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

[Dmitry Baryshkov]

On 27/03/2025 19:25, Christopher Obbard wrote:
> According to the eDP specification (e.g., VESA eDP 1.4b, section 3.3.10.2),
> if DP_EDP_PWMGEN_BIT_COUNT is less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN,
> the sink is required to use the MIN value as the effective bit count.
>
> Some eDP panels report DP_EDP_PWMGEN_BIT_COUNT as 0 while still providing
> valid non-zero MIN and MAX capability values. This patch updates the logic
> to use the CAP_MIN value in such cases, ensuring correct scaling of AUX-set
> backlight brightness values.
>
> This improves compatibility with panels like the Samsung ATNA40YK20 used
> on the Lenovo T14s Gen6 (Snapdragon variant with OLED) which report a
> bit count of 0 but declares an 11-bit PWM capability range.
>
> Co-developed-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
> Signed-off-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
> Signed-off-by: Christopher Obbard <christopher.obbard@xxxxxxxxxx>
> ---
> Changes in v2:
> - Split backlight brightness patch from T14s OLED enablement series.
> - Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
> - Rework commit message to reference eDP spec.
> - Rebase on drm-misc-next.
> - Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@xxxxxxxxxx/
> ---
>   drivers/gpu/drm/display/drm_dp_helper.c | 50 ++++++++++++++++++++++-----------
>   1 file changed, 33 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> index dbce1c3f49691fc687fee2404b723c73d533f23d..0b843d5b634f89f144b62b30311834d118b79ba9 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -4083,7 +4083,7 @@ 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;
> @@ -4094,8 +4094,39 @@ 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 use pn_min instead.
> +	 */
> +	if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
> +	    && pn_min)
> +		pn = pn_min;

I wonder, what stops you from implementing this part according to the 
standard, rather than adding a hack for 0 value.

> +
>   	bl->max = (1 << pn) - 1;
>   	if (!driver_pwm_freq_hz)
>   		return 0;

--
With best wishes
Dmitry