[PATCH v3] usb: typec: displayport: Reject DP Alt Mode VDO with no pin assignment for its capability

[madhu . m]

From: Madhu M <madhu.m@xxxxxxxxx>

Some docks/Type-C dongles expose a malformed DP Capabilities VDO: they
claim a DFP_D (source) or UFP_D (sink) capability but leave the
corresponding pin assignment field empty. Such a device can never have
Alt Mode configured. Currently the driver still proceeds, which is
misleading and offers no diagnostic.

Per VESA DPAM v2.1a Section 5.4.1 (Table 5-6):
Case 1 (receptacle): A DP Source device receptacle (DFP_D) declares its
lane routing in the DP Source Pin field (Bits 15:8); a DP Sink device
receptacle (UFP_D) declares its lanes in the DP Sink Pin field
(Bits 23:16).
Case 2 (direct-attach plug): A DP Sink device plug (UFP_D) declares its
lane routing in the DP Source Pin field (Bits 15:8), and a DP Source
device plug (DFP_D) declares its lanes in the DP Sink Pin field
(Bits 23:16).
In either case the field holds the supported pin assignment values
(e.g. C/D/E); 00000000b means no pin assignment is supported for that
capability.

Reject such a DP Alt Mode VDO in dp_altmode_probe(): if the claimed
capability has no matching pin assignments, fail probe with -ENODEV,
releasing the SOP' plug reference on the error path.

Signed-off-by: Madhu M <madhu.m@xxxxxxxxx>
Reviewed-by: Andrei Kuchynski <akuchynski@xxxxxxxxxxxx>
Reviewed-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>
---
Changes in v3:
- No functional change. Restore the version changelog below the --- line.

Note: v1 was an internal review (with Heikki) and was never posted to the
list, so v2 was effectively the first public submission.

Changes in v2 (vs internal v1):
- Drop the explicit "if (alt->vdo & DP_CAP_RECEPTACLE)" guard and the
  nested block. Use the receptacle-aware DP_CAP_PIN_ASSIGN_DFP_D()/
  DP_CAP_PIN_ASSIGN_UFP_D() macros instead of the raw
  DP_CAP_DFP_D_PIN_ASSIGN()/DP_CAP_UFP_D_PIN_ASSIGN() ones. They already
  apply the Bit 6 (Receptacle Indication) swap, so a single check now
  covers both receptacles and direct-attach plugs.
- Return -ENODEV instead of -EOPNOTSUPP, matching the existing
  pin-configuration rejection path right above.
- Drop the dev_err() diagnostic line (raw VDO + "DFP_D"/"UFP_D" string).
- Hoist "cap" to a function-scope u32 at the top of dp_altmode_probe()
  (was a local u8 inside the receptacle block).
- Reword subject: "usb: typec: altmodes/displayport: validate dp pin
  assignments for USB-C receptacles" -> "usb: typec: displayport: Reject
  DP Alt Mode VDO with no pin assignment for its capability".
- Rewrite commit body: describe the VESA Table 5-6 pin-field routing for
  both receptacles and direct-attach plugs (including the 00000000b
  "not supported" encoding).

 drivers/usb/typec/altmodes/displayport.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/usb/typec/altmodes/displayport.c b/drivers/usb/typec/altmodes/displayport.c
index 263a89c5f324..2a553cfcf61d 100644
--- a/drivers/usb/typec/altmodes/displayport.c
+++ b/drivers/usb/typec/altmodes/displayport.c
@@ -764,6 +764,7 @@ int dp_altmode_probe(struct typec_altmode *alt)
 	struct typec_altmode *plug = typec_altmode_get_plug(alt, TYPEC_PLUG_SOP_P);
 	struct fwnode_handle *fwnode;
 	struct dp_altmode *dp;
+	u32 cap = DP_CAP_CAPABILITY(alt->vdo);
 
 	/* Port can only be DFP_U. */
 	if (typec_altmode_get_data_role(alt) != TYPEC_HOST)
@@ -778,6 +779,18 @@ int dp_altmode_probe(struct typec_altmode *alt)
 		return -ENODEV;
 	}
 
+	/*
+	 * Make sure the DisplayPort VDO is valid (VESA DPAM v2.1a, Section
+	 * 5.4.1, Table 5-6, DP Capabilities VDO). A device exposing DP on a
+	 * USB-C receptacle must advertise at least one pin assignment for the
+	 * capability it claims, otherwise Alt Mode can never be configured.
+	 */
+	if ((cap == DP_CAP_DFP_D && !DP_CAP_PIN_ASSIGN_DFP_D(alt->vdo)) ||
+	    (cap == DP_CAP_UFP_D && !DP_CAP_PIN_ASSIGN_UFP_D(alt->vdo))) {
+		typec_altmode_put_plug(plug);
+		return -ENODEV;
+	}
+
 	dp = devm_kzalloc(&alt->dev, sizeof(*dp), GFP_KERNEL);
 	if (!dp) {
 		typec_altmode_put_plug(plug);
-- 
2.34.1