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

[Wentland, Harry]

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

> 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;
>