On Wed, Oct 26, 2022 at 01:27:25AM +0200, Hans de Goede wrote:
> this code should actually set the ACPI_VIDEO_BACKLIGHT flag:
> drivers/acpi/scan.c:
>
> static acpi_status
> acpi_backlight_cap_match(acpi_handle handle, u32 level, void *context,
> void **return_value)
> {
> long *cap = context;
>
> if (acpi_has_method(handle, "_BCM") &&
> acpi_has_method(handle, "_BCL")) {
> acpi_handle_debug(handle, "Found generic backlight support\n");
> *cap |= ACPI_VIDEO_BACKLIGHT;
> /* We have backlight support, no need to scan further */
> return AE_CTRL_TERMINATE;
> }
> return 0;
> }
Ah, yeah, my local tree no longer matches the upstream behaviour because
I've hacked the EC firmware to remove the backlight trigger because it
had an extremely poor brightness curve and also automatically changed it
on AC events - as a result I removed the backlight code from the DSDT
and just fell back to the native control. Like I said I'm a long way
from the normal setup, but this did previously work.
The "right" logic here seems pretty simple: if ACPI backlight control is
expected to work, use it. If it isn't, but there's a vendor interface,
use it. If there's no vendor interface, use the native interface. The
problem you're dealing with is that the knowledge of whether or not
there's a vendor interface isn't something the core kernel code knows
about. What you're proposing here is effectively for us to expose
additional information about whether or not there's a vendor interface
in the system firmware, but since we're talking in some cases about
hardware that's almost 20 years old, we're not realistically going to
get those old machines fixed. So, it feels like there's two choices:
1) Make a default policy decision, but then allow that decision to be
altered later on (eg, when a vendor-specific platform driver has been
loaded) - you've said this poses additional complexities.
2) Move the knowledge of whether or not there's a vendor interface into
the core code. Basically take every platform driver that exposes a
vendor interface, and move the detection code into the core.
I think any other approach is going to result in machines that
previously worked no longer working (and you can't just make the
vendor/native split dependent on the Coreboot DMI BIOS string, because
there are some Coreboot platforms that implement the vendor interface
for compatibility, and you also can't ask all Coreboot users to update
their firmware to fix things)
