[PATCH] drm/amd/display: set backlight level limit to 1

[Guttula]

On 10/26/2018 12:04 AM, Wentland, Harry wrote:
> On 2018-10-25 2:58 a.m., Guttula, Suresh wrote:
>> This patch will work as workaround for silicon limitation
>> related to PWM dutycycle when the backlight level goes to 0.
>>
>> Actually PWM value is 16 bit value and valid range from 1-65535.
>> when ever user requested to set this PWM value to 0 which is not
>> fall in the range, in VBIOS taken care this by limiting to 1.
>> This patch here will do the same. Either driver or VBIOS can not
>> pass 0 value as it is not a valid range for PWM and it will
>> give a high PWM pulse which is not the intented behaviour as
>> per HW constraints.
>>
> 
> 
> Comments from Windows dev, Anthony, CCed here as well:
>> In Windows, we typically never set backlight PWM value to 0.
>> Windows brightness slider is from 0 - 100% brightness.
>> 0% brightness maps to some non-zero value. So user never gets the chance to set value to 0.
>>
>> I'm not 100% sure what kind of flashing they are seeing.
>> I thought 0 PWM works, and just means PWM signal always low, I think it would equate to panel backlight completely being off.
>> I do know of a flashing issue caused by fractional PWM at low PWM values, but this would not even be at 0. The flashing problem would be anywhere from 1 - 300 for example. (out of 65535)
> 
> Do you see flickering when value is at 0 or in the 1 > 0 transition? 1 would be 65535/256 = 256 and put us right in the 1-300 range that has a flashing problem.
> 
> Harry
> 

We can not make out if flicker is at transition from 1 to 0 or exactly 
at 0, because the flickering is very instantaneous.But, I am not able to 
see the issue @1 , only seen the issue after going down from 1.

Thanks,
Suresh G

>> Signed-off-by: suresh guttula <suresh.guttula at amd.com>
>> ---
>>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> 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 492230c..38f84b2 100644
>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> @@ -1518,6 +1518,14 @@ static int amdgpu_dm_backlight_update_status(struct backlight_device *bd)
>>   {
>>   	struct amdgpu_display_manager *dm = bl_get_data(bd);
>>   
>> +	/*
>> +	 * When we use brightness low key to reduce the brightness,
>> +	 * brightness level reaching to 0, with which we can see flash
>> +	 * screen on ui beacuse of HW limitation.To avoid that  we are
>> +	 * limiting level to 1
>> +	 */
>> +	if (bd->props.brightness < 1)
>> +		return 1;
>>   	if (dc_link_set_backlight_level(dm->backlight_link,
>>   			bd->props.brightness, 0, 0))
>>   		return 0;
>>