little logical sense as the physical
> connectors are actually attached to the bridges.

Bridges aren't really drm objects in themselves, they can just be
thought of as entities that attach to an encoder. From a drm
perspective, the connector is only linked to an encoder. It
doesn't see any bridges. Therefore, it doesn't matter much
if the bridge driver doesn't create connectors. The DT bindings,
however, should be close to the physical connections.

> In any case, the board has the following layout:
>   - MT8173 HDMI bridge
>     - HDMI mux with 2 ports
>       1. ANX7688 for HDMI->DP over USB-C conversion
>       2. Native HDMI

So, the MTK SoC's HDMI output (TMDS lines) can be routed to the
connector on the board directly (native mode), or via the ANX7688
bridge using the gpio mux. Did I get this part right?

Is there only one connector at the end of both the output paths?


> The mux is controlled by hardware, looking at the HPD signals from both ANX7688
> and native HDMI, with a priority on the native HDMI output.

I didn't understand this. I can see that ANX7688 could generate a HPD
signal on behalf of the connected monitor, but why would the native MTK
HDMI controller generate a HPD signal? I would expect it to receive HPD
and trigger a CPU interrupt.

Could you also give an idea about why the hardware switches between the
two paths? It it based on what kind of device plugs into the connector?

Thanks,
Archit

> The whole setup works fairly well without any Linux kernel drivers, except the
> 2 following cases:
>   1. When ANX7688 is active, DP bandwidth may be limited, so we need to filter
>      resolutions that would exceed the available bandwidth.
>   2. When both outputs HPD signals are active, the kernel does not receive an
>      HPD pulse when the HDMI input is unplugged.
>
> ANX7688 driver fixes issue 1. The mux driver fixes 2 by forcing the kernel to
> re-read the EDID on mux output change, and also issue 1 by filtering only when
> ANX7688 is active.
>
> I understand this patch series might not be acceptable as-is, but I hope this
> sort of setup can be taken into account when better support for connector
> drivers is introduced.
>
> Thanks!
>
> Best,
>
> Nicolas
>
> Nicolas Boichat (4):
>    drm: bridge: anx7688: Add anx7688 bridge driver support.
>    devicetree: Add ANX7688 transmitter binding
>    drm: bridge: Generic GPIO mux driver
>    devicetree: Add GPIO display mux binding
>
>   .../devicetree/bindings/drm/bridge/anx7688.txt     |  32 ++
>   .../devicetree/bindings/drm/bridge/gpio-mux.txt    |  59 ++++
>   drivers/gpu/drm/bridge/Kconfig                     |  20 ++
>   drivers/gpu/drm/bridge/Makefile                    |   2 +
>   drivers/gpu/drm/bridge/analogix-anx7688.c          | 233 ++++++++++++++
>   drivers/gpu/drm/bridge/generic-gpio-mux.c          | 347 +++++++++++++++++++++
>   6 files changed, 693 insertions(+)
>   create mode 100644 Documentation/devicetree/bindings/drm/bridge/anx7688.txt
>   create mode 100644 Documentation/devicetree/bindings/drm/bridge/gpio-mux.txt
>   create mode 100644 drivers/gpu/drm/bridge/analogix-anx7688.c
>   create mode 100644 drivers/gpu/drm/bridge/generic-gpio-mux.c

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