[v8,6/6] drm/dp_mst: Add helpers for MST DSC and virtual DPCD aux
diff mbox series

Message ID 20190826180507.17802-7-David.Francis@amd.com
State New
Headers show
Series
  • DSC MST support in DRM
Related show

Commit Message

Francis, David Aug. 26, 2019, 6:05 p.m. UTC
Add drm_dp_mst_dsc_aux_for_port. To enable DSC, the DSC_ENABLED
register might have to be written on the leaf port's DPCD,
its parent's DPCD, or the MST manager's DPCD. This function
finds the correct aux for the job.

As part of this, add drm_dp_mst_is_virtual_dpcd. Virtual DPCD
is a DP feature new in DP v1.4, which exposes certain DPCD
registers on virtual ports.

v2: Remember to unlock mutex on all paths

Cc: Lyude Paul <lyude@redhat.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: David Francis <David.Francis@amd.com>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 152 ++++++++++++++++++++++++++
 include/drm/drm_dp_mst_helper.h       |   2 +
 2 files changed, 154 insertions(+)

Comments

Francis, David Aug. 26, 2019, 7:16 p.m. UTC | #1
Received offline review from AMD MST DSC (Non-Linux) expert Wenjing Liu, giving me permission  to mark this patch
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Lyude Paul Aug. 26, 2019, 8:27 p.m. UTC | #2
Please fix the indenting on this. I've mentioned this already in my previous
reviews.

On Mon, 2019-08-26 at 19:16 +0000, Francis, David wrote:
> Received offline review from AMD MST DSC (Non-Linux) expert Wenjing Liu,
> giving me permission  to mark this patch
> Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
> 
> ________________________________________
> From: David Francis <David.Francis@amd.com>
> Sent: August 26, 2019 2:05 PM
> To: dri-devel@lists.freedesktop.org
> Cc: Francis, David; Lyude Paul; Jani Nikula
> Subject: [PATCH v8 6/6] drm/dp_mst: Add helpers for MST DSC and virtual DPCD
> aux
> 
> Add drm_dp_mst_dsc_aux_for_port. To enable DSC, the DSC_ENABLED
> register might have to be written on the leaf port's DPCD,
> its parent's DPCD, or the MST manager's DPCD. This function
> finds the correct aux for the job.
> 
> As part of this, add drm_dp_mst_is_virtual_dpcd. Virtual DPCD
> is a DP feature new in DP v1.4, which exposes certain DPCD
> registers on virtual ports.
> 
> v2: Remember to unlock mutex on all paths
> 
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Signed-off-by: David Francis <David.Francis@amd.com>
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 152 ++++++++++++++++++++++++++
>  include/drm/drm_dp_mst_helper.h       |   2 +
>  2 files changed, 154 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 502923c24450..1fee92bd51f7 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -4150,3 +4150,155 @@ 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 DP Peer
> Device
> + * @port: The port to check
> + *
> + * A single physical MST hub object can be represented in the topology
> + * by multiple branches, with virtual ports between those branches.
> + *
> + * As of DP1.4, An MST hub with internal (virtual) ports must expose
> + * certain DPCD registers over those ports. See sections 2.6.1.1.1
> + * and 2.6.1.1.2 of Display Port specification v1.4 for details.
> + *
> + * May acquire mgr->lock
> + *
> + * Returns:
> + * true if the port is a virtual DP 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 */
> +       mutex_lock(&port->mgr->lock);
> +       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->pdt == DP_PEER_DEVICE_SST_SINK
> &&
> +                                       !downstream_port->input) {
> +                               mutex_unlock(&port->mgr->lock);
> +                               return true;
> +                       }
> +               }
> +       }
> +       mutex_unlock(&port->mgr->lock);
> +
> +       return false;
> +}
> +
> +/**
> + * 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.
> + *
> + * This is both the correct aux to read DSC_CAPABILITY and the
> + * correct aux to write DSC_ENABLED.
> + *
> + * This operation can be expensive (up to four aux reads), so
> + * the caller should cache the return.
> + *
> + * 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)
> +{
> +       struct drm_dp_mst_port *immediate_upstream_port;
> +       struct drm_dp_mst_port *fec_port;
> +       struct drm_dp_desc *desc = NULL;
> +
> +       if (port && port->parent)
> +               immediate_upstream_port = port->parent->port_parent;
> +       else
> +               immediate_upstream_port = NULL;
> +
> +       fec_port = immediate_upstream_port;
> +       while (fec_port) {
> +               /*
> +                * Each physical link (i.e. not a virtual port) between the
> +                * output and the primary device must support FEC
> +                */
> +               if (!drm_dp_mst_is_virtual_dpcd(fec_port) &&
> +                               !fec_port->fec_capable)
> +                       return NULL;
> +
> +               fec_port = fec_port->parent->port_parent;
> +       }
> +
> +       /* DP-to-DP peer device */
> +       if (drm_dp_mst_is_virtual_dpcd(immediate_upstream_port)) {
> +               u8 upstream_dsc;
> +               u8 endpoint_dsc;
> +               u8 endpoint_fec;
> +
> +               if (drm_dp_dpcd_read(&port->aux,
> +                                    DP_DSC_SUPPORT, &endpoint_dsc, 1) < 0)
> +                       return NULL;
> +               if (drm_dp_dpcd_read(&port->aux,
> +                                    DP_FEC_CAPABILITY, &endpoint_fec, 1) <
> 0)
> +                       return NULL;
> +               if (drm_dp_dpcd_read(&immediate_upstream_port->aux,
> +                                    DP_DSC_SUPPORT, &upstream_dsc, 1) < 0)
> +                       return NULL;
> +
> +               /* Enpoint decompression with DP-to-DP peer device */
> +               if ((endpoint_dsc & DP_DSC_DECOMPRESSION_IS_SUPPORTED)
> +                               && (endpoint_fec & DP_FEC_CAPABLE)
> +                               && (upstream_dsc & 0x2) /* DSC passthrough
> */)
> +                       return &port->aux;
> +
> +               /* Virtual DPCD decompression with DP-to-DP peer device */
> +               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;
> +
> +       /*
> +        * 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 false;
> +
> +       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;
> +
> +               drm_dp_dpcd_read(&port->aux, DP_DOWNSTREAMPORT_PRESENT,
> +                                &downstreamport, 1);
> +
> +               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_mst_helper.h
> b/include/drm/drm_dp_mst_helper.h
> index f113ae04fa88..4cf738545dfb 100644
> --- a/include/drm/drm_dp_mst_helper.h
> +++ b/include/drm/drm_dp_mst_helper.h
> @@ -673,6 +673,8 @@ 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);
> 
> +struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port
> *port);
> +
>  extern const struct drm_private_state_funcs
> drm_dp_mst_topology_state_funcs;
> 
>  /**
> --
> 2.17.1
>
Harry Wentland Aug. 26, 2019, 8:51 p.m. UTC | #3
On 2019-08-26 2:05 p.m., David Francis wrote:
> Add drm_dp_mst_dsc_aux_for_port. To enable DSC, the DSC_ENABLED
> register might have to be written on the leaf port's DPCD,
> its parent's DPCD, or the MST manager's DPCD. This function
> finds the correct aux for the job.
> 
> As part of this, add drm_dp_mst_is_virtual_dpcd. Virtual DPCD
> is a DP feature new in DP v1.4, which exposes certain DPCD
> registers on virtual ports.
> 
> v2: Remember to unlock mutex on all paths
> 
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Signed-off-by: David Francis <David.Francis@amd.com>
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 152 ++++++++++++++++++++++++++
>  include/drm/drm_dp_mst_helper.h       |   2 +
>  2 files changed, 154 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 502923c24450..1fee92bd51f7 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -4150,3 +4150,155 @@ 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 DP Peer Device
> + * @port: The port to check
> + *
> + * A single physical MST hub object can be represented in the topology
> + * by multiple branches, with virtual ports between those branches.
> + *
> + * As of DP1.4, An MST hub with internal (virtual) ports must expose
> + * certain DPCD registers over those ports. See sections 2.6.1.1.1
> + * and 2.6.1.1.2 of Display Port specification v1.4 for details.
> + *

I've briefly perused these sections and they seem quite vague. What we
have here is probably a reasonable implementation, though.

Might be worthwhile to mention that the spec is not very clear on this
and that this is the best interpretation we have.

> + * May acquire mgr->lock
> + *
> + * Returns:
> + * true if the port is a virtual DP 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;
> +

Can we early return with false if dpcd_rev < DP_DPCD_REV_14 rather than
check this in each if statement below?

> +	/* 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 */
> +	mutex_lock(&port->mgr->lock);
> +	if (port->pdt == DP_PEER_DEVICE_MST_BRANCHING &&
> +			port->mstb &&
> +			port->dpcd_rev >= DP_DPCD_REV_14 &&
> +			port->mstb->num_ports == 2) {

Is 'num_ports == 2' required? Maybe I don't follow this bit...

> +		list_for_each_entry(downstream_port, &port->mstb->ports, next) {
> +			if (downstream_port->pdt == DP_PEER_DEVICE_SST_SINK &&
> +					!downstream_port->input) {
> +				mutex_unlock(&port->mgr->lock);
> +				return true;
> +			}
> +		}
> +	}
> +	mutex_unlock(&port->mgr->lock);
> +
> +	return false;
> +}
> +
> +/**
> + * 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.
> + *
> + * This is both the correct aux to read DSC_CAPABILITY and the
> + * correct aux to write DSC_ENABLED.
> + *
> + * This operation can be expensive (up to four aux reads), so
> + * the caller should cache the return.
> + *
> + * 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)
> +{
> +	struct drm_dp_mst_port *immediate_upstream_port;
> +	struct drm_dp_mst_port *fec_port;
> +	struct drm_dp_desc *desc = NULL;
> +

Might be good to early return if !port to avoid proceeding through the
entire function if this is called without a 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) {
> +		/*
> +		 * Each physical link (i.e. not a virtual port) between the
> +		 * output and the primary device must support FEC
> +		 */
> +		if (!drm_dp_mst_is_virtual_dpcd(fec_port) &&
> +				!fec_port->fec_capable)
> +			return NULL;
> +
> +		fec_port = fec_port->parent->port_parent;
> +	}
> +
> +	/* DP-to-DP peer device */
> +	if (drm_dp_mst_is_virtual_dpcd(immediate_upstream_port)) {
> +		u8 upstream_dsc;
> +		u8 endpoint_dsc;
> +		u8 endpoint_fec;
> +
> +		if (drm_dp_dpcd_read(&port->aux,
> +				     DP_DSC_SUPPORT, &endpoint_dsc, 1) < 0)
> +			return NULL;
> +		if (drm_dp_dpcd_read(&port->aux,
> +				     DP_FEC_CAPABILITY, &endpoint_fec, 1) < 0)
> +			return NULL;
> +		if (drm_dp_dpcd_read(&immediate_upstream_port->aux,
> +				     DP_DSC_SUPPORT, &upstream_dsc, 1) < 0)
> +			return NULL;
> +
> +		/* Enpoint decompression with DP-to-DP peer device */
> +		if ((endpoint_dsc & DP_DSC_DECOMPRESSION_IS_SUPPORTED)
> +				&& (endpoint_fec & DP_FEC_CAPABLE)
> +				&& (upstream_dsc & 0x2) /* DSC passthrough */)
> +			return &port->aux;
> +
> +		/* Virtual DPCD decompression with DP-to-DP peer device */
> +		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;
> +
> +	/*
> +	 * 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 false;
> +
> +	if (drm_dp_has_quirk(desc, DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD &&
> +			port->mgr->dpcd[DP_DPCD_REV] >= DP_DPCD_REV_14) &&

Did you misplace that closing paranthesis? Shouldn't it be on the
previous line before the && ?

> +			port->parent == port->mgr->mst_primary) {

Use tabs & spaces to align with first element of if statement. Proper
indentation (with the function argument or elements of the if statement)
would make it obvious whether the above paranthesis has been replaced or
not.

Harry

> +		u8 downstreamport;
> +
> +		drm_dp_dpcd_read(&port->aux, DP_DOWNSTREAMPORT_PRESENT,
> +				 &downstreamport, 1);
> +
> +		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_mst_helper.h b/include/drm/drm_dp_mst_helper.h
> index f113ae04fa88..4cf738545dfb 100644
> --- a/include/drm/drm_dp_mst_helper.h
> +++ b/include/drm/drm_dp_mst_helper.h
> @@ -673,6 +673,8 @@ 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);
>  
> +struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port);
> +
>  extern const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs;
>  
>  /**
>

Patch
diff mbox series

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 502923c24450..1fee92bd51f7 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -4150,3 +4150,155 @@  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 DP Peer Device
+ * @port: The port to check
+ *
+ * A single physical MST hub object can be represented in the topology
+ * by multiple branches, with virtual ports between those branches.
+ *
+ * As of DP1.4, An MST hub with internal (virtual) ports must expose
+ * certain DPCD registers over those ports. See sections 2.6.1.1.1
+ * and 2.6.1.1.2 of Display Port specification v1.4 for details.
+ *
+ * May acquire mgr->lock
+ *
+ * Returns:
+ * true if the port is a virtual DP 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 */
+	mutex_lock(&port->mgr->lock);
+	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->pdt == DP_PEER_DEVICE_SST_SINK &&
+					!downstream_port->input) {
+				mutex_unlock(&port->mgr->lock);
+				return true;
+			}
+		}
+	}
+	mutex_unlock(&port->mgr->lock);
+
+	return false;
+}
+
+/**
+ * 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.
+ *
+ * This is both the correct aux to read DSC_CAPABILITY and the
+ * correct aux to write DSC_ENABLED.
+ *
+ * This operation can be expensive (up to four aux reads), so
+ * the caller should cache the return.
+ *
+ * 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)
+{
+	struct drm_dp_mst_port *immediate_upstream_port;
+	struct drm_dp_mst_port *fec_port;
+	struct drm_dp_desc *desc = NULL;
+
+	if (port && port->parent)
+		immediate_upstream_port = port->parent->port_parent;
+	else
+		immediate_upstream_port = NULL;
+
+	fec_port = immediate_upstream_port;
+	while (fec_port) {
+		/*
+		 * Each physical link (i.e. not a virtual port) between the
+		 * output and the primary device must support FEC
+		 */
+		if (!drm_dp_mst_is_virtual_dpcd(fec_port) &&
+				!fec_port->fec_capable)
+			return NULL;
+
+		fec_port = fec_port->parent->port_parent;
+	}
+
+	/* DP-to-DP peer device */
+	if (drm_dp_mst_is_virtual_dpcd(immediate_upstream_port)) {
+		u8 upstream_dsc;
+		u8 endpoint_dsc;
+		u8 endpoint_fec;
+
+		if (drm_dp_dpcd_read(&port->aux,
+				     DP_DSC_SUPPORT, &endpoint_dsc, 1) < 0)
+			return NULL;
+		if (drm_dp_dpcd_read(&port->aux,
+				     DP_FEC_CAPABILITY, &endpoint_fec, 1) < 0)
+			return NULL;
+		if (drm_dp_dpcd_read(&immediate_upstream_port->aux,
+				     DP_DSC_SUPPORT, &upstream_dsc, 1) < 0)
+			return NULL;
+
+		/* Enpoint decompression with DP-to-DP peer device */
+		if ((endpoint_dsc & DP_DSC_DECOMPRESSION_IS_SUPPORTED)
+				&& (endpoint_fec & DP_FEC_CAPABLE)
+				&& (upstream_dsc & 0x2) /* DSC passthrough */)
+			return &port->aux;
+
+		/* Virtual DPCD decompression with DP-to-DP peer device */
+		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;
+
+	/*
+	 * 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 false;
+
+	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;
+
+		drm_dp_dpcd_read(&port->aux, DP_DOWNSTREAMPORT_PRESENT,
+				 &downstreamport, 1);
+
+		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_mst_helper.h b/include/drm/drm_dp_mst_helper.h
index f113ae04fa88..4cf738545dfb 100644
--- a/include/drm/drm_dp_mst_helper.h
+++ b/include/drm/drm_dp_mst_helper.h
@@ -673,6 +673,8 @@  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);
 
+struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port);
+
 extern const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs;
 
 /**