linux-kernel - [PATCH 1/6] drm/dp: Don't zero PWMGEN_BIT_COUNT when driver_pwm_freq

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Message-Id: <20210728094511.1.If017efce7116ae8ba015ed7def840c390a0e0c77@changeid>
Date:   Wed, 28 Jul 2021 09:45:52 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     dri-devel@...ts.freedesktop.org
Cc:     tzimmermann@...e.de, linux-arm-msm@...r.kernel.org,
        a.hajda@...sung.com, sam@...nborg.org, airlied@...ux.ie,
        bjorn.andersson@...aro.org, daniel@...ll.ch,
        devicetree@...r.kernel.org, rodrigo.vivi@...el.com,
        jonas@...boo.se, narmstrong@...libre.com,
        Laurent.pinchart@...asonboard.com, robert.foss@...aro.org,
        jernej.skrabec@...il.com, mripard@...nel.org,
        linus.walleij@...aro.org, maarten.lankhorst@...ux.intel.com,
        rajeevny@...eaurora.org, lyude@...hat.com,
        thierry.reding@...il.com, steev@...i.org,
        Douglas Anderson <dianders@...omium.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/6] drm/dp: Don't zero PWMGEN_BIT_COUNT when driver_pwm_freq_hz not specified

The function drm_edp_backlight_init() is defined such that the
"driver_pwm_freq_hz" parameter is optional--it's 0 if you don't want
to futz with it. If you follow this variable through, you'll find out
that if it's 0 we won't ever set the "bl->pwmgen_bit_count", leaving
it as 0.

That means that before using it in drm_edp_backlight_enable() we need
to check to see if it's non-zero.

Programming this field to zero was confusing the panel I tested with
(Samsung ATNA33XC20). The backlight still worked somewhat but the
brightness values didn't correspond to what they should have been.

Fixes: 867cf9cd73c3 ("drm/dp: Extract i915's eDP backlight code into DRM helpers")
Signed-off-by: Douglas Anderson <dianders@...omium.org>
---

 drivers/gpu/drm/drm_dp_helper.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index b5f75ca05774..b8a5c5e49d74 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -3229,10 +3229,12 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
 		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
 		new_dpcd_buf |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
 
-		ret = drm_dp_dpcd_writeb(aux, DP_EDP_PWMGEN_BIT_COUNT, bl->pwmgen_bit_count);
-		if (ret != 1)
-			drm_dbg_kms(aux->drm_dev, "%s: Failed to write aux pwmgen bit count: %d\n",
-				    aux->name, ret);
+		if (bl->pwmgen_bit_count) {
+			ret = drm_dp_dpcd_writeb(aux, DP_EDP_PWMGEN_BIT_COUNT, bl->pwmgen_bit_count);
+			if (ret != 1)
+				drm_dbg_kms(aux->drm_dev, "%s: Failed to write aux pwmgen bit count: %d\n",
+					aux->name, ret);
+		}
 	}
 
 	if (bl->pwm_freq_pre_divider) {
-- 
2.32.0.432.gabb21c7263-goog