Messages in this thread | | | Date | Fri, 31 Mar 2023 10:39:32 +0300 | From | Matti Vaittinen <> | Subject | Re: [PATCH v6 2/7] iio: light: Add gain-time-scale helpers |
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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
~~ When things go utterly wrong vim users can always type :help! ~~
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