[RFC v4 08/11] drm/i915/dpcd_bl: Return early in vesa_calc_max_backlight if we can't read PWMGEN_BIT_COUNT

[Lyude Paul]

If we can't read DP_EDP_PWMGEN_BIT_COUNT in
intel_dp_aux_vesa_calc_max_backlight() but do have a valid PWM frequency
defined in the VBT, we'll keep going in the function until we inevitably
fail on reading DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN. There's not much point in
doing this, so just return early.

Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
---
 drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
index 611eb3a7cc08..a139f0e08839 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -449,11 +449,14 @@ static u32 intel_dp_aux_vesa_calc_max_backlight(struct intel_connector *connecto
 	int freq, fxp, fxp_min, fxp_max, fxp_actual, f = 1;
 	u8 pn, pn_min, pn_max;
 
-	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT, &pn) == 1) {
-		pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
-		max_backlight = (1 << pn) - 1;
+	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT, &pn) != 1) {
+		drm_dbg_kms(&i915->drm, "Failed to read pwmgen bit count cap\n");
+		return 0;
 	}
 
+	pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+	max_backlight = (1 << pn) - 1;
+
 	/* Find desired value of (F x P)
 	 * Note that, if F x P is out of supported range, the maximum value or
 	 * minimum value will applied automatically. So no need to check that.
-- 
2.29.2