drm: Stable identification of connectors?

[Dave Stevenson]

Hi Joerg

On Fri, 26 Jan 2024 at 10:45, Albert, Joerg (TT-U) <Joerg.Albert at iav.de>
wrote:

> Hi,
>
>
>
> I’m pretty new to DRM/DRI and wonder if there is a way to have a stable
> identification of connectors across changes in the Linux kernel and/or in
> the devicetree?
>
>
>
> Our hardware contains an iMX8QM with two displays, each one connected to a
> MIPI-DSI channel. We use kernel 6.1.38.
>
> In the output of “modetest -c” the connectors are called LVDS-1 and
> LVDS-2. These names are built in modetest.c from connector_type and
> connector_type_id.
>
>
>
> connector_type_id is set in the kernel in drivers/gpu/drm/drm_connector.c
> in __drm_connector_init():
>
>
>
>        *connector* <https://elixir.bootlin.com/linux/latest/C/ident/connector>->*connector_type_id* <https://elixir.bootlin.com/linux/latest/C/ident/connector_type_id> =
>
>               *ida_alloc_min* <https://elixir.bootlin.com/linux/latest/C/ident/ida_alloc_min>(*connector_ida* <https://elixir.bootlin.com/linux/latest/C/ident/connector_ida>, 1, *GFP_KERNEL* <https://elixir.bootlin.com/linux/latest/C/ident/GFP_KERNEL>);
>
>
>
> Seems to me that this number depends on initialization order only. Is
> there any other way to identify a connector?
>
>
>
> If not, will the type_id be stable as long as we don’t change the kernel
> version and the device tree?
>

My understanding is it all depends on initialisation order, and that isn't
guaranteed.

Raspberry Pi 5 has a similar issue that it has two independent DRM driver
instances each initialising a DSI connector. Which one gets assigned DSI-1
vs DSI-2 is down to probe order, and can change between boots.
You also have the situation that a panel connected to the second connector
is referred to as DSI-1 if nothing is connected to the first, but it'll
probably bump up to DSI-2 should you later configure a panel on the first
connector. Any configuration within a window manager based on display ID is
therefore near useless if there is any change to the system.

It's the same situation if you have more than one SPI display using the
mipi-dbi-spi or tinyDRM drivers - ordering of display to SPI-x IDs is down
to probe order, so near random.

I haven't attempted to upstream this yet, but on our tree [1] I've adopted
the same approach as the I2C and SPI subsystems take where a DT alias can
be used to set the desired connector ID. Devices with no alias get assigned
IDs above the last assigned alias.

Seeing as you've raised the same issue, it'd be interesting to know the
view of the maintainers as to whether my solution is acceptable. There's no
point in trying to upstream it if it's going to be immediately shot down.

Thanks.
  Dave

[1]
https://github.com/raspberrypi/linux/commit/3aa1f2477545ea6298bc6f2d7ffae68f090af9b8
and fixup
https://github.com/raspberrypi/linux/commit/f429fc1a072d4bb35e622a1012a5a52914eba4e3


>
> Best Regards,
>
> Joerg Albert
>
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