Messages in this thread | | | Date | Thu, 30 Mar 2023 19:48:06 +0300 | From | Matti Vaittinen <> | Subject | Re: [PATCH v6 2/7] iio: light: Add gain-time-scale helpers |
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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...
> + > + return 0; > +}
I will fix this for v7.
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