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

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Message-ID: <20180312123042.GA12967@w540>
Date:   Mon, 12 Mar 2018 13:30:42 +0100
From:   jacopo mondi <jacopo@...ndi.org>
To:     Andrzej Hajda <a.hajda@...sung.com>
Cc:     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,
        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

Hi Andrzej,

On Mon, Mar 12, 2018 at 10:07:27AM +0100, Andrzej Hajda wrote:
> 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.
>

I understand. The "transparent bridge" is of no actual use, but I don't see
how the "double bridge" thing is not an issue if I were to develop
support for the actual Thine chip.

Please see my reply from yesterday to Archit. I still think having two
bridges is somehow an issue...

While we clarify that, would it be fine an initial driver version for
the actualt Thine chip with a single input support[1]? I would ditch this
transparent bridge and resume working on a THC63LVD1024 driver from
comments received on v1.

Thanks
  j

[1] Single input support implies a single input port in DT bindings
even if the chip supports two, and my understanding was that you
didn't like this.


> 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
> >
> >
> >
> >
>

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