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Date:   Fri, 22 Jul 2022 10:33:47 +0100
From:   Paul Cercueil <paul@crapouillou.net>
Subject: Re: [PATCH 4/4] iio: afe/rescale: Implement write_raw
To:     Peter Rosin <peda@axentia.se>
Cc:     Jonathan Cameron <jic23@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>, list@opendingux.net,
        linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org
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        <20220721191526.374152-5-paul@crapouillou.net>
        <b8539e31-b20a-6eb5-4b86-59a17d7c6939@axentia.se>
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Hi Peter,

Le ven., juil. 22 2022 at 00:16:36 +0200, Peter Rosin <peda@axentia.se>=20
a =E9crit :
> Hi!
>=20
> 2022-07-21 at 21:15, Paul Cercueil wrote:
>>  Implement write_raw by converting the value if writing the scale, or
>>  just calling the managed channel driver's write_raw otherwise.
>>=20
>>  Signed-off-by: Paul Cercueil <paul@crapouillou.net>
>>  ---
>>   drivers/iio/afe/iio-rescale.c | 22 ++++++++++++++++++++++
>>   1 file changed, 22 insertions(+)
>>=20
>>  diff --git a/drivers/iio/afe/iio-rescale.c=20
>> b/drivers/iio/afe/iio-rescale.c
>>  index 5c9970b93384..0edb62ee4508 100644
>>  --- a/drivers/iio/afe/iio-rescale.c
>>  +++ b/drivers/iio/afe/iio-rescale.c
>>  @@ -141,6 +141,27 @@ int rescale_process_offset(struct rescale=20
>> *rescale, int scale_type,
>>   	}
>>   }
>>=20
>>  +static int rescale_write_raw(struct iio_dev *indio_dev,
>>  +			     struct iio_chan_spec const *chan,
>>  +			     int val, int val2, long mask)
>>  +{
>>  +	struct rescale *rescale =3D iio_priv(indio_dev);
>>  +	unsigned long long tmp;
>>  +
>>  +	switch (mask) {
>>  +	case IIO_CHAN_INFO_SCALE:
>>  +		tmp =3D val * 1000000000LL;
>>  +		do_div(tmp, rescale->numerator);
>>  +		tmp *=3D rescale->denominator;
>>  +		do_div(tmp, 1000000000LL);
>=20
> do_div is for unsigned operands. Can val never ever be negative?
> What about the numerator and denominator, can those be negative? I
> think this code should live in a new rescale_process_inverse_scale
> function, or something like that (and a few tests could be added to
> drivers/iio/test/iio-test-rescale.c)

I can do that.

>=20
>>  +		return iio_write_channel_attribute(rescale->source, tmp, 0,
>>  +						   IIO_CHAN_INFO_SCALE);
>>  +	default:
>=20
> What if the source driver has a .write_raw_get_fmt callback? That bit
> of info is silently dropped (with no comment that a shortcut has been
> taken). How does inverse rescaling mix with a .write_raw_get_fmt that
> returns e.g. IIO_VAL_INT_PLUS_MICRO_DB anyway? I think all cases might
> get a bit hairy to support, so I think you need to do some filtering
> and somehow fail the .write_raw call if the .write_raw_get_fmt of the
> source returns something that gets too difficult to support.

If the inverse rescale uses the same code as rescale_process_scale()=20
then it becomes problematic, yes, as it likes to change the type of the=20
value.

What I could try - compute the inverse of the value, then find the=20
closest scale value+type that the source driver supports, and use this=20
as the value+type. Then the only failure point would be if=20
.write_raw_get_fmt returns something different than the formats=20
returned by .read_avail, but that sounds unlikely to happen.

Cheers,
-Paul

>>  +		return iio_write_channel_attribute(rescale->source,
>>  +						   val, val2, mask);
>>  +	}
>>  +}
>>  +
>>   static int rescale_read_raw(struct iio_dev *indio_dev,
>>   			    struct iio_chan_spec const *chan,
>>   			    int *val, int *val2, long mask)
>>  @@ -250,6 +271,7 @@ static int rescale_read_avail(struct iio_dev=20
>> *indio_dev,
>>   }
>>=20
>>   static const struct iio_info rescale_info =3D {
>>  +	.write_raw =3D rescale_write_raw,
>>   	.read_raw =3D rescale_read_raw,
>>   	.read_avail =3D rescale_read_avail,
>>   };