linux-kernel - Re: [PATCH] drm/bridge: sii902x: add support for DRM_BRIDGE_ATTACH_NO

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Message-ID: <d115ad6e-1c23-e7d9-dc9f-a12903782850@baylibre.com>
Date:   Mon, 29 Aug 2022 15:36:24 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Dmitry Osipenko <digetx@...il.com>, andrzej.hajda@...el.com,
        robert.foss@...aro.org, Rob Herring <robh+dt@...nel.org>,
        jernej.skrabec@...il.com, jonas@...boo.se,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Laurent.pinchart@...asonboard.com,
        Sudeep Holla <Sudeep.Holla@....com>
Subject: Re: [PATCH] drm/bridge: sii902x: add support for
 DRM_BRIDGE_ATTACH_NO_CONNECTOR

On 25/08/2022 14:48, Linus Walleij wrote:
> On Wed, Aug 17, 2022 at 3:31 PM Neil Armstrong <narmstrong@...libre.com> wrote:
>> On 15/08/2022 02:15, Dmitry Osipenko wrote:
>>> 08.08.2022 12:51, Neil Armstrong пишет:
>>>> On 08/08/2022 11:15, Neil Armstrong wrote:
> 
>>>>>> This patch broke ARM/QEMU vexpress display because of_drm_find_bridge()
>>>>>> always fail with -EPROBE_DEFER. Reverting this patch returns display
>>>>>> back. Please fix or revert, thanks in advance.
>>>>>
>>>>> Can you share a QEMU cmdline to reproduce ?
>>>>
>>>> Actually the vexpress DT has multiple input ports instead of a single
>>>> input port at @0
>>>> and an output port at @1 like documented in the bindings:
>>>>
>>>> vexpress-v2m.dtsi#L303-L307:
>>>> ports {
>>>>       #address-cells = <1>;
>>>>       #size-cells = <0>;
>>>>
>>>>       /*
>>>>        * Both the core tile and the motherboard routes their output
>>>>        * pads to this transmitter. The motherboard system controller
>>>>        * can select one of them as input using a mux register in
>>>>        * "arm,vexpress-muxfpga". The Vexpress with the CA9 core tile is
>>>>        * the only platform with this specific set-up.
>>>>        */
>>>>       port@0 {
>>>>           reg = <0>;
>>>>           dvi_bridge_in_ct: endpoint {
>>>>               remote-endpoint = <&clcd_pads_ct>;
>>>>           };
>>>>       };
>>>>       port@1 {
>>>>           reg = <1>;
>>>>           dvi_bridge_in_mb: endpoint {
>>>>               remote-endpoint = <&clcd_pads_mb>;
>>>>           };
>>>>       };
>>>> };
>>>>
>>>> bindings:
>>>>     ports:
>>>>       $ref: /schemas/graph.yaml#/properties/ports
>>>>
>>>>       properties:
>>>>         port@0:
>>>>           $ref: /schemas/graph.yaml#/properties/port
>>>>           description: Parallel RGB input port
>>>>
>>>>         port@1:
>>>>           $ref: /schemas/graph.yaml#/properties/port
>>>>           description: HDMI output port
>>>>
>>>>         port@3:
>>>>           $ref: /schemas/graph.yaml#/properties/port
>>>>           description: Sound input port
>>>>
>>>> The patch is conform to the bindings, the DT was working but is actually
>>>> not valid.
>>>
>>> I haven't looked closely at how to fix this properly, but if we can fix
>>> it using of_machine_is_compatible("arm,vexpress") workaround in the
>>> driver, then it will be good enough at least as a temporal fix, IMO.
>>
>> If other maintainers are ok with that, it can be temporary fix until the DT gets fixed.
> 
> That's fine with me, will you send a patch?

Who, me ?

> 
> I don't know how you expect the DT to get "fixed" though.
> 
> The hardware looks like this, it's maybe not the most elegant
> electronics design but it exists, so... I wrote this DT with two
> inputs, see commit f1fe12c8bf332, the code handling this
> awkward mux is part of the DRM driver, see
> drivers/gpu/drm/pl111/pl111_versatile.c function
> pl111_vexpress_clcd_init() for an idea of how it works.

The proper fix would be the other way around, adding a mux bridge before the sii902x
returning the next bridge or nothing to the right controller.

> 
> Yours,
> Linus Walleij

Neil