linux-kernel - Re: [PATCH v2 0/3] drm: Add LVDS decoder bridge

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Date:   Mon, 12 Mar 2018 10:07:27 +0100
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Jacopo Mondi <jacopo+renesas@...ndi.org>, architt@...eaurora.org,
        Laurent.pinchart@...asonboard.com, airlied@...ux.ie,
        horms@...ge.net.au, magnus.damm@...il.com, geert@...ux-m68k.org,
        niklas.soderlund@...natech.se, sergei.shtylyov@...entembedded.com,
        robh+dt@...nel.org, mark.rutland@....com
Cc:     dri-devel@...ts.freedesktop.org, linux-renesas-soc@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/3] drm: Add LVDS decoder bridge

On 09.03.2018 14:51, Jacopo Mondi wrote:
> Hello,
>    after some discussion on the proposed bindings for generic lvds decoder and
> Thine THC63LVD1024, I decided to drop the THC63 specific part and just live with
> a transparent decoder that does not support any configuration from DT.
>
> Dropping THC63 support to avoid discussion on how to better implement support
> for a DRM bridge with 2 input ports and focus on LVDS mode propagation through
> bridges as explained in v1 cover letter (for DRM people: please see [1] as why
> I find difficult to implement support for bridges with multiple input endpoints)
>
> Same base branch as v1, with same patches for V3M Eagle applied on top.
> git://jmondi.org/linux v3m/v4.16-rc3/base
>
> Thanks
>    j
>
> v1 -> v2:
> - Drop support for THC63LVD1024
>
> [1] I had a quick at how to model a DRM bridge with multiple input
> ports, and I see a blocker in how DRM identifies and matches bridges using
> the devices node in place of the endpoint nodes.
>
> As THC63LVD1024 supports up to 2 LVDS inputs and 2 LVDS outputs, I see only
> a few ways to support that:
>  1) register 2 drm bridges from the same driver (one for each input/output pair)
>     but they would both be matches on the same device node when the preceding
>     bridge calls "of_drm_find_bridge()".
>  2) register a single bridge with multiple "next bridges", but when the bridge
>     gets attached I don't see a way on how to identify on which next bridge
>     "drm_bridge_attach()" on, as it depends on the endpoint the current bridge
>     has been attached on first, and we don't have that information.
>  3) Register more instances of the same chip in DTS, one for each input/output
>     pair. They gonna share supplies and gpios, and I don't like that.
>
> I had a quick look at the currently in mainline bridges and none of them has
> multiple input endpoints, except for HDMI audio endpoint, which I haven't found
> in use in any DTS. I guess the problem has been already debated and maybe solved
> in the past, so feel free to point me to other sources.

I think this is is a step in wrong direction, IMHO. Your previous
patchset was quite OK, at least bindings, IMHO. Few things needed only
polishing.
Here we have unmanaged/transparent bridge, which is totally different,
what happened to regulators and gpios from previous iteration.
I do not have schematics of the board, but I guess respective pins of
the bridge must be connected somehow.
I think the problem you want to avoid (double bridge) should not be a
problem at all.
I suppose the most important is to have correct bindings - as they need
to be stable.
If you really must to do hacks better is to put them into driver.

Regards
Andrzej

>
> Jacopo Mondi (3):
>   dt-bindings: display: bridge: Document LVDS to parallel decoder
>   drm: bridge: Add LVDS decoder driver
>   arm64: dts: renesas: Add LVDS decoder to R-Car V3M Eagle
>
>  .../bindings/display/bridge/lvds-decoder.txt       |  42 ++++++
>  arch/arm64/boot/dts/renesas/r8a77970-eagle.dts     |  31 +++-
>  drivers/gpu/drm/bridge/Kconfig                     |   8 ++
>  drivers/gpu/drm/bridge/Makefile                    |   1 +
>  drivers/gpu/drm/bridge/lvds-decoder.c              | 157 +++++++++++++++++++++
>  5 files changed, 237 insertions(+), 2 deletions(-)
>  create mode 100644 Documentation/devicetree/bindings/display/bridge/lvds-decoder.txt
>  create mode 100644 drivers/gpu/drm/bridge/lvds-decoder.c
>
> --
> 2.7.4
>
>
>
>