[PATCH v3 12/16] drm/dp-mst: Add helpers for querying and enabling MST DSC

[David Francis <David.Francis-5C7GfCeVMHo@public.gmane.org>]

Add drm_dp_mst_dsc_caps_for_port and drm_dp_mst_dsc_enable,
two helper functions for MST DSC

The former, given a port, returns the raw DPCD DSC caps off
that port.

The latter, given a port, enables or disables DSC on that port.

In both cases, the port given as input should be a leaf of
the MST tree with an attached display.

The logic for this is somewhat complicated, as DSC can be
enabled in 4 different ways.

Case 1: DP-to-DP peer device
if the branch immediately upstream has
 - PDT = DP_PEER_DEVICE_DP_MST_BRANCHING (2)
 - DPCD rev. >= DP 1.4
 - Exactly one input and one output
 - The output has PDT = DP_PEER_DEVICE_SST_SINK (3)

In this case, DSC could be possible either on the endpoint
or the peer device. Prefer the endpoint, which is possible if
 - The endpoint has DP_DSC_DECOMPRESSION_IS_SUPPORTED bit set
 - The endpoint has DP_FEC_CAPABLE bit set
 - The peer device has DSC_PASSTHROUGH_CAPABILITY bit set (from DP v2.0)

Otherwise, use the peer device

Case 2: DP-to-HDMI peer device
If the output port has
 - PDT = DP_PEER_DEVICE_DP_LEGACY_CONV (4)
 - DPCD rev. >= DP 1.4
 - LDPS = true
 - MCS = false

In this case, DSC can only be attempted on the peer device
(the output port)

Case 3: Virtual DP Sink (Internal Display Panel)
If the output port has
 - DPCD rev. >= DP 1.4
 - port_num >= 8

In this case, DSC can only be attempted on the peer device
(the output port)

Case 4: Synaptix Workaround
If the output has
 - link DPCD rev. >= DP 1.4
 - link branch_dev_id = 0x90CC24 (Synaptix)
 - There is exactly one branch device between the link and output

In this case, DSC can be attempted, but only using the *link*
aux device's caps. This is a quirk.

Cc: Lyude Paul <lyude@redhat.com>
Cc: Wenjing Liu <Wenjing.Liu@amd.com>
Cc: Nikola Cornij <Nikola.Cornij@amd.com>
Signed-off-by: David Francis <David.Francis@amd.com>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 192 ++++++++++++++++++++++++++
 include/drm/drm_dp_mst_helper.h       |   3 +
 2 files changed, 195 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 7decb5bef062..94742538551e 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -4183,3 +4183,195 @@ static void drm_dp_mst_unregister_i2c_bus(struct drm_dp_aux *aux)
 {
 	i2c_del_adapter(&aux->ddc);
 }
+
+/**
+ * drm_dp_mst_is_virtual_dpcd() - Is the given port a virtual DPCD device?
+ * @port: The port to check
+ *
+ * Returns:
+ * true if the port is a virtual DPCD peer device, false otherwise
+ */
+static bool drm_dp_mst_is_virtual_dpcd(struct drm_dp_mst_port *port)
+{
+	struct drm_dp_mst_port *downstream_port;
+
+	if (!port)
+		return false;
+
+	/* Virtual DP Sink (Internal Display Panel) */
+	if (port->port_num >= 8 && port->dpcd_rev >= DP_DPCD_REV_14)
+		return true;
+
+	/* DP-to-HDMI Protocol Converter */
+	if (port->pdt == DP_PEER_DEVICE_DP_LEGACY_CONV &&
+			!port->mcs &&
+			port->ldps &&
+			port->dpcd_rev >= DP_DPCD_REV_14)
+		return true;
+
+	/* DP-to-DP */
+	if (port->pdt == DP_PEER_DEVICE_MST_BRANCHING &&
+			port->mstb &&
+			port->dpcd_rev >= DP_DPCD_REV_14 &&
+			port->mstb->num_ports == 2) {
+		list_for_each_entry(downstream_port, &port->mstb->ports, next) {
+			if (!downstream_port->input &&
+				downstream_port->pdt == DP_PEER_DEVICE_SST_SINK)
+				return true;
+		}
+	}
+
+	return false;
+}
+
+/**
+ * drm_dp_mst_is_virtual_dpcd() - Does this port require Synaptix DSC workaround?
+ * @port: The port to check
+ *
+ * Some Synaptix MST hubs support DSC even though they do not support virtual
+ * DPCD. This is a quirk.
+ *
+ * Returns:
+ * true if the Synaptix workaround is required, false otherwise
+ */
+static bool drm_dp_mst_dsc_synaptix_workaround(struct drm_dp_mst_port *port)
+{
+	u8 data[3] = { 0 };
+	u32 dev_id;
+	struct drm_dp_aux *phys_aux;
+
+	/* The hub must be directly connected to the connector */
+	if (port->mgr->mst_primary != port->parent)
+		return false;
+
+	phys_aux = port->mgr->aux;
+	if (drm_dp_dpcd_read(phys_aux, DP_BRANCH_OUI, data, 3) < 0)
+		return false;
+	dev_id = (data[0] << 16) & (data[1] << 8) & data[3];
+	/* Synaptix device ID */
+	if (dev_id != 0x90CC24)
+		return false;
+
+	if (drm_dp_dpcd_read(phys_aux, DP_DPCD_REV, data, 1) < 0)
+		return false;
+	/* Must be DPCD rev. 1.4 or later */
+	if (data[0] < DP_DPCD_REV_14)
+		return false;
+
+	if (drm_dp_dpcd_read(&port->aux, DP_DOWNSTREAMPORT_PRESENT, data, 1) < 0)
+		return false;
+	/* Must not be a VGA converter */
+	if ((data[0] & 7) == 3)
+		return false;
+
+	return true;
+}
+
+/**
+ * drm_dp_mst_dsc_aux_for_port() - Find the correct aux for DSC
+ * @port: The port to check. A leaf of the MST tree with an attached display.
+ *
+ * Depending on the situation, DSC may be enabled via the endpoint aux,
+ * the immediately upstream aux, or the connector's physical aux.
+ *
+ * Returns:
+ * NULL if DSC cannot be enabled on this port, otherwise the aux device
+ */
+struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port)
+{
+	u8 upstream_dsc_caps = 0;
+	u8 endpoint_dsc_caps = 0;
+	u8 endpoint_fec_caps = 0;
+	struct drm_dp_mst_port *immediate_upstream_port;
+	struct drm_dp_mst_port *fec_port;
+
+	if (port && port->parent)
+		immediate_upstream_port = port->parent->port_parent;
+	else
+		immediate_upstream_port = NULL;
+
+	fec_port = immediate_upstream_port;
+	while (fec_port) {
+		if (!fec_port->fec_capable)
+			return NULL;
+
+		fec_port = fec_port->parent->port_parent;
+	}
+
+	if (immediate_upstream_port) {
+		if (drm_dp_dpcd_read(&immediate_upstream_port->aux,
+				DP_DSC_SUPPORT, &upstream_dsc_caps, 1) < 0)
+			return NULL;
+	}
+
+	if (drm_dp_dpcd_read(&port->aux, DP_DSC_SUPPORT, &endpoint_dsc_caps, 1) < 0)
+		return NULL;
+	if (drm_dp_dpcd_read(&port->aux, DP_FEC_CAPABILITY, &endpoint_fec_caps, 1) < 0)
+		return NULL;
+
+	/* Enpoint decompression with DP-to-DP peer device */
+	if (drm_dp_mst_is_virtual_dpcd(immediate_upstream_port)
+			&& (upstream_dsc_caps & 0x2) /* DSC passthrough capability */
+			&& (endpoint_fec_caps & DP_FEC_CAPABLE)
+			&& (endpoint_dsc_caps & DP_DSC_DECOMPRESSION_IS_SUPPORTED))
+		return &port->aux;
+
+	/* Virtual DPCD decompression with DP-to-DP peer device */
+	if (drm_dp_mst_is_virtual_dpcd(immediate_upstream_port))
+		return &immediate_upstream_port->aux;
+
+	/* Virtual DPCD decompression with DP-to-HDMI or Virtual DP Sink */
+	if (drm_dp_mst_is_virtual_dpcd(port))
+		return &port->aux;
+
+	/* Synaptix workaround */
+	if (drm_dp_mst_dsc_synaptix_workaround(port))
+		return port->mgr->aux;
+
+	return NULL;
+}
+
+/**
+ * drm_dp_mst_dsc_aux_for_port() - Retrieve the DSC capability registers
+ * @port: The port to check. A leaf of the MST tree with an attached display.
+ * @caps: Output.  A pointer to an array at least 16 bytes long
+ *
+ * Reads the DSC capability registers (DSC_SUPPORT through
+ * BITS_PER_PIXEL_INCREMENT) and store them in the given pointer. Use
+ * the correct aux for DSC on the given port.
+ *
+ * Returns:
+ * The number of bytes read on success, or a negative error code on failure
+ */
+int drm_dp_mst_dsc_caps_for_port(struct drm_dp_mst_port *port, u8 *caps)
+{
+	struct drm_dp_aux *aux = drm_dp_mst_dsc_aux_for_port(port);
+
+	if (!aux)
+		return -EINVAL;
+
+	return drm_dp_dpcd_read(aux, DP_DSC_SUPPORT, caps, 16);
+}
+EXPORT_SYMBOL(drm_dp_mst_dsc_caps_for_port);
+
+/**
+ * drm_dp_mst_dsc_aux_for_port() - Enable DSC on an MST endpoint
+ * @port: The port to check. A leaf of the MST tree with an attached display.
+ * @enable: true for turn on DSC, false for turn off DSC
+ *
+ * Writes DP_DSC_ENABLE on the correct aux for the given port.
+ *
+ * Returns:
+ * The number of bytes written on success, or a negative error code on failure
+ */
+int drm_dp_mst_dsc_enable(struct drm_dp_mst_port *port, bool enable)
+{
+	struct drm_dp_aux *aux = drm_dp_mst_dsc_aux_for_port(port);
+	u8 enable_dsc = enable ? 1 : 0;
+
+	if (!aux)
+		return -EINVAL;
+
+	return drm_dp_dpcd_write(aux, DP_DSC_ENABLE, &enable_dsc, 1);
+}
+EXPORT_SYMBOL(drm_dp_mst_dsc_enable);
diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
index fa973773a4a7..0f70dc8dfbeb 100644
--- a/include/drm/drm_dp_mst_helper.h
+++ b/include/drm/drm_dp_mst_helper.h
@@ -674,6 +674,9 @@ int __must_check drm_dp_mst_atomic_check(struct drm_atomic_state *state);
 void drm_dp_mst_get_port_malloc(struct drm_dp_mst_port *port);
 void drm_dp_mst_put_port_malloc(struct drm_dp_mst_port *port);
 
+int drm_dp_mst_dsc_caps_for_port(struct drm_dp_mst_port *port, u8 *caps);
+int drm_dp_mst_dsc_enable(struct drm_dp_mst_port *port, bool enable);
+
 extern const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs;
 
 /**
-- 
2.17.1

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