drm/i915: Force DPCD backlight mode for some Dell CML 2020 panels

According to Dell, trying to match their panels via OUI is not reliable
enough and we've been told that we should check against the EDID
instead. As well, Dell seems to have some panels that are actually
intended to switch between using PWM for backlight controls and DPCD for
backlight controls depending on whether or not the panel is in HDR or
SDR mode. Yikes.

Regardless, we need to add quirks for these so that DPCD backlight
controls get enabled by default, since without additional driver support
that's the only form of brightness control that will work. Hopefully in
the future we can remove these quirks once we have a better way of
probing for this.

Changes since v1:
* Add one more EDID per Dell's request
* Remove model number (which is possibly wrong) and replace with Dell
  CML 2020 systems

Signed-off-by: Lyude Paul <[email protected]>
Cc: Jani Nikula <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Adam Jackson <[email protected]>

drivers/gpu/drm/drm_dp_helper.c

@@ -1240,6 +1240,20 @@ static const struct edid_quirk edid_quirk_list[] = {
 	 * only supports DPCD backlight controls
 	 */
 	{ MFG(0x4c, 0x83), PROD_ID(0x41, 0x41), BIT(DP_QUIRK_FORCE_DPCD_BACKLIGHT) },
+	/*
+	 * Some Dell CML 2020 systems have panels support both AUX and PWM
+	 * backlight control, and some only support AUX backlight control. All
+	 * said panels start up in AUX mode by default, and we don't have any
+	 * support for disabling HDR mode on these panels which would be
+	 * required to switch to PWM backlight control mode (plus, I'm not
+	 * even sure we want PWM backlight controls over DPCD backlight
+	 * controls anyway...). Until we have a better way of detecting these,
+	 * force DPCD backlight mode on all of them.
+	 */
+	{ MFG(0x06, 0xaf), PROD_ID(0x9b, 0x32), BIT(DP_QUIRK_FORCE_DPCD_BACKLIGHT) },
+	{ MFG(0x06, 0xaf), PROD_ID(0xeb, 0x41), BIT(DP_QUIRK_FORCE_DPCD_BACKLIGHT) },
+	{ MFG(0x4d, 0x10), PROD_ID(0xc7, 0x14), BIT(DP_QUIRK_FORCE_DPCD_BACKLIGHT) },
+	{ MFG(0x4d, 0x10), PROD_ID(0xe6, 0x14), BIT(DP_QUIRK_FORCE_DPCD_BACKLIGHT) },
 };
 
 #undef MFG