From: Mikita Lipski <mikita.lip...@amd.com>

Synaptics DP1.4 hubs (BRANCH_ID 0x90CC24) do not
support virtual DPCD registers, but do support DSC.
The DSC caps can be read from the physical aux,
like in SST DSC. These hubs have many different
DEVICE_IDs.  Add a new quirk to detect this case.

Reviewed-by: Wenjing Liu <wenjing....@amd.com>
Reviewed-by: Lyude Paul <ly...@redhat.com>
Signed-off-by: David Francis <david.fran...@amd.com>
Signed-off-by: Mikita Lipski <mikita.lip...@amd.com>
---
 drivers/gpu/drm/drm_dp_helper.c       |  2 ++
 drivers/gpu/drm/drm_dp_mst_topology.c | 27 +++++++++++++++++++++++++++
 include/drm/drm_dp_helper.h           |  7 +++++++
 3 files changed, 36 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 2faef8bd911f..374cedc6c2ac 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -1165,6 +1165,8 @@ static const struct dpcd_quirk dpcd_quirk_list[] = {
        { OUI(0x00, 0x10, 0xfa), DEVICE_ID_ANY, false, 
BIT(DP_DPCD_QUIRK_NO_PSR) },
        /* CH7511 seems to leave SINK_COUNT zeroed */
        { OUI(0x00, 0x00, 0x00), DEVICE_ID('C', 'H', '7', '5', '1', '1'), 
false, BIT(DP_DPCD_QUIRK_NO_SINK_COUNT) },
+       /* Synaptics DP1.4 MST hubs can support DSC without virtual DPCD */
+       { OUI(0x90, 0xCC, 0x24), DEVICE_ID_ANY, true, 
BIT(DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD) },
 };
 
 #undef OUI
diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c 
b/drivers/gpu/drm/drm_dp_mst_topology.c
index 9df02090aa9f..018921c4ba98 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -5101,6 +5101,7 @@ struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct 
drm_dp_mst_port *port)
 {
        struct drm_dp_mst_port *immediate_upstream_port;
        struct drm_dp_mst_port *fec_port;
+       struct drm_dp_desc desc = { 0 };
 
        if (!port)
                return NULL;
@@ -5153,6 +5154,32 @@ struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct 
drm_dp_mst_port *port)
        if (drm_dp_mst_is_virtual_dpcd(port))
                return &port->aux;
 
+       /*
+        * Synaptics quirk
+        * Applies to ports for which:
+        * - Physical aux has Synaptics OUI
+        * - DPv1.4 or higher
+        * - Port is on primary branch device
+        * - Not a VGA adapter (DP_DWN_STRM_PORT_TYPE_ANALOG)
+        */
+       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) &&
+           port->mgr->dpcd[DP_DPCD_REV] >= DP_DPCD_REV_14 &&
+           port->parent == port->mgr->mst_primary) {
+               u8 downstreamport;
+
+               if (drm_dp_dpcd_read(&port->aux, DP_DOWNSTREAMPORT_PRESENT,
+                                    &downstreamport, 1) < 0)
+                       return NULL;
+
+               if ((downstreamport & DP_DWN_STRM_PORT_PRESENT) &&
+                  ((downstreamport & DP_DWN_STRM_PORT_TYPE_MASK)
+                    != DP_DWN_STRM_PORT_TYPE_ANALOG))
+                       return port->mgr->aux;
+       }
+
        return NULL;
 }
 EXPORT_SYMBOL(drm_dp_mst_dsc_aux_for_port);
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 51ecb5112ef8..e63cd508882b 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1520,6 +1520,13 @@ enum drm_dp_quirk {
         * The driver should ignore SINK_COUNT during detection.
         */
        DP_DPCD_QUIRK_NO_SINK_COUNT,
+       /**
+        * @DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD:
+        *
+        * The device supports MST DSC despite not supporting Virtual DPCD.
+        * The DSC caps can be read from the physical aux instead.
+        */
+       DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD,
 };
 
 /**
-- 
2.17.1

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