[PATCH 2/3] drm/i915/dp: Try to find proper bpc for DP->legacy converters.

[Mario Kleiner]

This fixes a regression in output precision for DVI and VGA
video sinks connected to Intel hw via active DisplayPort->DVI/VGA
converters.

The regression was indirectly introduced by commit 013dd9e03872
("drm/i915/dp: fall back to 18 bpp when sink capability is unknown").

Our current drm edid 1.3 handling can't reliably assign a proper
minimum supported display depth of 8 bpc to all DVI sinks, as
mandated by DVI 1.0 spec, section 2.2.11.2 "Monitor data format
support", but returns 0 bpc = "Don't know" instead. For analog VGA
sinks it also returns 0 bpc, although those sinks themselves have
infinite color depth, only restricted by the DAC resolution of
the encoder.

If a VGA or dual-link DVI display is connected via DisplayPort
connector then due to above commit the driver would fall back to
only 6 bpc, which would cause degradation for DVI and VGA displays,
annoying in general, but especially harmful for application of display
devices used in neuroscience research and for medical diagnosic
which absolutely need native non-dithered 8 bpc at a minimum to
operate correctly.

For DP connectors with bpc == 0 according to EDID, fix this problem
by checking the dpcd data to find out if a DP->legacy converter
is connected. If the converter is DP->DVI/HDMI assume 8 bpc
depth. If the converter is DP->VGA assume at least 8 bpc, but
try to get a more accurate value (8, 10, 12 or 16 bpc) if the
converter exposes this info.

As the "fall back to 18 bpp" patch was backported to stable we should
include this one also into stable to fix the regression in color depth
for such panels.

Tested with MiniDP->DP adapter, MiniDP->HDMI adapter,
MiniDP->single-link DVI adapter, MiniDP->dual-link DVI active adapter,
and Apple MiniDP->VGA active adapter.

Signed-off-by: Mario Kleiner <mario.kleiner.de at gmail.com>
Cc: Jani Nikula <jani.nikula at intel.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: stable at vger.kernel.org
---
 drivers/gpu/drm/i915/intel_display.c | 12 ++++++++
 drivers/gpu/drm/i915/intel_dp.c      | 56 ++++++++++++++++++++++++++++++++++++
 drivers/gpu/drm/i915/intel_drv.h     |  1 +
 3 files changed, 69 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 6e0d828..9903949 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -12018,12 +12018,24 @@ connected_sink_compute_bpp(struct intel_connector *connector,
 	if (connector->base.display_info.bpc == 0) {
 		int type = connector->base.connector_type;
 		int clamp_bpp = 24;
+		int legacy_bpc = 0;
 
 		/* Fall back to 18 bpp when DP sink capability is unknown. */
 		if (type == DRM_MODE_CONNECTOR_DisplayPort ||
 		    type == DRM_MODE_CONNECTOR_eDP)
 			clamp_bpp = 18;
 
+		/* On DP to legacy converter, try harder to find sink bpc */
+		if (type == DRM_MODE_CONNECTOR_DisplayPort) {
+			legacy_bpc = intel_dp_legacy_bpc(&connector->base);
+
+			if (legacy_bpc) {
+				DRM_DEBUG_KMS("DP to VGA/DVI/HDMI converter with bpc %d\n",
+					      legacy_bpc);
+				clamp_bpp = 3 * legacy_bpc;
+			}
+		}
+
 		if (bpp > clamp_bpp) {
 			DRM_DEBUG_KMS("clamping display bpp (was %d) to default limit of %d\n",
 				      bpp, clamp_bpp);
diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index f069a82..963a8f8 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -6117,3 +6117,59 @@ void intel_dp_mst_resume(struct drm_device *dev)
 		}
 	}
 }
+
+/* XXX Needs work for more than 1 downstream port */
+int intel_dp_legacy_bpc(struct drm_connector *connector)
+{
+	struct intel_dp *intel_dp = intel_attached_dp(connector);
+	uint8_t *dpcd = intel_dp->dpcd;
+	uint8_t type;
+	int bpc = 0;
+
+	/* if there's no downstream port, then this can't be DP->legacy */
+	if (!(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT))
+		return bpc;
+
+	/* Basic type of downstream ports */
+	type = intel_dp->dpcd[DP_DOWNSTREAMPORT_PRESENT] &
+		DP_DWN_STRM_PORT_TYPE_MASK;
+
+	/*
+	 * Lacking other info, 8 bpc is a reasonable start for analog out.
+	 * E.g., Apple MiniDP->VGA adaptors don't provide more info than
+	 * that. Despite having DP_DPCD_REV == 0x11 their downstream_ports
+	 * descriptor is empty - all zeros.
+	 */
+	if (type == DP_DWN_STRM_PORT_TYPE_ANALOG)
+		bpc = 8;
+
+	if (intel_dp->dpcd[DP_DPCD_REV] < 0x11)
+		return bpc;
+
+	/* Rev 1.1+. More specific downstream port type available */
+	type = intel_dp->downstream_ports[0] & DP_DS_PORT_TYPE_MASK;
+
+	/* VGA, DVI and HDMI support at least 8 bpc */
+	if (type == DP_DS_PORT_TYPE_VGA || type == DP_DS_PORT_TYPE_DVI ||
+	    type == DP_DS_PORT_TYPE_HDMI)
+		bpc = 8;
+
+	/* As of DP interop v1.1a only VGA defines additional detail */
+	if (type != DP_DS_PORT_TYPE_VGA ||
+	    !(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DETAILED_CAP_INFO_AVAILABLE))
+		return bpc;
+
+	/* VGA with DP_DETAILED_CAP_INFO_AVAILABLE provides bpc info */
+	switch (intel_dp->downstream_ports[2] & DP_DS_VGA_MAX_BPC_MASK) {
+	case DP_DS_VGA_8BPC:
+		return 8;
+	case DP_DS_VGA_10BPC:
+		return 10;
+	case DP_DS_VGA_12BPC:
+		return 12;
+	case DP_DS_VGA_16BPC:
+		return 16;
+	}
+
+	return bpc;
+}
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 4c027d6..6692788 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -1253,6 +1253,7 @@ void intel_edp_panel_off(struct intel_dp *intel_dp);
 void intel_dp_add_properties(struct intel_dp *intel_dp, struct drm_connector *connector);
 void intel_dp_mst_suspend(struct drm_device *dev);
 void intel_dp_mst_resume(struct drm_device *dev);
+int intel_dp_legacy_bpc(struct drm_connector *connector);
 int intel_dp_max_link_rate(struct intel_dp *intel_dp);
 int intel_dp_rate_select(struct intel_dp *intel_dp, int rate);
 void intel_dp_hot_plug(struct intel_encoder *intel_encoder);
-- 
2.7.0