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@xxxxxxx>
>> ---
>> 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;
>>
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