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

From: Paul Cercueil
Date: Thu Sep 23 2021 - 09:30:40 EST

Hi Nikolaus,

Le jeu., sept. 23 2021 at 13:41:28 +0200, H. Nikolaus Schaller <hns@xxxxxxxxxxxxx> a écrit :
Hi Laurent,

Am 23.09.2021 um 12:03 schrieb Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>:

Hi Nikolaus,

On Thu, Sep 23, 2021 at 11:55:56AM +0200, H. Nikolaus Schaller wrote:
Am 23.09.2021 um 11:27 schrieb Laurent Pinchart:
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.

That doesn't require creating a connector though. The driver implements
drm_bridge_funcs.get_edid(), which is used to get the EDID without the
need to create a connector in the dw-hdmi driver.

Ah, ok.

But then we still have issues.

Firstly I would assume that get_edid only works properly if it is initialized
through dw_hdmi_connector_create().

Next, in the current code, passing DRM_BRIDGE_ATTACH_NO_CONNECTOR to
dw_hdmi_bridge_attach() indeed does not call dw_hdmi_connector_create()
but returns 0.

This patch 6/6 makes drm/ingenic unconditionally require a connector
to be attached which is defined somewhere else (device tree e.g. "connector-hdmi")
unrelated to dw-hdmi. Current upstream code for drm/ingenic does not init/attach
such a connector on its own so it did work before.

I.e. I think we can't just use parts of dw-hdmi.

The fact that Laurent is using dw-hdmi with DRM_BRIDGE_ATTACH_NO_CONNECTOR on Renesas makes me think that it's possible here as well. There's no reason why it shouldn't work with ingenic-drm.

The ingenic-drm driver does not need to create any connector. The "connector-hdmi" is connected to dw-hdmi as the "next bridge" in the list.

If drm_bridge_attach() would return some errno if DRM_BRIDGE_ATTACH_NO_CONNECTOR
is set, initialization in ingenic_drm_bind() would fail likewise with "Unable to attach bridge".

So in any case dw-hdmi is broken by this drm/ingenic patch unless someone
reworks it to make it compatible.

Where would the errno be returned? Why would drm_bridge_attach() return an error code?

Another issue is that dw_hdmi_connector_create() does not only do dcd/edid
but appears to detects hot plug and does some special initialization.
So we probably loose hotplug detect if we just use drm_bridge_funcs.get_edid().

There's drm_bridge_funcs.detect().

Cheers,
-Paul

I come to the conclusion that not creating a specific connector in dw-hdmi
and relying on a generic connector does not seem to be an option with current
code proposals.

In such a situation the question is what the least invasive surgery is to
avoid complications and lenghty regression tests on unknown platforms.
IMHO it is leaving (mature) dw-hdmi untouched and make attachment of a connector
in ingenic_drm_bind() depend on some condition.

BR and thanks,
Nikolaus