linux-kernel - Re: [PATCH v2 3/3] usb: typec: tcpm: add get max power support

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Message-ID: <20230321050048.3p7xtyikf2lkqwrg@mercury.elektranox.org>
Date:   Tue, 21 Mar 2023 06:00:48 +0100
From:   Sebastian Reichel <sebastian.reichel@...labora.com>
To:     Frank Wang <frank.wang@...k-chips.com>
Cc:     linux@...ck-us.net, heikki.krogerus@...ux.intel.com,
        gregkh@...uxfoundation.org, heiko@...ech.de,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-rockchip@...ts.infradead.org, huangtao@...k-chips.com,
        william.wu@...k-chips.com, jianwei.zheng@...k-chips.com,
        yubing.zhang@...k-chips.com, wmc@...k-chips.com
Subject: Re: [PATCH v2 3/3] usb: typec: tcpm: add get max power support

Hi,

On Tue, Mar 21, 2023 at 09:32:53AM +0800, Frank Wang wrote:
> On 2023/3/21 4:31, Sebastian Reichel wrote:
> > On Mon, Mar 20, 2023 at 06:07:11PM +0800, Frank Wang wrote:
> > > Traverse fixed pdos to calculate the maximum power that the charger
> > > can provide, and it can be get by POWER_SUPPLY_PROP_INPUT_POWER_LIMIT
> > > property.
> > > 
> > > Signed-off-by: Frank Wang <frank.wang@...k-chips.com>
> > > ---
> > >   drivers/usb/typec/tcpm/tcpm.c | 24 ++++++++++++++++++++++++
> > >   1 file changed, 24 insertions(+)
> > > 
> > > diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
> > > index 13830b5e2d09f..d6ad3cdf9e4af 100644
> > > --- a/drivers/usb/typec/tcpm/tcpm.c
> > > +++ b/drivers/usb/typec/tcpm/tcpm.c
> > > @@ -6320,6 +6320,27 @@ static int tcpm_psy_get_current_now(struct tcpm_port *port,
> > >   	return 0;
> > >   }
> > > +static int tcpm_psy_get_input_power_limit(struct tcpm_port *port,
> > > +					  union power_supply_propval *val)
> > > +{
> > > +	unsigned int src_mv, src_ma, max_src_mw = 0;
> > > +	unsigned int i, tmp;
> > > +
> > > +	for (i = 0; i < port->nr_source_caps; i++) {
> > > +		u32 pdo = port->source_caps[i];
> > > +
> > > +		if (pdo_type(pdo) == PDO_TYPE_FIXED) {
> > > +			src_mv = pdo_fixed_voltage(pdo);
> > > +			src_ma = pdo_max_current(pdo);
> > > +			tmp = src_mv * src_ma / 1000;
> > > +			max_src_mw = tmp > max_src_mw ? tmp : max_src_mw;
> > > +		}
> > > +	}
> > > +
> > > +	val->intval = max_src_mw;
> > The power-supply subsystem expects Microwatts and not Milliwatts.
> 
> Yes, but I see the 'power_supply_propval' member 'intval' is an integer
> type, I worry about it may be overflowed that uses Microwatts.

Data being encoded in Microwatts is part of the ABI. The data
you are supplying will be interpreted in µW. If you submit your
data in mW it is basically always wrong even without an overflow.

Now regarding the overflow: A signed int can store 2^31 bit, so
2,147,483,648 µW = 2147 W. Looking at your code you effectively
calculate Microwatts in an unsigned int and then divide by 1000.
Since the intermediate value (before dividing by 1000) needs to be
stored you gain only one bit. That raises the question: Why do you
expect data to be between 2147 W and 4294 W when the latest released
USB PD spec allows 5A@48V = 240W?

-- Sebastian

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