Re: [PATCH v2 4/4] drm/dp: fallback to maximum when PWM bit count is zero

[Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>]

On 25/03/2025 21:21, Christopher Obbard wrote:
> Some eDP devices report DP_EDP_PWMGEN_BIT_COUNT as 0, but still provide
> valid non-zero MIN and MAX values. This patch reworks the logic to
> fallback to the max value in such cases, ensuring correct backlight PWM
> configuration even when the bit count value is not explicitly set.

I don't think this matches the eDP standard. It tells to use MIN if 
BIT_COUNT is less than MIN, if I understand it correctly.

> 
> This improves compatibility with eDP panels (e.g. Samsung ATNA40YK20
> used on the Lenovo T14s Gen6 Snapdragon with OLED panel) which reports
> DP_EDP_PWMGEN_BIT_COUNT as 0 but still provides valid non-zero MIN/MAX
> values.
> 
> Co-developed-by: Rui Miguel Silva <rui.silva@linaro.org>
> Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
> Signed-off-by: Christopher Obbard <christopher.obbard@linaro.org>
> ---
>   drivers/gpu/drm/display/drm_dp_helper.c | 51 ++++++++++++++++++++++-----------
>   1 file changed, 34 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> index da3c8521a7fa7d3c9761377363cdd4b44ab1106e..734b7b8e46394de21837cda6ca1b189413b25cd8 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -3964,7 +3964,7 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
>   {
>   	int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
>   	int ret;
> -	u8 pn, pn_min, pn_max;
> +	u8 pn, pn_min, pn_max, bl_caps;
>   
>   	if (!bl->aux_set)
>   		return 0;
> @@ -3975,8 +3975,40 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
>   			    aux->name, ret);
>   		return -ENODEV;
>   	}
> -
>   	pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> +
> +	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
> +	if (ret != 1) {
> +		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
> +			    aux->name, ret);
> +		return 0;
> +	}
> +	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> +
> +	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
> +	if (ret != 1) {
> +		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
> +			    aux->name, ret);
> +		return 0;
> +	}
> +	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> +
> +	ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_ADJUSTMENT_CAP, &bl_caps);
> +	if (ret != 1) {
> +		bl_caps = 0;
> +		drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight adjustment cap: %d\n",
> +			aux->name, ret);
> +	}
> +
> +	/*
> +	 * Some eDP panels report brightness byte count support, but the byte count
> +	 * reading is 0 (e.g. Samsung ATNA40YK20) so in these cases use pn_max
> +	 * for pn.
> +	 */
> +	if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
> +	    && pn_max)
> +		pn = pn_max;
> +
>   	bl->max = (1 << pn) - 1;
>   	if (!driver_pwm_freq_hz)
>   		return 0;
> @@ -4003,21 +4035,6 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
>   	 * - FxP is within 25% of desired value.
>   	 *   Note: 25% is arbitrary value and may need some tweak.
>   	 */
> -	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
> -	if (ret != 1) {
> -		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
> -			    aux->name, ret);
> -		return 0;
> -	}
> -	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
> -	if (ret != 1) {
> -		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
> -			    aux->name, ret);
> -		return 0;
> -	}
> -	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> -	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> -
>   	/* Ensure frequency is within 25% of desired value */
>   	fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4);
>   	fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);
> 


-- 
With best wishes
Dmitry