Re: [PATCH v2] usb: typec: tcpm: Correct the PDO counting in pd_set

[Heikki Krogerus <heikki.krogerus@linux.intel.com>]

On Tue, Mar 26, 2024 at 11:19:09PM +0800, Kyle Tso wrote:
> Off-by-one errors happen because nr_snk_pdo and nr_src_pdo are
> incorrectly added one. The index of the loop is equal to the number of
> PDOs to be updated when leaving the loop and it doesn't need to be added
> one.
> 
> When doing the power negotiation, TCPM relies on the "nr_snk_pdo" as
> the size of the local sink PDO array to match the Source capabilities
> of the partner port. If the off-by-one overflow occurs, a wrong RDO
> might be sent and unexpected power transfer might happen such as over
> voltage or over current (than expected).
> 
> "nr_src_pdo" is used to set the Rp level when the port is in Source
> role. It is also the array size of the local Source capabilities when
> filling up the buffer which will be sent as the Source PDOs (such as
> in Power Negotiation). If the off-by-one overflow occurs, a wrong Rp
> level might be set and wrong Source PDOs will be sent to the partner
> port. This could potentially cause over current or port resets.
> 
> Fixes: cd099cde4ed2 ("usb: typec: tcpm: Support multiple capabilities")
> Cc: stable@vger.kernel.org
> Signed-off-by: Kyle Tso <kyletso@google.com>

Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>

> ---
> v1 -> v2:
> - update the commit message (adding the problems this patch solves)
> 
>  drivers/usb/typec/tcpm/tcpm.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
> index ae2b6c94482d..2464710ea0c8 100644
> --- a/drivers/usb/typec/tcpm/tcpm.c
> +++ b/drivers/usb/typec/tcpm/tcpm.c
> @@ -6855,14 +6855,14 @@ static int tcpm_pd_set(struct typec_port *p, struct usb_power_delivery *pd)
>  	if (data->sink_desc.pdo[0]) {
>  		for (i = 0; i < PDO_MAX_OBJECTS && data->sink_desc.pdo[i]; i++)
>  			port->snk_pdo[i] = data->sink_desc.pdo[i];
> -		port->nr_snk_pdo = i + 1;
> +		port->nr_snk_pdo = i;
>  		port->operating_snk_mw = data->operating_snk_mw;
>  	}
>  
>  	if (data->source_desc.pdo[0]) {
>  		for (i = 0; i < PDO_MAX_OBJECTS && data->source_desc.pdo[i]; i++)
>  			port->snk_pdo[i] = data->source_desc.pdo[i];
> -		port->nr_src_pdo = i + 1;
> +		port->nr_src_pdo = i;
>  	}
>  
>  	switch (port->state) {
> -- 
> 2.44.0.396.g6e790dbe36-goog

-- 
heikki