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Date:   Thu, 23 Sep 2021 11:55:56 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Paul Cercueil <paul@...pouillou.net>,
        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>,
        Paul Boddie <paul@...die.org.uk>
Subject: Re: [PATCH v3 6/6] drm/ingenic: Attach bridge chain to encoders

Hi Laurent,

> Am 23.09.2021 um 11:27 schrieb Laurent Pinchart <laurent.pinchart@...asonboard.com>:
> 
> Hi Nikolaus,
> 
> On Thu, Sep 23, 2021 at 11:19:23AM +0200, H. Nikolaus Schaller wrote:
>> 
>>>>> +		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?
> 
> "Maintainer" would be an overstatement. I've worked on that driver in
> the past, and I still use it, but don't have time to really maintain it.
> I've also been told that Synopsys required all patches for that driver
> developed using documentation under NDA to be submitted internally to
> them first before being published, so I decided to stop contributing
> instead of agreeing with this insane process. There's public
> documentation about the IP in some NXP reference manuals though, so it
> should be possible to still move forward without abiding by this rule.
> 
>>>> 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?
> 
> I haven't followed the ful conversation, what the reason why
> DRM_BRIDGE_ATTACH_NO_CONNECTOR can't always be use here ?

The synopsys driver creates its own connector through dw_hdmi_connector_create()
because the IP handles DDC/EDID directly.

Hence it checks for ! DRM_BRIDGE_ATTACH_NO_CONNECTOR which seems to be the
right thing to do on current platforms that use it.

For CI20/jz4780 we just add a specialisation of the generic dw-hdmi to
make HDMI work.

Now this patch for drm/ingenic wants the opposite definition and create its own
connector. This fails even if we remove the check (then we have two interfering
connectors).

> We're moving
> towards requiring DRM_BRIDGE_ATTACH_NO_CONNECTOR for all new code, so it
> will have to be done eventually.

So from my view drm/ingenic wants to already enforce this rule and breaks dw-hdmi.

IMHO it should either handle this situation gracefully or include a fix for
dw-hdmi.c to keep it compatible.

BR and thanks,
Nikolaus

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