linux-kernel - RE: [PATCH] usb: typec: tcpm: Extend the matching rules on PPS APDO selection

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Date:   Mon, 17 Dec 2018 12:23:32 +0000
From:   Adam Thomson <Adam.Thomson.Opensource@...semi.com>
To:     Kyle Tso <kyletso@...gle.com>,
        "linux@...ck-us.net" <linux@...ck-us.net>,
        "heikki.krogerus@...ux.intel.com" <heikki.krogerus@...ux.intel.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Adam Thomson <Adam.Thomson.Opensource@...semi.com>
CC:     "badhri@...gle.com" <badhri@...gle.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] usb: typec: tcpm: Extend the matching rules on PPS APDO
 selection

On 17 December 2018 02:48, Kyle Tso wrote:

> Current matching rules ensure that the voltage range of selected Source
> Capability is entirely within the range defined in one of the Sink Capabilities. This
> is reasonable but not practical because Sink may not support wide range of
> voltage when sinking power while Source could advertise its capabilities in
> raletively wider range. For example, a Source PDO advertising 3.3V-11V@3A (9V

relatively

> Prog of Fixed Nominal Voltage) will not be selected if the Sink requires 5V-
> 12V@3A PPS power. However, the Sink could work well if the requested voltage
> range in RDOs is 5V-11V@3A.
> 
> Currently accepted:
> 		|--------- source -----|
> 	|----------- sink ---------------|
> 
> Currently not accepted:
> 			|--------- source -----|
> 	|----------- sink ---------------|
> 
> 	|--------- source -----|
> 		|----------- sink ---------------|
> 
> 	|--------- source -----------------|
> 		|------ sink -------|
> 
> To improve the usability, change the matching rules to what listed
> below:
> a. The Source PDO is selectable if any portion of the voltage range
>    overlaps one of the Sink PDO's voltage range.
> b. The maximum operational voltage will be the lower one between the
>    selected Source PDO and the matching Sink PDO.
> c. The maximum power will be the maximum operational voltage times the
>    maximum current defined in the selected Source PDO d. Select the Source PDO
> with the highest maximum power
> 
> Signed-off-by: Kyle Tso <kyletso@...gle.com>
> 
> ---
> Changelog since v1:
> - updated the commit message as suggested by Guenter Roeck <linux@...ck-
> us.net>
> ---
>  drivers/usb/typec/tcpm/tcpm.c | 29 +++++++++++++++++------------
>  1 file changed, 17 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
> index 3620efee2688..3001df7bd602 100644
> --- a/drivers/usb/typec/tcpm/tcpm.c
> +++ b/drivers/usb/typec/tcpm/tcpm.c
> @@ -2213,7 +2213,8 @@ static unsigned int tcpm_pd_select_pps_apdo(struct
> tcpm_port *port)
>  	unsigned int i, j, max_mw = 0, max_mv = 0;
>  	unsigned int min_src_mv, max_src_mv, src_ma, src_mw;
>  	unsigned int min_snk_mv, max_snk_mv;
> -	u32 pdo;
> +	unsigned int max_op_mv;
> +	u32 pdo, src, snk;
>  	unsigned int src_pdo = 0, snk_pdo = 0;
> 
>  	/*
> @@ -2263,16 +2264,18 @@ static unsigned int tcpm_pd_select_pps_apdo(struct
> tcpm_port *port)
>  					continue;
>  				}
> 
> -				if (max_src_mv <= max_snk_mv &&
> -				    min_src_mv >= min_snk_mv) {
> +				if (min_src_mv <= max_snk_mv &&
> +				    max_src_mv >= min_snk_mv) {
> +					max_op_mv = min(max_src_mv,
> max_snk_mv);
> +					src_mw = (max_op_mv * src_ma) / 1000;
>  					/* Prefer higher voltages if available */
>  					if ((src_mw == max_mw &&
> -					     min_src_mv > max_mv) ||
> +					     max_op_mv > max_mv) ||
>  					    src_mw > max_mw) {

Sorry I didn't raise this before, but came to mind this morning when I was
considering your updates again. What happens if the Source validly provides two
PPS APDOs, for example:

	3.3V - 11V, 3A (9V programmable)
	3.3V - 16V, 3A (15V programmable)

and the sink APDO is:

	5V - 9V, 3A

I think the code here will now select the higher range (15V programmable) as it
gives a larger power output value, even if the sink can only support a voltage
that's far smaller. I really don't think this is correct. *If* you are going to
allow selection of PPS APDOs that provide a larger voltage range than the Sink
can actually cope with, then I think you should at least select the lower of
those advertised which fulfils the needs of the Sink.

>  						src_pdo = i;
>  						snk_pdo = j;
>  						max_mw = src_mw;
> -						max_mv = max_src_mv;
> +						max_mv = max_op_mv;
>  					}
>  				}
>  			}
> @@ -2285,14 +2288,16 @@ static unsigned int tcpm_pd_select_pps_apdo(struct
> tcpm_port *port)
>  	}
> 
>  	if (src_pdo) {
> -		pdo = port->source_caps[src_pdo];
> -
> -		port->pps_data.min_volt = pdo_pps_apdo_min_voltage(pdo);
> -		port->pps_data.max_volt = pdo_pps_apdo_max_voltage(pdo);
> -		port->pps_data.max_curr =
> -			min_pps_apdo_current(pdo, port->snk_pdo[snk_pdo]);
> +		src = port->source_caps[src_pdo];
> +		snk = port->snk_pdo[snk_pdo];
> +
> +		port->pps_data.min_volt =
> max(pdo_pps_apdo_min_voltage(src),
> +					      pdo_pps_apdo_min_voltage(snk));
> +		port->pps_data.max_volt =
> min(pdo_pps_apdo_max_voltage(src),
> +					      pdo_pps_apdo_max_voltage(snk));
> +		port->pps_data.max_curr = min_pps_apdo_current(src, snk);
>  		port->pps_data.out_volt =
> -			min(pdo_pps_apdo_max_voltage(pdo), port-
> >pps_data.out_volt);
> +			min(port->pps_data.max_volt, port-
> >pps_data.out_volt);
>  		port->pps_data.op_curr =
>  			min(port->pps_data.max_curr, port->pps_data.op_curr);
>  	}
> --
> 2.20.0.405.gbc1bbc6f85-goog