On 21, April 2018 19:07, Jonathan Cameron wrote:
> On Fri, 20 Apr 2018 21:31:09 +0200
> David Veenstra <davidjulianveenstra@xxxxxxxxx> wrote:
>
>> The sysfs iio ABI states radians per second is expected as the unit for
>> angular velocity, but the 12-bit angular velocity register has rps
>> as its unit. So a fractional scaling factor of approximately 2 * Pi is
>> added to the angular velocity channel.
>>
>> The added comments will also be relevant for the scaling factor of
>> the angle channel.
>>
>> Signed-off-by: David Veenstra <davidjulianveenstra@xxxxxxxxx>
> Comment inline. The function you are talking about isn't used
> in the majority of likely use cases for this part. The maths will actually
> be done in userspace (which can use floating point).
>
> Thanks,
>
> Jonathan
>
>> ---
>> Changes in v2:
>> - Move explanation of Pi approximation to top of switch statement,
>> as this will also be relevant to angle channel.
>> - Replaced 33102 / 2 with 16551 on line 84.
>>
>> drivers/staging/iio/resolver/ad2s1200.c | 84 +++++++++++++++++++++++----------
>> 1 file changed, 59 insertions(+), 25 deletions(-)
>>
>> diff --git a/drivers/staging/iio/resolver/ad2s1200.c b/drivers/staging/iio/resolver/ad2s1200.c
>> index 29a9bb666e7b..6c56257be3b1 100644
>> --- a/drivers/staging/iio/resolver/ad2s1200.c
>> +++ b/drivers/staging/iio/resolver/ad2s1200.c
>> @@ -60,38 +60,71 @@ static int ad2s1200_read_raw(struct iio_dev *indio_dev,
>> int ret = 0;
>> u16 vel;
>>
>> - mutex_lock(&st->lock);
>> - gpiod_set_value(st->sample, 0);
>> + /*
>> + * Below a fractional approximation of Pi is needed.
>> + * The following approximation will be used: 103993 / 33102.
>> + * This is accurate in 9 decimals places.
>> + *
>> + * This fraction is based on OEIS series of nominator/denominator
>> + * of convergents to Pi (A002485 and A002486).
>> + */
>> + switch (m) {
>> + case IIO_CHAN_INFO_SCALE:
>> + switch (chan->type) {
>> + case IIO_ANGL_VEL:
>> + /*
>> + * 2 * Pi ~= 2 * 103993 / 33102
>> + *
>> + * iio_convert_raw_to_processed uses integer
>> + * division. This will cause at most 5% error
>> + * (for very small values). But for 99.5% of the values
>> + * it will cause less that 1% error.
> This is an interesting comment, but relies on anyone actually
> using iio_convert_raw_to_processed with this device.
>
> I would imagine that 'in kernel' users of a resolver (who would use
> that function) will be few and far between. Mostly this will just
> get passed to userspace. That involves this being converted to
> a decimal. I would just specify it as one in the first place.
Hmm, I didn't realize that it might never be used in kernel. A
decimal representation is indeed a lot more clear. I'll change
scale value type to IIO_VAL_INT_PLUS_MICRO for both the angular
velocity and angel channel.
Best regards,
David Veenstra
>
>> + */
>> + *val = 103993;
>> + *val2 = 16551;
>> + return IIO_VAL_FRACTIONAL;
>> + default:
>> + return -EINVAL;
>> + }
>> + break;
>> + case IIO_CHAN_INFO_RAW:
>> + mutex_lock(&st->lock);
>> + gpiod_set_value(st->sample, 0);
>> +
>> + /* delay (6 * AD2S1200_TSCLK + 20) nano seconds */
>> + udelay(1);
>> + gpiod_set_value(st->sample, 1);
>> + gpiod_set_value(st->rdvel, !!(chan->type == IIO_ANGL));
>> +
>> + ret = spi_read(st->sdev, st->rx, 2);
>> + if (ret < 0) {
>> + mutex_unlock(&st->lock);
>> + return ret;
>> + }
>>
>> - /* delay (6 * AD2S1200_TSCLK + 20) nano seconds */
>> - udelay(1);
>> - gpiod_set_value(st->sample, 1);
>> - gpiod_set_value(st->rdvel, !!(chan->type == IIO_ANGL));
>> + vel = be16_to_cpup((__be16 *)st->rx);
>> + switch (chan->type) {
>> + case IIO_ANGL:
>> + *val = vel >> 4;
>> + break;
>> + case IIO_ANGL_VEL:
>> + *val = sign_extend32((s16)vel >> 4, 11);
>> + break;
>> + default:
>> + mutex_unlock(&st->lock);
>> + return -EINVAL;
>> + }
>>
>> - ret = spi_read(st->sdev, st->rx, 2);
>> - if (ret < 0) {
>> + /* delay (2 * AD2S1200_TSCLK + 20) ns for sample pulse */
>> + udelay(1);
>> mutex_unlock(&st->lock);
>> - return ret;
>> - }
>>
>> - vel = be16_to_cpup((__be16 *)st->rx);
>> - switch (chan->type) {
>> - case IIO_ANGL:
>> - *val = vel >> 4;
>> - break;
>> - case IIO_ANGL_VEL:
>> - *val = sign_extend32((s16)vel >> 4, 11);
>> - break;
>> + return IIO_VAL_INT;
>> default:
>> - mutex_unlock(&st->lock);
>> - return -EINVAL;
>> + break;
>> }
>>
>> - /* delay (2 * AD2S1200_TSCLK + 20) ns for sample pulse */
>> - udelay(1);
>> - mutex_unlock(&st->lock);
>> -
>> - return IIO_VAL_INT;
>> + return -EINVAL;
>> }
>>
>> static const struct iio_chan_spec ad2s1200_channels[] = {
>> @@ -105,6 +138,7 @@ static const struct iio_chan_spec ad2s1200_channels[] = {
>> .indexed = 1,
>> .channel = 0,
>> .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
>> + .info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),
>> }
>> };
>>
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