linux-kernel - Re: [PATCH v3 6/6] drm/ingenic: Attach bridge chain to encoders

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Message-Id: <896D04E4-4058-474B-8BD2-7F21B1C754E4@goldelico.com>
Date:   Thu, 23 Sep 2021 11:19:23 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        linux-mips <linux-mips@...r.kernel.org>, list@...ndingux.net,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v3 6/6] drm/ingenic: Attach bridge chain to encoders

Hi Paul,

> Am 23.09.2021 um 10:49 schrieb Paul Cercueil <paul@...pouillou.net>:
> 
> Hi Nikolaus,
> 
> Le jeu., sept. 23 2021 at 07:52:08 +0200, H. Nikolaus Schaller <hns@...delico.com> a écrit :
>> Hi Paul,
>> thanks for another update.
>> We have been delayed to rework the CI20 HDMI code on top of your series
>> but it basically works in some situations. There is for example a problem
>> if the EDID reports DRM_COLOR_FORMAT_YCRCB422 but it appears to be outside
>> of your series.
> 
> I think the SoC can output YCbCr as well, but I never tried to use it.

Maybe there is code missing or something else. We have not yet deeply researched.
Except that when ignoring DRM_COLOR_FORMAT_YCRCB422 capability it uses RGB
and works.

> 
>>> +		ret = drm_bridge_attach(encoder, &ib->bridge, NULL,
>>> +					DRM_BRIDGE_ATTACH_NO_CONNECTOR);
>> DRM_BRIDGE_ATTACH_NO_CONNECTOR makes it fundamentally incompatible
>> with synopsys/dw_hdmi.c
>> That driver checks for DRM_BRIDGE_ATTACH_NO_CONNECTOR being NOT present,
>> since it wants to register its own connector through dw_hdmi_connector_create().
>> It does it for a reason: the dw-hdmi is a multi-function driver which does
>> HDMI and DDC/EDID stuff in a single driver (because I/O registers and power
>> management seem to be shared).
> 
> The IT66121 driver does all of that too, and does not need DRM_BRIDGE_ATTACH_NO_CONNECTOR. The drm_bridge_funcs struct has callbacks to handle cable detection and DDC stuff.
> 
>> Since I do not see who could split this into a separate bridge and a connector driver
>> and test it on multiple SoC platforms (there are at least 3 or 4), I think modifying
>> the fundamentals of the dw-hdmi architecture just to get CI20 HDMI working is not
>> our turf.
> 
> You could have a field in the dw-hdmi pdata structure, that would instruct the driver whether or not it should use the new API. Ugly, I know, and would probably duplicate a lot of code, but that would allow other drivers to be updated at a later date.

Yes, would be very ugly.

But generally who has the knowledge (and time) to do this work?
And has a working platform to test (jz4780 isn't a good development environment)?

The driver seems to have a turbulent history starting 2013 in staging/imx and
apparently it was generalized since then... Is Laurent currently dw-hdmi maintainer?

> 
>> Therefore the code here should be able to detect if drm_bridge_attach() already
>> creates and attaches a connector and then skip the code below.
> 
> Not that easy, unfortunately. On one side we have dw-hdmi which checks that DRM_BRIDGE_ATTACH_NO_CONNECTOR is not set, and on the other side we have other drivers like the IT66121 which will fail if this flag is not set.

Ok, I see. You have to handle contradicting cases here.

Would it be possible to run it with DRM_BRIDGE_ATTACH_NO_CONNECTOR first
and retry if it fails without?

But IMHO the return value (in error case) is not well defined. So there
must be a test if a connector has been created (I do not know how this
would work).

Another suggestion: can you check if there is a downstream connector defined in
device tree (dw-hdmi does not need such a definition)?
If not we call it with 0 and if there is one we call it with
DRM_BRIDGE_ATTACH_NO_CONNECTOR and create one?

Just some ideas how to solve without touching hdmi drivers.

BR and thanks,
Nikolaus