linux-kernel - Re: [PATCH v1] ACPI: video: Fix missing native backlight on Chromebooks

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Message-ID: <92e9d424-0672-b37e-b8b3-cac431ade7f7@redhat.com>
Date:   Mon, 24 Oct 2022 15:46:16 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Akihiko Odaki <akihiko.odaki@...nix.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Sean Paul <seanpaul@...omium.org>
Cc:     kernel@...labora.com, linux-acpi@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] ACPI: video: Fix missing native backlight on
 Chromebooks

Hi,

On 10/24/22 15:32, Dmitry Osipenko wrote:
> Chromebooks don't have backlight in ACPI table, they suppose to use
> native backlight in this case. Check presence of the CrOS embedded
> controller ACPI device and prefer the native backlight if EC found.

Thank you for this patch!

> Suggested-by: Hans de Goede <hdegoede@...hat.com>
> Fixes: b1d36e73cc1c ("drm/i915: Don't register backlight when another backlight should be used (v2)")
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> ---
>  drivers/acpi/video_detect.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/acpi/video_detect.c b/drivers/acpi/video_detect.c
> index 0d9064a9804c..8ed5021de6fb 100644
> --- a/drivers/acpi/video_detect.c
> +++ b/drivers/acpi/video_detect.c
> @@ -668,6 +668,11 @@ static const struct dmi_system_id video_detect_dmi_table[] = {
>  	{ },
>  };
>  
> +static bool google_cros_ec_present(void)
> +{
> +	return acpi_dev_found("GOOG0004");
> +}
> +
>  /*
>   * Determine which type of backlight interface to use on this system,
>   * First check cmdline, then dmi quirks, then do autodetect.
> @@ -730,6 +735,9 @@ static enum acpi_backlight_type __acpi_video_get_backlight_type(bool native)
>  			return acpi_backlight_video;
>  	}
>  
> +	if (google_cros_ec_present())
> +		return acpi_backlight_native;
> +

Nice, a couple of remarks:

1. Maybe add a small comment explaining why, like all the other tests in the function have a small comment ?

2. I think it would be better to do:

	if (google_cros_ec_present() && native_available)
		return acpi_backlight_native;

I can e.g. imagine in the future some chromebooks where for some reason native
GPU backlight control is not available using the EC for backlight control
and then having the chrome-ec code register a backlight with "vendor" type ?

3. This will also trigger on the Framework laptops and possible other new
non Chromebook designs which choose to use the Chrome EC code for their EC,
I don't expect these devices to get to this point of __acpi_video_get_backlight_type()
(they will hit the earlier acpi_video / native paths) but still I want to
at least point this out in case someone sees a potential issue with this?

If you can address 1. and 2. from above (or explain why 2. is a bad idea)
then I believe that the next version of this can get merged to resolve
the chromebook backlight issues introduced in 6.1-rc1, thank you!

>  	/* No ACPI video (old hw), use vendor specific fw methods. */
>  	return acpi_backlight_vendor;
>  }

Regards,

Hans