Re: [PATCH v3 3/4] hwmon: (ltc2945): add support for sense resistor

From: Alexandru Ardelean
Date: Thu Jan 07 2021 - 10:45:42 EST

On Thu, Jan 7, 2021 at 5:25 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>
> On Thu, Jan 07, 2021 at 12:34:16PM +0200, Alexandru Ardelean wrote:
> > The sense resistor is a parameter of the board. It should be configured in
> > the driver via a device-tree / ACPI property, so that the proper current
> > measurements can be done in the driver.
> >
> > It shouldn't be necessary that userspace need to know about the value of
> > the resistor. It makes things a bit harder to make the application code
> > portable from one board to another.
> >
> > This change implements support for the sense resistor to be configured from
> > DT/ACPI and used in current calculations.
> > Also, the maximum power and current that can be represented by the driver
> > are scaled with the value of the sense resistor.
> >
>
> In a way you are correct, but that applies to all hwmon
> drivers, not just to this one. A solution which only applies
> to a single driver doesn't solve the underlying problem,
> which would be the desire or need to provide kernel-based
> scaling for hwmon drivers. As such, I am not inclined to accept
> this patch. We should think about a generic solution instead.

I agree that scaling is an issue with hwmon [seeing as how IIO does it already].
Maybe I can adapt some things from IIO.
But I can't promise/commit to it quite yet.

>
> Note that this patch doesn't compile on 32-bit targets.

Yeah, my bad.
I only tested with make allmodconfig, and that doesn't do a good job
at providing linker issues.

>
> Thanks,
> Guenter
>
> > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>
> > ---
> > drivers/hwmon/ltc2945.c | 60 ++++++++++++++++++++++++++---------------
> > 1 file changed, 39 insertions(+), 21 deletions(-)
> >
> > diff --git a/drivers/hwmon/ltc2945.c b/drivers/hwmon/ltc2945.c
> > index 41df2c8b7673..e60b15832b0e 100644
> > --- a/drivers/hwmon/ltc2945.c
> > +++ b/drivers/hwmon/ltc2945.c
> > @@ -70,9 +70,15 @@
> > /**
> > * struct ltc2945_state - driver instance specific data
> > * @regmap: regmap object to access device registers
> > + * @max_power_uw: maximum power that can be represented based on sense resistor
> > + * @max_current_ma: maximum current that can be represented based on sense resistor
> > + * @r_sense_mohm: current sense resistor value
> > */
> > struct ltc2945_state {
> > struct regmap *regmap;
> > + u32 max_power_uw;
> > + u32 max_current_ma;
> > + u32 r_sense_mohm;
> > };
> >
> > static inline bool is_power_reg(u8 reg)
> > @@ -110,9 +116,8 @@ static long long ltc2945_reg_to_val(struct device *dev, u8 reg)
> > case LTC2945_MAX_POWER_THRES_H:
> > case LTC2945_MIN_POWER_THRES_H:
> > /*
> > - * Convert to uW by assuming current is measured with
> > - * an 1mOhm sense resistor, similar to current
> > - * measurements.
> > + * Convert to uW by and scale it with the configured
> > + * sense resistor, similar to current measurements.
> > * Control register bit 0 selects if voltage at SENSE+/VDD
> > * or voltage at ADIN is used to measure power.
> > */
> > @@ -126,6 +131,7 @@ static long long ltc2945_reg_to_val(struct device *dev, u8 reg)
> > /* 0.5 mV * 25 uV = 0.0125 uV resolution. */
> > val = (val * 25LL) >> 1;
> > }
> > + val /= st->r_sense_mohm;
> > break;
> > case LTC2945_VIN_H:
> > case LTC2945_MAX_VIN_H:
> > @@ -149,13 +155,11 @@ static long long ltc2945_reg_to_val(struct device *dev, u8 reg)
> > case LTC2945_MAX_SENSE_THRES_H:
> > case LTC2945_MIN_SENSE_THRES_H:
> > /*
> > - * 25 uV resolution. Convert to current as measured with
> > - * an 1 mOhm sense resistor, in mA. If a different sense
> > - * resistor is installed, calculate the actual current by
> > - * dividing the reported current by the sense resistor value
> > - * in mOhm.
> > + * 25 uV resolution. Convert to current and scale it
> > + * with the value of the sense resistor.
> > */
> > val *= 25;
> > + val /= st->r_sense_mohm;
> > break;
> > default:
> > return -EINVAL;
> > @@ -163,7 +167,8 @@ static long long ltc2945_reg_to_val(struct device *dev, u8 reg)
> > return val;
> > }
> >
> > -static unsigned long ltc2945_val_clamp(u8 reg, unsigned long val)
> > +static unsigned long ltc2945_val_clamp(struct ltc2945_state *st, u8 reg,
> > + unsigned long val)
> > {
> > switch (reg) {
> > case LTC2945_POWER_H:
> > @@ -171,8 +176,7 @@ static unsigned long ltc2945_val_clamp(u8 reg, unsigned long val)
> > case LTC2945_MIN_POWER_H:
> > case LTC2945_MAX_POWER_THRES_H:
> > case LTC2945_MIN_POWER_THRES_H:
> > - /* No sense in clamping now, LTC2945_POWER_FULL_SCALE_UW is larger than UINT32_MAX */
> > - return val;
> > + return clamp_val(val, 0, st->max_power_uw);
> > case LTC2945_VIN_H:
> > case LTC2945_MAX_VIN_H:
> > case LTC2945_MIN_VIN_H:
> > @@ -190,7 +194,7 @@ static unsigned long ltc2945_val_clamp(u8 reg, unsigned long val)
> > case LTC2945_MIN_SENSE_H:
> > case LTC2945_MAX_SENSE_THRES_H:
> > case LTC2945_MIN_SENSE_THRES_H:
> > - return clamp_val(val, 0, LTC2945_SENSE_FULL_SCALE_MA);
> > + return clamp_val(val, 0, st->max_current_ma);
> > default:
> > /*
> > * This is unlikely to happen, and if it does, it should
> > @@ -215,9 +219,8 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> > case LTC2945_MAX_POWER_THRES_H:
> > case LTC2945_MIN_POWER_THRES_H:
> > /*
> > - * Convert to register value by assuming current is measured
> > - * with an 1mOhm sense resistor, similar to current
> > - * measurements.
> > + * Convert to register value, scale it with the configured sense
> > + * resistor value, similar to current measurements.
> > * Control register bit 0 selects if voltage at SENSE+/VDD
> > * or voltage at ADIN is used to measure power, which in turn
> > * determines register calculations.
> > @@ -236,6 +239,7 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> > */
> > val = DIV_ROUND_CLOSEST(val, 25) * 2;
> > }
> > + val *= st->r_sense_mohm;
> > break;
> > case LTC2945_VIN_H:
> > case LTC2945_MAX_VIN_H:
> > @@ -259,12 +263,10 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> > case LTC2945_MAX_SENSE_THRES_H:
> > case LTC2945_MIN_SENSE_THRES_H:
> > /*
> > - * 25 uV resolution. Convert to current as measured with
> > - * an 1 mOhm sense resistor, in mA. If a different sense
> > - * resistor is installed, calculate the actual current by
> > - * dividing the reported current by the sense resistor value
> > - * in mOhm.
> > + * 25 uV resolution. Convert to current and scale it
> > + * with the value of the sense resistor, in mA.
> > */
> > + val *= st->r_sense_mohm;
> > val = DIV_ROUND_CLOSEST(val, 25);
> > break;
> > default:
> > @@ -303,7 +305,7 @@ static ssize_t ltc2945_value_store(struct device *dev,
> > if (ret)
> > return ret;
> >
> > - val = ltc2945_val_clamp(reg, val);
> > + val = ltc2945_val_clamp(st, reg, val);
> >
> > /* convert to register value, then clamp and write result */
> > regval = ltc2945_val_to_reg(dev, reg, val);
> > @@ -512,6 +514,7 @@ static int ltc2945_probe(struct i2c_client *client)
> > struct ltc2945_state *st;
> > struct device *hwmon_dev;
> > struct regmap *regmap;
> > + u64 val64;
> >
> > st = devm_kzalloc(dev, sizeof(*st), GFP_KERNEL);
> > if (!st)
> > @@ -523,7 +526,22 @@ static int ltc2945_probe(struct i2c_client *client)
> > return PTR_ERR(regmap);
> > }
> >
> > + if (device_property_read_u32(dev, "shunt-resistor-micro-ohms",
> > + &st->r_sense_mohm))
> > + st->r_sense_mohm = 1000;
> > +
> > + if (st->r_sense_mohm < 1000) {
> > + dev_err(dev, "Value too small for sense resistor, minimum 1000\n");
> > + return -EINVAL;
> > + }
> > + st->r_sense_mohm /= 1000;
> > +
> > st->regmap = regmap;
> > + val64 = LTC2945_POWER_FULL_SCALE_UW / st->r_sense_mohm;
> > + /* clamp power to ULONG_MAX, since we represent it on 32 bits */
> > + st->max_power_uw = clamp_val(val64, 0, ULONG_MAX);
> > +
> > + st->max_current_ma = LTC2945_SENSE_FULL_SCALE_MA / st->r_sense_mohm;
> >
> > /* Clear faults */
> > regmap_write(regmap, LTC2945_FAULT, 0x00);
> > --
> > 2.17.1
> >