linux-kernel - Re: [PATCH 2/4] iio: adc: ad4695: implement calibration support

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Message-ID: <e2401290-5504-4a17-93bd-af8684a82a7a@baylibre.com>
Date: Wed, 21 Aug 2024 11:12:12 -0500
From: David Lechner <dlechner@...libre.com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: Jonathan Cameron <jic23@...nel.org>,
 Michael Hennerich <Michael.Hennerich@...log.com>,
 Nuno Sá <nuno.sa@...log.com>,
 Jonathan Corbet <corbet@....net>, linux-iio@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/4] iio: adc: ad4695: implement calibration support

On 8/21/24 8:16 AM, Jonathan Cameron wrote:
> On Tue, 20 Aug 2024 10:58:36 -0500
> David Lechner <dlechner@...libre.com> wrote:
> 
>> The AD4695 has a calibration feature that allows the user to compensate
>> for variations in the analog front end. This implements this feature in
>> the driver using the standard `calibgain` and `calibbias` attributes.
>>
>> Signed-off-by: David Lechner <dlechner@...libre.com>
> Hi David,
> 
> Whilst some of the messy value manipulation is unavoidable
> (oh for signed integer zero!), I wonder if we can at least
> move one case into the core.
> 
> See below.
> 
>> +
>> +static int ad4695_write_raw(struct iio_dev *indio_dev,
>> +			    struct iio_chan_spec const *chan,
>> +			    int val, int val2, long mask)
>> +{
>> +	struct ad4695_state *st = iio_priv(indio_dev);
>> +	unsigned int reg_val;
>> +	int ret;
>> +
>> +	iio_device_claim_direct_scoped(return -EBUSY, indio_dev) {
>> +		switch (mask) {
>> +		case IIO_CHAN_INFO_CALIBSCALE:
>> +			switch (chan->type) {
>> +			case IIO_VOLTAGE:
>> +				if (val < 0 || val2 < 0)
>> +					reg_val = 0;
>> +				else if (val > 1)
>> +					reg_val = U16_MAX;
>> +				else
>> +					reg_val = (val * (1 << 16) +
>> +						   mul_u64_u32_div(val2, 1 << 16,
>> +								   MICRO)) / 2;
> Maybe worth extending iio_write_channel_info() to handle
> IIO_VAL_FRACTIONAL_LOG2()?
> It'll look much like this and you'll need to provide write_raw_get_fmt()
> so the core know what to do with the value formatting.
> 
> I don't really like the mixture here between the read path being able
> to rely on the core code to deal with the /2^X and the write path not.

Sounds like a good idea to me.

It seems like we would need to add an extra out parameter to 
write_raw_get_fmt to say what we want the X in 2^X to be. For
example, we would want to make sure the val2 we get in write_raw
for this driver is 15.

>> +
>> +				return regmap_write(st->regmap16,
>> +					AD4695_REG_GAIN_IN(chan->scan_index),
>> +					reg_val);
>> +			default:
>> +				return -EINVAL;
> 
> 
>