Re: [PATCH v6 2/7] iio: light: Add gain-time-scale helpers

[Matti Vaittinen <mazziesaccount@gmail.com>]

On 3/30/23 19:48, Matti Vaittinen wrote:
> On 3/27/23 14:28, Matti Vaittinen wrote:
>> Some light sensors can adjust both the HW-gain and integration time.
>> There are cases where adjusting the integration time has similar impact
>> to the scale of the reported values as gain setting has.
>>
>> IIO users do typically expect to handle scale by a single writable 
>> 'scale'
>> entry. Driver should then adjust the gain/time accordingly.
>>
>> It however is difficult for a driver to know whether it should change
>> gain or integration time to meet the requested scale. Usually it is
>> preferred to have longer integration time which usually improves
>> accuracy, but there may be use-cases where long measurement times can be
>> an issue. Thus it can be preferable to allow also changing the
>> integration time - but mitigate the scale impact by also changing the 
>> gain
>> underneath. Eg, if integration time change doubles the measured values,
>> the driver can reduce the HW-gain to half.
>>
>> The theory of the computations of gain-time-scale is simple. However,
>> some people (undersigned) got that implemented wrong for more than once.
>>
>> Add some gain-time-scale helpers in order to not dublicate errors in all
>> drivers needing these computations.
>>
>> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
>>
>> ---
>> Currently it is only BU27034 using these in this series. I am however 
>> working
>> with drivers for RGB sensors BU27008 and BU27010 which have similar
>> [gain - integration time - scale] - relation. I hope sending those
>> follows soon after the BU27034 is done.
>>
> 
>> +/**
>> + * iio_gts_find_new_gain_sel_by_old_gain_time - compensate for time 
>> change
>> + * @gts:        Gain time scale descriptor
>> + * @old_gain:        Previously set gain
>> + * @old_time_sel:    Selector corresponding previously set time
>> + * @new_time_sel:    Selector corresponding new time to be set
>> + * @new_gain:        Pointer to value where new gain is to be written
>> + *
>> + * We may want to mitigate the scale change caused by setting a new 
>> integration
>> + * time (for a light sensor) by also updating the (HW)gain. This 
>> helper computes
>> + * new gain value to maintain the scale with new integration time.
>> + *
>> + * Return: 0 on success. -EINVAL if gain matching the new time is not 
>> found.
> 
> Here we need to document another return value denote whether the 
> @new_gain was updated.
> 
>> + */
>> +int iio_gts_find_new_gain_sel_by_old_gain_time(struct iio_gts *gts,
>> +                           int old_gain, int old_time_sel,
>> +                           int new_time_sel, int *new_gain)
>> +{
>> +    const struct iio_itime_sel_mul *itime_old, *itime_new;
>> +    u64 scale;
>> +    int ret;
>> +
>> +    itime_old = iio_gts_find_itime_by_sel(gts, old_time_sel);
>> +    if (!itime_old)
>> +        return -EINVAL;
>> +
>> +    itime_new = iio_gts_find_itime_by_sel(gts, new_time_sel);
>> +    if (!itime_new)
>> +        return -EINVAL;
>> +
>> +    ret = iio_gts_get_scale_linear(gts, old_gain, itime_old->time_us,
>> +                       &scale);
>> +    if (ret)
>> +        return ret;
>> +
>> +    ret = gain_get_scale_fraction(gts->max_scale, scale, itime_new->mul,
>> +                      new_gain);
>> +    if (ret)
>> +        return ret;
>> +
>> +    if (!iio_gts_valid_gain(gts, *new_gain))
>> +        return -EINVAL;
> 
> I would change this to -ERANGE to differentiate the case where the new 
> gain was computed but was not valid. The bu27034 (and 
> not-yet-fully-finished bu27008) driver uses the computed gain to find 
> closest matching gain the hardware supports. I am not super happy with 
> the -ERANGE, as it is also possible the gain is in the "range" of 
> supported gains but not _exactly_ supported one. In a sense -EINVAL 
> would be more correct. The invalid time could in a sense be interpreted 
> as an "time selector not found" - so maybe the -ENOENT could be somehow 
> tolerated. Still, in my opinion the invalid integration time is very 
> much more an -EINVAL than anything else...

Looks like I keep discussing with myself. This however was not a good 
solution as we might detect non integer gain to be required in the 
gain_get_scale_fraction(). And deciding if that function should return 
-ERANGE or -EINVAL got things even worse.

So, the take N (where N is a positive integer, much greater than 1) is 
that I'll do:

int iio_gts_find_new_gain_sel_by_old_gain_time(struct iio_gts *gts,
                            int old_gain, int old_time_sel,
                            int new_time_sel, int *new_gain)
{
     const struct iio_itime_sel_mul *itime_old, *itime_new;
     u64 scale;
     int ret;

     *new_gain = -1;

and add return value doc like:

  * Return: 0 if an exactly matching supported new gain was found. When 
a
  * non-zero value is returned, the @new_gain will be set to a negative or
  * positive value. The negative value means that no gain could be computed.
  * Positive value will be the "best possible new gain there could be". 
There
  * can be two reasons why finding the "best possible" new gain is not 
deemed
  * successful. 1) This new value cannot be supported by the hardware. 
2) The new
  * gain required to maintain the scale would not be an integer. In this 
case,
  * the "best possible" new gain will be a floored optimal gain, which 
may or
  * may not be supported by the hardware.

> 
> I will fix this for v7.
> 

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

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