linux-kernel - Re: [PATCH v2 5/6] staging:iio:ad2s90: Add IIO_CHAN_INFO_SCALE to channel spec and read

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Message-ID: <20181028165040.27c69fc5@archlinux>
Date:   Sun, 28 Oct 2018 16:50:40 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Matheus Tavares <matheus.bernardino@....br>
Cc:     Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org, kernel-usp@...glegroups.com,
        Victor Colombo <victorcolombo@...il.com>
Subject: Re: [PATCH v2 5/6] staging:iio:ad2s90: Add IIO_CHAN_INFO_SCALE to
 channel spec and read_raw

On Fri, 26 Oct 2018 23:00:04 -0300
Matheus Tavares <matheus.bernardino@....br> wrote:

> This patch adds the IIO_CHAN_INFO_SCALE mask to ad2s90_chan and
> implements the relative read behavior at ad2s90_read_raw.
> 
> Signed-off-by: Victor Colombo <victorcolombo@...il.com>
> Signed-off-by: Matheus Tavares <matheus.bernardino@....br>

Hi,

A suggestion inline.  This is a common case that we have infrastructure
to simplify.  + I think your scale factor is very slightly wrong.

Jonathan

> ---
>  drivers/staging/iio/resolver/ad2s90.c | 32 ++++++++++++++++++---------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/staging/iio/resolver/ad2s90.c b/drivers/staging/iio/resolver/ad2s90.c
> index b4a6a89c11b0..52b656875ed1 100644
> --- a/drivers/staging/iio/resolver/ad2s90.c
> +++ b/drivers/staging/iio/resolver/ad2s90.c
> @@ -34,19 +34,31 @@ static int ad2s90_read_raw(struct iio_dev *indio_dev,
>  	int ret;
>  	struct ad2s90_state *st = iio_priv(indio_dev);
>  
> -	mutex_lock(&st->lock);
> +	switch (m) {
> +	case IIO_CHAN_INFO_SCALE:
> +		/* 2 * Pi / (2^12 - 1) ~= 0.001534355 */
> +		*val = 0;
> +		*val2 = 1534355;
Definitely 2^12 - 1?  That's a bit unusual if true as it would imply
that 2^12 - 1 and 0 were the same value.

Imagine a smaller version with on 2^2 bits so 0, 1, 2, 3
Values of each are

0, M_PI/2, M_PI, 3*M_PI/2

So the multiplier is 2*M_PI/(2^2) not 2*M_PI/(2^2 - 1)
1/2 vs 2/3 * M_PI

Now this is a very common case so we have the return type
IIO_VAL_FRACTIONAL_LOG2 to give a more obvious and potentially
more accurate representation.

> +		return IIO_VAL_INT_PLUS_NANO;
> +	case IIO_CHAN_INFO_RAW:
> +		mutex_lock(&st->lock);
> +
> +		ret = spi_read(st->sdev, st->rx, 2);
> +		if (ret < 0) {
> +			mutex_unlock(&st->lock);
> +			return ret;
> +		}
> +
> +		*val = (((u16)(st->rx[0])) << 4) | ((st->rx[1] & 0xF0) >> 4);
>  
> -	ret = spi_read(st->sdev, st->rx, 2);
> -	if (ret < 0) {
>  		mutex_unlock(&st->lock);
> -		return ret;
> -	}
>  
> -	*val = (((u16)(st->rx[0])) << 4) | ((st->rx[1] & 0xF0) >> 4);
> -
> -	mutex_unlock(&st->lock);
> +		return IIO_VAL_INT;
> +	default:
> +		break;
> +	}
>  
> -	return IIO_VAL_INT;
> +	return -EINVAL;
>  }
>  
>  static const struct iio_info ad2s90_info = {
> @@ -57,7 +69,7 @@ static const struct iio_chan_spec ad2s90_chan = {
>  	.type = IIO_ANGL,
>  	.indexed = 1,
>  	.channel = 0,
> -	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
> +	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) | BIT(IIO_CHAN_INFO_SCALE),
>  };
>  
>  static int ad2s90_probe(struct spi_device *spi)