[PATCH v1] usb: typec: altmodes/displayport: verify compatible source/sink role combination

[PATCH v1] usb: typec: altmodes/displayport: verify compatible source/sink role combination

[RD Babiera]

DisplayPort Alt Mode CTS test 10.3.8 states that both sides of the
connection shall be compatible with one another such that the connection
is not Source to Source or Sink to Sink.

The DisplayPort driver currently checks for a compatible pin configuration
that resolves into a source and sink combination. The CTS test is designed
to send a Discover Modes message that has a compatible pin configuration
but advertises the same port capability as the device; the current check
fails this.

Verify that the port and port partner resolve into a valid source and sink
combination before checking for a compatible pin configuration.

Fixes: 0e3bb7d6894d ("usb: typec: Add driver for DisplayPort alternate mode")
Cc: stable@vger.kernel.org
Signed-off-by: RD Babiera <rdbabiera@google.com>
---
 drivers/usb/typec/altmodes/displayport.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/usb/typec/altmodes/displayport.c b/drivers/usb/typec/altmodes/displayport.c
index 718da02036d8..3b35a6b8cb72 100644
--- a/drivers/usb/typec/altmodes/displayport.c
+++ b/drivers/usb/typec/altmodes/displayport.c
@@ -575,9 +575,18 @@ int dp_altmode_probe(struct typec_altmode *alt)
 	struct fwnode_handle *fwnode;
 	struct dp_altmode *dp;
 	int ret;
+	int port_cap, partner_cap;
 
 	/* FIXME: Port can only be DFP_U. */
 
+	/* Make sure that the port and partner can resolve into source and sink */
+	port_cap = DP_CAP_CAPABILITY(port->vdo);
+	partner_cap = DP_CAP_CAPABILITY(alt->vdo);
+	if (!((port_cap & DP_CAP_DFP_D) && (partner_cap & DP_CAP_UFP_D)) &&
+	    !((port_cap & DP_CAP_UFP_D) && (partner_cap & DP_CAP_DFP_D))) {
+		return -ENODEV;
+	}
+
 	/* Make sure we have compatiple pin configurations */
 	if (!(DP_CAP_PIN_ASSIGN_DFP_D(port->vdo) &
 	      DP_CAP_PIN_ASSIGN_UFP_D(alt->vdo)) &&

base-commit: d0d27ef87e1ca974ed93ed4f7d3c123cbd392ba6
-- 
2.42.0.655.g421f12c284-goog

Re: [PATCH v1] usb: typec: altmodes/displayport: verify compatible source/sink role combination

[Prashant Malani]

Hi RD,

On Mon, Oct 16, 2023 at 3:56 PM RD Babiera <rdbabiera@google.com> wrote:
>
> DisplayPort Alt Mode CTS test 10.3.8 states that both sides of the
> connection shall be compatible with one another such that the connection
> is not Source to Source or Sink to Sink.
>
> The DisplayPort driver currently checks for a compatible pin configuration
> that resolves into a source and sink combination. The CTS test is designed
> to send a Discover Modes message that has a compatible pin configuration
> but advertises the same port capability as the device; the current check
> fails this.
>
> Verify that the port and port partner resolve into a valid source and sink
> combination before checking for a compatible pin configuration.
>
> Fixes: 0e3bb7d6894d ("usb: typec: Add driver for DisplayPort alternate mode")
> Cc: stable@vger.kernel.org
> Signed-off-by: RD Babiera <rdbabiera@google.com>
> ---
>  drivers/usb/typec/altmodes/displayport.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/drivers/usb/typec/altmodes/displayport.c b/drivers/usb/typec/altmodes/displayport.c
> index 718da02036d8..3b35a6b8cb72 100644
> --- a/drivers/usb/typec/altmodes/displayport.c
> +++ b/drivers/usb/typec/altmodes/displayport.c
> @@ -575,9 +575,18 @@ int dp_altmode_probe(struct typec_altmode *alt)
>         struct fwnode_handle *fwnode;
>         struct dp_altmode *dp;
>         int ret;
> +       int port_cap, partner_cap;

VDOs are 32-bit, so u32 is probably better here.

>
>         /* FIXME: Port can only be DFP_U. */
>
> +       /* Make sure that the port and partner can resolve into source and sink */
> +       port_cap = DP_CAP_CAPABILITY(port->vdo);
> +       partner_cap = DP_CAP_CAPABILITY(alt->vdo);
> +       if (!((port_cap & DP_CAP_DFP_D) && (partner_cap & DP_CAP_UFP_D)) &&

nit: bitwise '&' has a higher precedence than logical '&&', so the
innermost parentheses shouldn't be necessary:

           if (!(port_cap & DP_CAP_DFP_D && partner_cap & DP_CAP_UFP_D) &&
               !(port_cap & DP_CAP_UFP_D && partner_cap & DP_CAP_DFP_D))
                   return -ENODEV;
                ...

OTOH, perhaps you should just introduce a macro that performs this
bitwise operation for even better
readability. Something like

#define DP_CAP_IS_DFP_D(_cap_)        (!!(DP_CAP_CAPABILITY(_cap_) &
DP_CAP_DFP_D))

(not sure if "!!" is tolerated in kernel style, but you get the gist...)


> +           !((port_cap & DP_CAP_UFP_D) && (partner_cap & DP_CAP_DFP_D))) {
> +               return -ENODEV;

Single line if statements can drop curly braces [1]

Best regards,

-Prashant

[1] https://www.kernel.org/doc/html/v4.10/process/coding-style.html#placing-braces-and-spaces

Re: [PATCH v1] usb: typec: altmodes/displayport: verify compatible source/sink role combination

[RD Babiera]

On Tue, Oct 17, 2023 at 2:33 PM Prashant Malani <pmalani@chromium.org> wrote:
> OTOH, perhaps you should just introduce a macro that performs this
> bitwise operation for even better
> readability. Something like
>
> #define DP_CAP_IS_DFP_D(_cap_)        (!!(DP_CAP_CAPABILITY(_cap_) &
> DP_CAP_DFP_D))

I'll take this approach, thanks for the feedback Prashant!

---

Best,
RD