[PATCH v5 02/31] drm/i915: Don't register backlight when another backlight should be used (v2)

[Limonciello, Mario]

On 10/25/2022 15:25, Hans de Goede wrote:
> Hi Matthew,
> 
> On 10/25/22 21:32, Matthew Garrett wrote:
>> On Tue, Oct 25, 2022 at 08:50:54PM +0200, Hans de Goede wrote:
>>
>>> That is a valid point, but keep in mind that this is only used on ACPI
>>> platforms and then only on devices with a builtin LCD panel and then
>>> only by GPU drivers which actually call acpi_video_get_backlight_type(),
>>> so e.g. not by all the ARM specific display drivers.
>>>
>>> So I believe that Chromebooks quite likely are the only devices with
>>> this issue.
>>
>> My laptop is, uh, weird, but it falls into this category.
>>   
>>>> I think for this to work correctly you need to have
>>>> the infrastructure be aware of whether or not a vendor interface exists,
>>>> which means having to handle cleanup if a vendor-specific module gets
>>>> loaded later.
>>>
>>> Getting rid of the whole ping-ponging of which backlight drivers
>>> get loaded during boot was actually one of the goals of the rework
>>> which landed in 6.1 this actually allowed us to remove some quirks
>>> because some hw/firmware did not like us changing our mind and
>>> switching backlight interfaces after first poking another one.
>>> So we definitely don't want to go back to the ping-pong thing.
>>
>> Defaulting to native but then having a vendor driver be able to disable
>> native drivers seems easiest? It shouldn't be a regression over the
>> previous state of affairs since both drivers were being loaded already.
> 
> Part of the reason for the ACPI backlight detect refactor is
> because of a planned new backlight uAPI where the brightness
> control becomes a property on the drm connector object, for a
> RFC including the rationale behind this planned uAPI change see:
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2Fb61d3eeb-6213-afac-2e70-7b9791c86d2e%40redhat.com%2F&data=05%7C01%7Cmario.limonciello%40amd.com%7C6380e44c87e447eedc3f08dab6c7180a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638023263541559113%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=K4oMmVl51kT9V%2B4GAdx%2FS7tXWHPKyFe5WXZzC3CPeOU%3D&reserved=0
> 
> These plans require that there is only 1 backlight device
> registered (per panel).
> 
> Having the native driver come and then go and be replaced
> with the vendor driver would also be quite inconvenient
> for these planned changes.
> 
> As such I would rather find a solution for your "weird"
> laptop so that acpi_video_get_backlight_type() just always
> returns vendor there.

What exactly is this "weird" laptop?  Is it something running coreboot 
that doesn't "normally" ship with coreboot perhaps?

If that's the category it falls in, I think what we really need is 
something to land in coreboot source for indicating how it should behave 
and then also a change in the kernel to react to that.

That's a similar approach to what was used fore the chromebook laptops 
that run coreboot.

> 
> Note that drivers/acpi/video_detect.c already has a DMI
> quirk tables for models where the heuristics from
> acpi_video_get_backlight_type() don't work. In general
> we (mostly me) try to make it so that the heuristics
> work on most models, to avoid needing to add every model
> under the sun to the DMI quirk table, but if your laptop
> is somehow special then adding a DMI quirk for it should
> be fine ?
> 
> Can you perhaps explain a bit in what way your laptop
> is weird ?
> 
> Note that technically if the native backlight does not work,
> then the GPU driver really should not even try to register
> it. But sometimes the video-bios-tables claim the backlight
> pwm input is attached to the GPU while it is not and things
> have evolved in such a way that the DMI quirks for that
> live in acpi/video_detect.c rather then in the GPU driver.
> 
> Regards,
> 
> Hans
>