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

[Christopher Obbard]

Hi Dmitry,

On Tue, 25 Mar 2025 at 22:53, Dmitry Baryshkov
<dmitry.baryshkov@xxxxxxxxxxxxxxxx> wrote:
>
> 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.

Thanks for your comment; that's a good point.

I need to re-read this section of the spec; but at least on this
hardware I printed the values of the registers and it seems like
MIN and MAX are the same, so I could switch the patch around to use
MIN in the next version.

   drm_edp_backlight_probe_max: pn=0, pn_min=11, pn_max=11, bl_caps=134


diff --git a/drivers/gpu/drm/display/drm_dp_helper.c
b/drivers/gpu/drm/display/drm_dp_helper.c
index 6e519c58c2e84..2be2b00c8a531 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -4061,6 +4061,8 @@ drm_edp_backlight_probe_max(struct drm_dp_aux
*aux, struct drm_edp_backlight_inf
                        aux->name, ret);
        }

+       pr_info("%s: pn=%d, pn_min=%d, pn_max=%d, bl_caps=%d\n",
__func__, pn, pn_min, pn_max, bl_caps);
+
        /*
         * 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