Re: [PATCH v11] iio: adc: pac1921: Add ACPI support to Microchip pac1921

[Matteo Martelli <matteomartelli3@gmail.com>]

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On Thu, 14 Nov 2024 12:52:12 +0000, <Victor.Duicu@microchip.com> wrote:
> On Thu, 2024-11-14 at 12:00 +0100, Matteo Martelli wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you
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> > 
> 
> Hi Matteo,
> 
> > On Thu, 14 Nov 2024 10:47:02 +0200, <victor.duicu@microchip.com>
> > wrote:
> > > From: Victor Duicu <victor.duicu@microchip.com>
> > > 
> > > This patch implements ACPI support to Microchip pac1921.
> > > The driver can read the shunt resistor value and label from the
> > > ACPI table.
> > > 
> > > Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
> > > Signed-off-by: Victor Duicu <victor.duicu@microchip.com>
> > > ---
> > > 
> 
> ....
> 
> > > 
> > > 
> > > +#define PAC1921_ACPI_GET_uOHMS_VALS             0
> > > +#define PAC1921_ACPI_GET_LABEL                       1
> > > +/*
> > > + * The maximum acceptable shunt value is 2146.999999 OHM.
> > > + * This value, which is below INT_MAX, was chosen in order to
> > > + * allow the readings from dt and ACPI to share the same range
> > > + * and to simplify the checks.
> > > + * With this value the maximum current that can be read is
> > > + * 0.1V / 2146.999999OHM = 46.576 uA
> > > + * If we use INT_MAX the maximum current that can be read is
> > > + * 0.1V / 2147.483647OHM = 46.566 uA
> > > + * The relative error between the two values is
> > > + * |(46.566 - 46.576) / 46.566| * 100 = 0.0214
> > > + */
> > > +#define PAC1921_MAX_SHUNT_VALUE_uOHMS                2146999999UL
> > > +
> > 
> > Just a minor point about this: if I understand correctly that value
> > comes from (INT_MAX / MICRO - 1) * MICRO + MAX_MICRO. This was to
> > simplify the check in a single statement in
> > pac1921_write_shunt_resistor()
> > which is called when the shunt resistor is set from *sysfs* (neither
> > from DT nor ACPI). I'm fine with this value and the new check but I
> > find
> > the explanation comment a bit confusing. If you could come up with a
> > bit
> > more clear explanation about the reason of such value I think it
> > would be
> > better otherwise I am fine with it as it is. Also, maybe use the full
> > room
> > for 80 characters per line and UOHMS instead of uOHMS to avoid mixed
> > case if
> > you are going with a new version.
> 
> We could completely remove the need to use a constant below INT_MAX
> with this check in pac1921_write_shunt_resistor:
> 
> if ((!val && !val_fract) || 
> ((val >= INT_MAX / MICRO) && (val_fract > INT_MAX % MICRO)))
> 	return -EINVAL;
> 
> Do you agree with this approach?

Yes, something like this would be clearer to me.

Anyway, I think you also need to check for val > INT_MAX / MICRO when
val_fract is < INT_MAX % MICRO, right?

Also, I think you can remove a couple of parenthesis.

So something like the following maybe (but please double check it):

	if ((!val && !val_fract) || val > INT_MAX / MICRO ||
	   (val == INT_MAX / MICRO && val_fract > INT_MAX % MICRO))

I think that usually it would be better to use pre-computed constants
instead of run-time divisions for efficiency but since the shunt
resistor is likely going to be set rarely, I would go for this code for
better clarity.


> Also, the use of mixed case was suggested by Andy to increase
> readability.

Ah, sorry for missing Andy's comment. I am fine with it if you also find
it more readable.

> 
> ...
> 
> > 
> > 
> > Best regards,
> > Matteo Martelli
> 
> With Best Regards,
> Duicu Victor

Thanks,
Matteo Martelli