acpi_video(4): mention that acpi_video should be loaded after any drm driver

[driesm]

Summary

When not adhering to this order, brightness sysctl's do not show up (tested on Lenovo T490). Thanks to Ulrich for pointing this out! Wouldn't have found it without him.

Test Plan

mandoc

[uqs]

While I had to do this as well to make it attach, I'm not sure if this is intentional or simply a bug that should be fixed (instead of documented).

Could you try roping in someone knowledgeable in acpi_video or graphics and ACPI in general to have a look? Is this also common for laptops other than Thinkpads for example?

[zeising]

Interestingly, I have acpi_video in my kernel, and I've never had any issues with this, even though acpi_video attaches long before the DRM kernel modules.

[driesm]

I tagged @manu and @avg as they seem to have touched acpi_video not so long ago.
acpi_video can attach before, but it just wont show brightness sysctls. (atleast on our laptops T490)
So in the end maybe a bug rather than intended behavior.

[manu]

Indeed it needs to be loaded after on some platform (Like my Lenovo X390).
Thanks.