Re: [PATCH] drm/amd/display: Set AMDGPU_DM_DEFAULT_MIN_BACKLIGHT to 0

From: Harry Wentland <harry.wentland@amd.com>
To: Alex Deucher <alexdeucher@gmail.com>, Evan Benn <evanbenn@chromium.org>
Cc: "Stylon Wang" <stylon.wang@amd.com>,
	"Eryk Brol" <eryk.brol@amd.com>,
	"David Airlie" <airlied@linux.ie>,
	"Rodrigo Siqueira" <Rodrigo.Siqueira@amd.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"amd-gfx list" <amd-gfx@lists.freedesktop.org>,
	"Christian König" <christian.koenig@amd.com>,
	"Leo Li" <sunpeng.li@amd.com>,
	"Aurabindo Pillai" <aurabindo.pillai@amd.com>,
	"Maling list - DRI developers" <dri-devel@lists.freedesktop.org>,
	"Alex Deucher" <alexander.deucher@amd.com>,
	"Bhawanpreet Lakha" <Bhawanpreet.Lakha@amd.com>,
	"Nicholas Kazlauskas" <nicholas.kazlauskas@amd.com>,
	Anand <amistry@chromium.org>
Subject: Re: [PATCH] drm/amd/display: Set AMDGPU_DM_DEFAULT_MIN_BACKLIGHT to 0
Date: Fri, 19 Mar 2021 11:07:32 -0400	[thread overview]
Message-ID: <54fc883a-c149-3f43-fb79-3cbff13e7b6a@amd.com> (raw)
In-Reply-To: <CADnq5_OguuMsqT7MVC=ieNZm9mqyVUsGpQDHr59BWtBJJUvFoA@mail.gmail.com>

On 2021-03-19 10:22 a.m., Alex Deucher wrote:
> On Fri, Mar 19, 2021 at 3:23 AM Evan Benn <evanbenn@chromium.org> wrote:
>>
>> AMDGPU_DM_DEFAULT_MIN_BACKLIGHT was set to the value of 12
>> to ensure no display backlight will flicker at low user brightness
>> settings. However this value is quite bright, so for devices that do not
>> implement the ACPI ATIF
>> ATIF_FUNCTION_QUERY_BRIGHTNESS_TRANSFER_CHARACTERISTICS
>> functionality the user cannot set the brightness to a low level even if
>> the display would support such a low PWM.
>>
>> This ATIF feature is not implemented on for example AMD grunt chromebooks.
>>
>> Signed-off-by: Evan Benn <evanbenn@chromium.org>
>>
>> ---
>> I could not find a justification for the reason for the value. It has
>> caused some noticable regression for users: https://bugzilla.kernel.org/show_bug.cgi?id=203439>>>
>> Maybe this can be either user controlled or userspace configured, but
>> preventing users from turning their backlight dim seems wrong.
> 
> My understanding is that some panels flicker if you set the min to a
> value too low.  This was a safe minimum if the platform didn't specify
> it's own safe minimum.  I think we'd just be trading one bug for
> another (flickering vs not dim enough).  Maybe a whitelist or
> blacklist would be a better solution?
> 

Yeah, this is a NACK from me as-is for the reasons Alex described.

I agree a whitelist approach might be best.

Is this fix perhaps for OLED panels? If so we could use a different 
min-value for OLED panels that don't do PWM, but use 12 for everything else.

Harry

> Alex
> 
> 
>>
>> Also reviewed here: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2748377>>>
>>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> index 573cf17262da..0129bd69b94e 100644
>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> @@ -3151,7 +3151,7 @@ static int amdgpu_dm_mode_config_init(struct amdgpu_device *adev)
>>          return 0;
>>   }
>>
>> -#define AMDGPU_DM_DEFAULT_MIN_BACKLIGHT 12
>> +#define AMDGPU_DM_DEFAULT_MIN_BACKLIGHT 0
>>   #define AMDGPU_DM_DEFAULT_MAX_BACKLIGHT 255
>>   #define AUX_BL_DEFAULT_TRANSITION_TIME_MS 50
>>
>> --
>> 2.31.0.291.g576ba9dcdaf-goog
>>
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