drm: Stable identification of connectors?

[Pekka Paalanen]

On Fri, 26 Jan 2024 14:18:36 +0000
Dave Stevenson <dave.stevenson at raspberrypi.com> wrote:

> 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.

Hi Dave,

how does the DT make sure that the alias is always assigned to the same
physical connector?

Is there some persistent, permanent, stable hardware path that the DT
is keying off of?

How about finding a consensus on exposing such hardware paths in the
first place so that there is no need to set aliases in DT. That could
also help systems that don't use DT in the first place.

I think Ville had a serious attempt in
https://lists.freedesktop.org/archives/dri-devel/2019-June/221902.html
but I guess people were not interested?


Thanks,
pq

> 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
> 
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