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

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Date:   Mon, 24 Oct 2022 17:07:17 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Hans de Goede <hdegoede@...hat.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

On 10/24/22 16:46, Hans de Goede wrote:
> 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!

I don't have anything against 2., it also works.

Thank you for the review!

-- 
Best regards,
Dmitry