Re: [PATCH v5 7/8] iio: light: ROHM BU27034 Ambient Light Sensor

[Jonathan Cameron <jic23@kernel.org>]

On Wed, 22 Mar 2023 11:07:56 +0200
Matti Vaittinen <mazziesaccount@gmail.com> wrote:

> ROHM BU27034 is an ambient light sensor with 3 channels and 3 photo diodes
> capable of detecting a very wide range of illuminance. Typical application
> is adjusting LCD and backlight power of TVs and mobile phones.
> 
> Add initial  support for the ROHM BU27034 ambient light sensor.
> 
> NOTE:
> 	- Driver exposes 4 channels. One IIO_LIGHT channel providing the
> 	  calculated lux values based on measured data from diodes #0 and
> 	  #1. In addition, 3 IIO_INTENSITY channels are emitting the raw
> 	  register data from all diodes for more intense user-space
> 	  computations.
> 	- Sensor has GAIN values that can be adjusted from 1x to 4096x.
> 	- Sensor has adjustible measurement times of 5, 55, 100, 200 and
> 	  400 mS. Driver does not support 5 mS which has special
> 	  limitations.
> 	- Driver exposes standard 'scale' adjustment which is
> 	  implemented by:
> 		1) Trying to adjust only the GAIN
> 		2) If GAIN adjustment alone can't provide requested
> 		   scale, adjusting both the time and the gain is
> 		   attempted.
> 	- Driver exposes writable INT_TIME property that can be used
> 	  for adjusting the measurement time. Time adjustment will also
> 	  cause the driver to try to adjust the GAIN so that the
> 	  overall scale is kept as close to the original as possible.
> 
> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
> 
Hi Matti,

A few minor comments inline.  I'll take a closer look at the rest of the
series when the discussions around the tests and devices to be used
for them settle down.

Thanks,

Jonathan

> +
> +static u64 bu27034_fixp_calc_t1(unsigned int coeff, unsigned int ch0,
> +				unsigned int ch1, unsigned int gain0,
> +				unsigned int gain1)
> +{
> +	unsigned int helper, tmp;
> +	u64 helper64;
> +
> +	/*
> +	 * Here we could overflow even the 64bit value. Hence we
> +	 * multiply with gain0 only after the divisions - even though
> +	 * it may result loss of accuracy
> +	 */
> +	helper64 = (u64)coeff * (u64)ch1 * (u64)ch1;
> +	helper = coeff * ch1 * ch1;
> +	tmp = helper * gain0;
> +
> +	if (helper == helper64 && (tmp / gain0 == helper))

Similar to below.  Don't bother with the non 64 bit version.

> +		return tmp / (gain1 * gain1) / ch0;
> +
> +	helper = gain1 * gain1;
> +	if (helper > ch0) {
> +		do_div(helper64, helper);
> +
> +		return gain_mul_div_helper(helper64, gain0, ch0);
> +	}
> +
> +	do_div(helper64, ch0);
> +
> +	return gain_mul_div_helper(helper64, gain0, helper);
> +}
> +
> +static u64 bu27034_fixp_calc_t23(unsigned int coeff, unsigned int ch,
> +				 unsigned int gain)
> +{
> +	unsigned int helper;
> +	u64 helper64;
> +
> +	helper64 = (u64)coeff * (u64)ch;
> +	helper = coeff * ch;
> +
> +	if (helper == helper64)
> +		return helper / gain;
> +
> +	do_div(helper64, gain);
> +
> +	return helper64;

I suspect that this is a premature bit of optimization so I'd just
do it in 64 bits always.

Also, if you did want to do this, check_mul_overflow() etc would help.
(linux/overflow.h)


> +}

> +
> +static int bu27034_calc_mlux(struct bu27034_data *data, __le16 *res, int *val)
> +{
> +	unsigned int gain0, gain1, meastime;
> +	unsigned int d1_d0_ratio_scaled;
> +	u16  ch0, ch1;

Stray space after the u16

> +	u64 helper64;
> +	int ret;
> +
> +	/*
> +	 * We return 0 luxes if calculation fails. This should be reasonably

0 lux 
(I think)

> +	 * easy to spot from the buffers especially if raw-data channels show
> +	 * valid values
> +	 */