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

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Message-ID: <CACr-zFBFpqgHVRiH37ooeVJ4Jk1UA4AhP5J5L5yV8_CHBTG07g@mail.gmail.com>
Date: Wed, 26 Mar 2025 15:08:30 +0000
From: Christopher Obbard <christopher.obbard@...aro.org>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Douglas Anderson <dianders@...omium.org>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Jessica Zhang <quic_jesszhan@...cinc.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio <konradybcio@...nel.org>, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	linux-arm-msm@...r.kernel.org, Johan Hovold <johan@...nel.org>, 
	Rui Miguel Silva <rui.silva@...aro.org>, Abel Vesa <abel.vesa@...aro.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 4/4] drm/dp: fallback to maximum when PWM bit count is zero

Hi Dmitry,

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