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Subject[PATCH 1/6] drm/dp: Don't zero PWMGEN_BIT_COUNT when driver_pwm_freq_hz not specified
Date
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@chromium.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
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