[PATCH v2 0/3] drm/dp,i915: eDP DPCD backlight control detection fixes

[PATCH v2 0/3] drm/dp,i915: eDP DPCD backlight control detection fixes

[Lyude Paul]

Unfortunately, it turned out that the DPCD is also not a reliable way of
probing for DPCD backlight support as some panels will lie and say they
have DPCD backlight controls when they don't actually.

So, revert back to the old behavior and add a bunch of EDID-based DP
quirks for the panels that we know need this. As you might have already
guessed, OUI quirks didn't seem to be a very safe bet for these panels
due to them not having their device IDs filled out.

Lyude Paul (3):
  drm/dp: Introduce EDID-based quirks
  drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED
    panel
  drm/i915: Force DPCD backlight mode for some Dell CML 2020 panels

 drivers/gpu/drm/drm_dp_helper.c               | 79 +++++++++++++++++++
 drivers/gpu/drm/drm_dp_mst_topology.c         |  3 +-
 .../drm/i915/display/intel_display_types.h    |  1 +
 drivers/gpu/drm/i915/display/intel_dp.c       | 11 ++-
 .../drm/i915/display/intel_dp_aux_backlight.c | 25 +++++-
 drivers/gpu/drm/i915/display/intel_dp_mst.c   |  2 +-
 drivers/gpu/drm/i915/display/intel_psr.c      |  2 +-
 include/drm/drm_dp_helper.h                   | 21 ++++-
 8 files changed, 130 insertions(+), 14 deletions(-)

-- 
2.24.1

[PATCH v2 1/3] drm/dp: Introduce EDID-based quirks

[Lyude Paul]

The whole point of using OUIs is so that we can recognize certain
devices and potentially apply quirks for them. Normally this should work
quite well, but there appears to be quite a number of laptop panels out
there that will fill the OUI but not the device ID. As such, for devices
like this I can't imagine it's a very good idea to try relying on OUIs
for applying quirks. As well, some laptop vendors have confirmed to us
that their panels have this exact issue.

So, let's introduce the ability to apply DP quirks based on EDID
identification. We reuse the same quirk bits for OUI-based quirks, so
that callers can simply check all possible quirks using
drm_dp_has_quirk().

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
---
 drivers/gpu/drm/drm_dp_helper.c               | 61 +++++++++++++++++++
 drivers/gpu/drm/drm_dp_mst_topology.c         |  3 +-
 .../drm/i915/display/intel_display_types.h    |  1 +
 drivers/gpu/drm/i915/display/intel_dp.c       | 11 ++--
 drivers/gpu/drm/i915/display/intel_dp_mst.c   |  2 +-
 drivers/gpu/drm/i915/display/intel_psr.c      |  2 +-
 include/drm/drm_dp_helper.h                   | 11 +++-
 7 files changed, 81 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 5a103e9b3c86..db23308f4d8b 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -1221,6 +1221,67 @@ drm_dp_get_quirks(const struct drm_dp_dpcd_ident *ident, bool is_branch)
 #undef DEVICE_ID_ANY
 #undef DEVICE_ID
 
+struct edid_quirk {
+	u8 mfg_id[2];
+	u8 prod_id[2];
+	u32 quirks;
+};
+
+#define MFG(first, second) { (first), (second) }
+#define PROD_ID(first, second) { (first), (second) }
+
+/*
+ * Some devices have unreliable OUIDs where they don't set the device ID
+ * correctly, and as a result we need to use the EDID for finding additional
+ * DP quirks in such cases.
+ */
+static const struct edid_quirk edid_quirk_list[] = {
+};
+
+#undef MFG
+#undef PROD_ID
+
+/**
+ * drm_dp_get_edid_quirks() - Check the EDID of a DP device to find additional
+ * DP-specific quirks
+ * @edid: The EDID to check
+ *
+ * While OUIDs are meant to be used to recognize a DisplayPort device, a lot
+ * of manufacturers don't seem to like following standards and neglect to fill
+ * the dev-ID in, making it impossible to only use OUIDs for determining
+ * quirks in some cases. This function can be used to check the EDID and look
+ * up any additional DP quirks. The bits returned by this function correspond
+ * to the quirk bits in &drm_dp_quirk.
+ *
+ * Returns: a bitmask of quirks, if any. The driver can check this using
+ * drm_dp_has_quirk().
+ */
+u32 drm_dp_get_edid_quirks(const struct edid *edid)
+{
+	const struct edid_quirk *quirk;
+	u32 quirks = 0;
+	int i;
+
+	if (!edid)
+		return 0;
+
+	for (i = 0; i < ARRAY_SIZE(edid_quirk_list); i++) {
+		quirk = &edid_quirk_list[i];
+		if (memcmp(quirk->mfg_id, edid->mfg_id,
+			   sizeof(edid->mfg_id)) == 0 &&
+		    memcmp(quirk->prod_id, edid->prod_code,
+			   sizeof(edid->prod_code)) == 0)
+			quirks |= quirk->quirks;
+	}
+
+	DRM_DEBUG_KMS("DP sink: EDID mfg %*phD prod-ID %*phD quirks: 0x%04x\n",
+		      (int)sizeof(edid->mfg_id), edid->mfg_id,
+		      (int)sizeof(edid->prod_code), edid->prod_code, quirks);
+
+	return quirks;
+}
+EXPORT_SYMBOL(drm_dp_get_edid_quirks);
+
 /**
  * drm_dp_read_desc - read sink/branch descriptor from DPCD
  * @aux: DisplayPort AUX channel
diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index a811247cecfe..68247670427a 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -5452,7 +5452,8 @@ struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port)
 	if (drm_dp_read_desc(port->mgr->aux, &desc, true))
 		return NULL;
 
-	if (drm_dp_has_quirk(&desc, DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD) &&
+	if (drm_dp_has_quirk(&desc, 0,
+			     DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD) &&
 	    port->mgr->dpcd[DP_DPCD_REV] >= DP_DPCD_REV_14 &&
 	    port->parent == port->mgr->mst_primary) {
 		u8 downstreamport;
diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h b/drivers/gpu/drm/i915/display/intel_display_types.h
index 283c622f8ba1..d80e7d74179d 100644
--- a/drivers/gpu/drm/i915/display/intel_display_types.h
+++ b/drivers/gpu/drm/i915/display/intel_display_types.h
@@ -1232,6 +1232,7 @@ struct intel_dp {
 	int max_link_rate;
 	/* sink or branch descriptor */
 	struct drm_dp_desc desc;
+	u32 edid_quirks;
 	struct drm_dp_aux aux;
 	u32 aux_busy_last_status;
 	u8 train_set[4];
diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index f4dede6253f8..245e21d1cadd 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -2386,7 +2386,7 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 	struct intel_connector *intel_connector = intel_dp->attached_connector;
 	struct intel_digital_connector_state *intel_conn_state =
 		to_intel_digital_connector_state(conn_state);
-	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
+	bool constant_n = drm_dp_has_quirk(&intel_dp->desc, 0,
 					   DP_DPCD_QUIRK_CONSTANT_N);
 	int ret = 0, output_bpp;
 
@@ -4499,7 +4499,8 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
 	 * it don't care about read it here and in intel_edp_init_dpcd().
 	 */
 	if (!intel_dp_is_edp(intel_dp) &&
-	    !drm_dp_has_quirk(&intel_dp->desc, DP_DPCD_QUIRK_NO_SINK_COUNT)) {
+	    !drm_dp_has_quirk(&intel_dp->desc, 0,
+			      DP_DPCD_QUIRK_NO_SINK_COUNT)) {
 		u8 count;
 		ssize_t r;
 
@@ -5666,6 +5667,7 @@ intel_dp_set_edid(struct intel_dp *intel_dp)
 
 	intel_dp->has_audio = drm_detect_monitor_audio(edid);
 	drm_dp_cec_set_edid(&intel_dp->aux, edid);
+	intel_dp->edid_quirks = drm_dp_get_edid_quirks(edid);
 }
 
 static void
@@ -5678,6 +5680,7 @@ intel_dp_unset_edid(struct intel_dp *intel_dp)
 	intel_connector->detect_edid = NULL;
 
 	intel_dp->has_audio = false;
+	intel_dp->edid_quirks = 0;
 }
 
 static int
@@ -7409,8 +7412,8 @@ static bool intel_edp_init_connector(struct intel_dp *intel_dp,
 	edid = drm_get_edid(connector, &intel_dp->aux.ddc);
 	if (edid) {
 		if (drm_add_edid_modes(connector, edid)) {
-			drm_connector_update_edid_property(connector,
-								edid);
+			drm_connector_update_edid_property(connector, edid);
+			intel_dp->edid_quirks = drm_dp_get_edid_quirks(edid);
 		} else {
 			kfree(edid);
 			edid = ERR_PTR(-EINVAL);
diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
index 9cd59141953d..ce787dae6411 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
@@ -50,7 +50,7 @@ static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
 	const struct drm_display_mode *adjusted_mode =
 		&crtc_state->hw.adjusted_mode;
 	void *port = connector->port;
-	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
+	bool constant_n = drm_dp_has_quirk(&intel_dp->desc, 0,
 					   DP_DPCD_QUIRK_CONSTANT_N);
 	int bpp, slots = -EINVAL;
 
diff --git a/drivers/gpu/drm/i915/display/intel_psr.c b/drivers/gpu/drm/i915/display/intel_psr.c
index b4942b6445ae..028f27a3db74 100644
--- a/drivers/gpu/drm/i915/display/intel_psr.c
+++ b/drivers/gpu/drm/i915/display/intel_psr.c
@@ -304,7 +304,7 @@ void intel_psr_init_dpcd(struct intel_dp *intel_dp)
 	drm_dbg_kms(&dev_priv->drm, "eDP panel supports PSR version %x\n",
 		    intel_dp->psr_dpcd[0]);
 
-	if (drm_dp_has_quirk(&intel_dp->desc, DP_DPCD_QUIRK_NO_PSR)) {
+	if (drm_dp_has_quirk(&intel_dp->desc, 0, DP_DPCD_QUIRK_NO_PSR)) {
 		drm_dbg_kms(&dev_priv->drm,
 			    "PSR support not currently available for this panel\n");
 		return;
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 262faf9e5e94..7f5dd2ee4a94 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1495,13 +1495,16 @@ struct drm_dp_desc {
 
 int drm_dp_read_desc(struct drm_dp_aux *aux, struct drm_dp_desc *desc,
 		     bool is_branch);
+u32 drm_dp_get_edid_quirks(const struct edid *edid);
 
 /**
  * enum drm_dp_quirk - Display Port sink/branch device specific quirks
  *
  * Display Port sink and branch devices in the wild have a variety of bugs, try
  * to collect them here. The quirks are shared, but it's up to the drivers to
- * implement workarounds for them.
+ * implement workarounds for them. Note that because some devices have
+ * unreliable OUIDs, the EDID of sinks should also be checked for quirks using
+ * drm_dp_get_edid_quirks().
  */
 enum drm_dp_quirk {
 	/**
@@ -1537,14 +1540,16 @@ enum drm_dp_quirk {
 /**
  * drm_dp_has_quirk() - does the DP device have a specific quirk
  * @desc: Device decriptor filled by drm_dp_read_desc()
+ * @edid_quirks: Optional quirk bitmask filled by drm_dp_get_edid_quirks()
  * @quirk: Quirk to query for
  *
  * Return true if DP device identified by @desc has @quirk.
  */
 static inline bool
-drm_dp_has_quirk(const struct drm_dp_desc *desc, enum drm_dp_quirk quirk)
+drm_dp_has_quirk(const struct drm_dp_desc *desc, u32 edid_quirks,
+		 enum drm_dp_quirk quirk)
 {
-	return desc->quirks & BIT(quirk);
+	return (desc->quirks | edid_quirks) & BIT(quirk);
 }
 
 #ifdef CONFIG_DRM_DP_CEC
-- 
2.24.1

[PATCH v2 2/3] drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED panel

[Lyude Paul]

The X1 Extreme is one of the systems that lies about which backlight
interface that it uses in its VBIOS as PWM backlight controls don't work
at all on this machine. It's possible that this panel could be one of
the infamous ones that can switch between PWM mode and DPCD backlight
control mode, but we haven't gotten any more details on this from Lenovo
just yet. For the time being though, making sure the backlight 'just
works' is a bit more important.

So, add a quirk to force DPCD backlight controls on for these systems
based on EDID (since this panel doesn't appear to fill in the device ID).
Hopefully in the future we'll figure out a better way of probing this.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
---
 drivers/gpu/drm/drm_dp_helper.c               |  4 +++
 .../drm/i915/display/intel_dp_aux_backlight.c | 25 ++++++++++++++++---
 include/drm/drm_dp_helper.h                   | 10 ++++++++
 3 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index db23308f4d8b..a39c3cdacb20 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -1236,6 +1236,10 @@ struct edid_quirk {
  * DP quirks in such cases.
  */
 static const struct edid_quirk edid_quirk_list[] = {
+	/* Optional 4K AMOLED panel in the ThinkPad X1 Extreme 2nd Generation
+	 * only supports DPCD backlight controls
+	 */
+	{ MFG(0x4c, 0x83), PROD_ID(0x41, 0x41), BIT(DP_QUIRK_FORCE_DPCD_BACKLIGHT) },
 };
 
 #undef MFG
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 48276237b362..35a19c0314fa 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -328,15 +328,32 @@ intel_dp_aux_display_control_capable(struct intel_connector *connector)
 int intel_dp_aux_init_backlight_funcs(struct intel_connector *intel_connector)
 {
 	struct intel_panel *panel = &intel_connector->panel;
-	struct drm_i915_private *dev_priv = to_i915(intel_connector->base.dev);
+	struct intel_dp *intel_dp = enc_to_intel_dp(intel_connector->encoder);
+	struct drm_device *dev = intel_connector->base.dev;
+	struct drm_i915_private *dev_priv = to_i915(dev);
 
 	if (i915_modparams.enable_dpcd_backlight == 0 ||
-	    (i915_modparams.enable_dpcd_backlight == -1 &&
-	    dev_priv->vbt.backlight.type != INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE))
+	    !intel_dp_aux_display_control_capable(intel_connector))
 		return -ENODEV;
 
-	if (!intel_dp_aux_display_control_capable(intel_connector))
+	/*
+	 * There are a lot of machines that don't advertise the backlight
+	 * control interface to use properly in their VBIOS, :\
+	 */
+	if (dev_priv->vbt.backlight.type !=
+	    INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE &&
+	    !drm_dp_has_quirk(&intel_dp->desc, intel_dp->edid_quirks,
+			      DP_QUIRK_FORCE_DPCD_BACKLIGHT)) {
+		DRM_DEV_INFO(dev->dev,
+			     "Panel advertises DPCD backlight support, but "
+			     "VBT disagrees. If your backlight controls "
+			     "don't work try booting with "
+			     "i915.enable_dpcd_backlight=1. If your machine "
+			     "needs this, please file a _new_ bug report on "
+			     "bugs.freedesktop.org against DRI -> "
+			     "DRM/Intel\n");
 		return -ENODEV;
+	}
 
 	panel->backlight.setup = intel_dp_aux_setup_backlight;
 	panel->backlight.enable = intel_dp_aux_enable_backlight;
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 7f5dd2ee4a94..c5580e988826 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1535,6 +1535,16 @@ enum drm_dp_quirk {
 	 * The DSC caps can be read from the physical aux instead.
 	 */
 	DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD,
+	/**
+	 * @DP_QUIRK_FORCE_DPCD_BACKLIGHT:
+	 *
+	 * The device is telling the truth when it says that it uses DPCD
+	 * backlight controls, even if the system's firmware disagrees. This
+	 * quirk should be checked against both the ident and panel EDID.
+	 * When present, the driver should honor the DPCD backlight
+	 * capabilities advertised.
+	 */
+	DP_QUIRK_FORCE_DPCD_BACKLIGHT,
 };
 
 /**
-- 
2.24.1

[PATCH v2 3/3] drm/i915: Force DPCD backlight mode for some Dell CML 2020 panels

[Lyude Paul]

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 <lyude@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
---
 drivers/gpu/drm/drm_dp_helper.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index a39c3cdacb20..620d78ff2706 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/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
-- 
2.24.1

Re: [v2,2/3] drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED panel

[Adam Jackson]

On Tue, 2020-02-11 at 13:33 -0500, Lyude Paul wrote:

> -	if (!intel_dp_aux_display_control_capable(intel_connector))
> +	/*
> +	 * There are a lot of machines that don't advertise the backlight
> +	 * control interface to use properly in their VBIOS, :\
> +	 */
> +	if (dev_priv->vbt.backlight.type !=
> +	    INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE &&
> +	    !drm_dp_has_quirk(&intel_dp->desc, intel_dp->edid_quirks,
> +			      DP_QUIRK_FORCE_DPCD_BACKLIGHT)) {
> +		DRM_DEV_INFO(dev->dev,
> +			     "Panel advertises DPCD backlight support, but "
> +			     "VBT disagrees. If your backlight controls "
> +			     "don't work try booting with "
> +			     "i915.enable_dpcd_backlight=1. If your machine "
> +			     "needs this, please file a _new_ bug report on "
> +			     "bugs.freedesktop.org against DRI -> "
> +			     "DRM/Intel\n");

Bugzilla's been put out of our misery, probably this should point to
gitlab instead.

- ajax

Re: [v2,1/3] drm/dp: Introduce EDID-based quirks

[Adam Jackson]

On Tue, 2020-02-11 at 13:33 -0500, Lyude Paul wrote:
> The whole point of using OUIs is so that we can recognize certain
> devices and potentially apply quirks for them. Normally this should work
> quite well, but there appears to be quite a number of laptop panels out
> there that will fill the OUI but not the device ID. As such, for devices
> like this I can't imagine it's a very good idea to try relying on OUIs
> for applying quirks. As well, some laptop vendors have confirmed to us
> that their panels have this exact issue.
> 
> So, let's introduce the ability to apply DP quirks based on EDID
> identification. We reuse the same quirk bits for OUI-based quirks, so
> that callers can simply check all possible quirks using
> drm_dp_has_quirk().

With the bug URL fixed in 2/3, series is:

Reviewed-by: Adam Jackson <ajax@redhat.com>

- ajax

[PATCH v3] drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED panel

[Lyude Paul]

The X1 Extreme is one of the systems that lies about which backlight
interface that it uses in its VBIOS as PWM backlight controls don't work
at all on this machine. It's possible that this panel could be one of
the infamous ones that can switch between PWM mode and DPCD backlight
control mode, but we haven't gotten any more details on this from Lenovo
just yet. For the time being though, making sure the backlight 'just
works' is a bit more important.

So, add a quirk to force DPCD backlight controls on for these systems
based on EDID (since this panel doesn't appear to fill in the device ID).
Hopefully in the future we'll figure out a better way of probing this.

Changes since v2:
* The bugzilla URL is deprecated, bug reporting happens on gitlab now.
  Update the messages we print to reflect this
* Also, take the opportunity to move FDO_BUG_URL out of i915_utils.c and
  into i915_utils.h so that other places which print things that aren't
  traditional errors but are worth filing bugs about, can actually use
  it.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
---
 drivers/gpu/drm/drm_dp_helper.c               |  4 ++++
 .../drm/i915/display/intel_dp_aux_backlight.c | 24 +++++++++++++++----
 drivers/gpu/drm/i915/i915_utils.c             |  1 -
 drivers/gpu/drm/i915/i915_utils.h             |  2 ++
 include/drm/drm_dp_helper.h                   | 10 ++++++++
 5 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 7be712c67aae..671f4d3968fc 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -1295,6 +1295,10 @@ struct edid_quirk {
  * DP quirks in such cases.
  */
 static const struct edid_quirk edid_quirk_list[] = {
+	/* Optional 4K AMOLED panel in the ThinkPad X1 Extreme 2nd Generation
+	 * only supports DPCD backlight controls
+	 */
+	{ MFG(0x4c, 0x83), PROD_ID(0x41, 0x41), BIT(DP_QUIRK_FORCE_DPCD_BACKLIGHT) },
 };
 
 #undef MFG
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 48276237b362..3e706bb850a8 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -328,15 +328,31 @@ intel_dp_aux_display_control_capable(struct intel_connector *connector)
 int intel_dp_aux_init_backlight_funcs(struct intel_connector *intel_connector)
 {
 	struct intel_panel *panel = &intel_connector->panel;
-	struct drm_i915_private *dev_priv = to_i915(intel_connector->base.dev);
+	struct intel_dp *intel_dp = enc_to_intel_dp(intel_connector->encoder);
+	struct drm_device *dev = intel_connector->base.dev;
+	struct drm_i915_private *dev_priv = to_i915(dev);
 
 	if (i915_modparams.enable_dpcd_backlight == 0 ||
-	    (i915_modparams.enable_dpcd_backlight == -1 &&
-	    dev_priv->vbt.backlight.type != INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE))
+	    !intel_dp_aux_display_control_capable(intel_connector))
 		return -ENODEV;
 
-	if (!intel_dp_aux_display_control_capable(intel_connector))
+	/*
+	 * There are a lot of machines that don't advertise the backlight
+	 * control interface to use properly in their VBIOS, :\
+	 */
+	if (dev_priv->vbt.backlight.type !=
+	    INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE &&
+	    !drm_dp_has_quirk(&intel_dp->desc, intel_dp->edid_quirks,
+			      DP_QUIRK_FORCE_DPCD_BACKLIGHT)) {
+		DRM_DEV_INFO(dev->dev,
+			     "Panel advertises DPCD backlight support, but "
+			     "VBT disagrees. If your backlight controls "
+			     "don't work try booting with "
+			     "i915.enable_dpcd_backlight=1. If your machine "
+			     "needs this, please file a _new_ bug report on "
+			     "drm/i915, see " FDO_BUG_URL " for details.\n");
 		return -ENODEV;
+	}
 
 	panel->backlight.setup = intel_dp_aux_setup_backlight;
 	panel->backlight.enable = intel_dp_aux_enable_backlight;
diff --git a/drivers/gpu/drm/i915/i915_utils.c b/drivers/gpu/drm/i915/i915_utils.c
index 632d6953c78d..029854ae65fc 100644
--- a/drivers/gpu/drm/i915/i915_utils.c
+++ b/drivers/gpu/drm/i915/i915_utils.c
@@ -8,7 +8,6 @@
 #include "i915_drv.h"
 #include "i915_utils.h"
 
-#define FDO_BUG_URL "https://gitlab.freedesktop.org/drm/intel/-/wikis/How-to-file-i915-bugs"
 #define FDO_BUG_MSG "Please file a bug on drm/i915; see " FDO_BUG_URL " for details."
 
 void
diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
index b0ade76bec90..cae0ae520398 100644
--- a/drivers/gpu/drm/i915/i915_utils.h
+++ b/drivers/gpu/drm/i915/i915_utils.h
@@ -34,6 +34,8 @@
 struct drm_i915_private;
 struct timer_list;
 
+#define FDO_BUG_URL "https://gitlab.freedesktop.org/drm/intel/-/wikis/How-to-file-i915-bugs"
+
 #undef WARN_ON
 /* Many gcc seem to no see through this and fall over :( */
 #if 0
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index abb04650f210..c6119e4c169a 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1538,6 +1538,16 @@ enum drm_dp_quirk {
 	 * The DSC caps can be read from the physical aux instead.
 	 */
 	DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD,
+	/**
+	 * @DP_QUIRK_FORCE_DPCD_BACKLIGHT:
+	 *
+	 * The device is telling the truth when it says that it uses DPCD
+	 * backlight controls, even if the system's firmware disagrees. This
+	 * quirk should be checked against both the ident and panel EDID.
+	 * When present, the driver should honor the DPCD backlight
+	 * capabilities advertised.
+	 */
+	DP_QUIRK_FORCE_DPCD_BACKLIGHT,
 };
 
 /**
-- 
2.24.1