Re: [PATCH 2/4] staging:iio:dac:ad5791: Allow asymmetrical reference voltages

[Lars-Peter Clausen <lars@metafoo.de>]

On 10/19/2011 03:01 PM, Jonathan Cameron wrote:
> On 10/19/11 13:23, Lars-Peter Clausen wrote:
>> The ad5791 currently assumes that the negative and positive supply have the
>> same absolute value, which is not necessarily true. This patch introduces a
>> offset attribute which will contain the negative supply voltage scaled
>> according to the iio spec. The raw attribute now accepts values in the range
>> of 0 to max instead of -max/2 to max/2.
>>
>> While we are at it also fix the vref span calculation. Since both positive and
>> negative reference voltages are specificed as absolute values we need to add
>> them and not subtract them to get the reference voltage span.
> Everything commented here is fine, but one odd looking removal...
>> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
>> ---
>>  drivers/staging/iio/dac/ad5791.c |   24 ++++++++++++++++--------
>>  drivers/staging/iio/dac/ad5791.h |    2 ++
>>  2 files changed, 18 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/staging/iio/dac/ad5791.c b/drivers/staging/iio/dac/ad5791.c
>> index 9a76c43..c465fcf 100644
>> --- a/drivers/staging/iio/dac/ad5791.c
>> +++ b/drivers/staging/iio/dac/ad5791.c
>> @@ -77,7 +77,8 @@ static int ad5791_spi_read(struct spi_device *spi, u8 addr, u32 *val)
>>  	.indexed = 1,					\
>>  	.address = AD5791_ADDR_DAC0,			\
>>  	.channel = 0,					\
>> -	.info_mask = (1 << IIO_CHAN_INFO_SCALE_SHARED),	\
>> +	.info_mask = (1 << IIO_CHAN_INFO_SCALE_SHARED) | \
>> +		(1 << IIO_CHAN_INFO_OFFSET_SHARED),	\
>>  	.scan_type = IIO_ST('u', bits, 24, shift)	\
>>  }
>>  
>> @@ -225,6 +226,7 @@ static int ad5791_read_raw(struct iio_dev *indio_dev,
>>  			   long m)
>>  {
>>  	struct ad5791_state *st = iio_priv(indio_dev);
>> +	u64 val64;
>>  	int ret;
>>  
>>  	switch (m) {
>> @@ -234,12 +236,16 @@ static int ad5791_read_raw(struct iio_dev *indio_dev,
>>  			return ret;
>>  		*val &= AD5791_DAC_MASK;
>>  		*val >>= chan->scan_type.shift;
>> -		*val -= (1 << (chan->scan_type.realbits - 1));
>>  		return IIO_VAL_INT;
>>  	case (1 << IIO_CHAN_INFO_SCALE_SHARED):
>>  		*val = 0;
>>  		*val2 = (st->vref_mv * 1000) >> chan->scan_type.realbits;
>>  		return IIO_VAL_INT_PLUS_MICRO;
>> +	case (1 << IIO_CHAN_INFO_OFFSET_SHARED):
>> +		val64 = (((u64)st->vref_neg_mv) << chan->scan_type.realbits);
>> +		do_div(val64, st->vref_mv);
>> +		*val = -val64;
>> +		return IIO_VAL_INT;
>>  	default:
>>  		return -EINVAL;
>>  	}
>> @@ -257,7 +263,6 @@ static int ad5791_write_raw(struct iio_dev *indio_dev,
>>  
>>  	switch (mask) {
>>  	case 0:
>> -		val += (1 << (chan->scan_type.realbits - 1));
> This line looks suspicious.  What is it doing in this patch?
> I'll be honest and say I'm not sure why it was there in the first place, but
> I can't immediately see what it has to do with the rest of the patch.

The driver would previously accept values from -(1 << realbits) / 2 to (1 <<
realbits) / 2
for the raw attribute. But this only really works if the the reference
voltages are symmetrical.
So what this patch does is to change the raw attribute to accept values from
0 to (1 << realbits) -1
and add the offset attribute which contains the negative offset.

>>  		val &= AD5791_RES_MASK(chan->scan_type.realbits);
>>  		val <<= chan->scan_type.shift;
>>  
>> @@ -309,12 +314,15 @@ static int __devinit ad5791_probe(struct spi_device *spi)
>>  	st->pwr_down = true;
>>  	st->spi = spi;
>>  
>> -	if (!IS_ERR(st->reg_vss) && !IS_ERR(st->reg_vdd))
>> -		st->vref_mv = (pos_voltage_uv - neg_voltage_uv) / 1000;
>> -	else if (pdata)
>> -		st->vref_mv = pdata->vref_pos_mv - pdata->vref_neg_mv;
>> -	else
>> +	if (!IS_ERR(st->reg_vss) && !IS_ERR(st->reg_vdd)) {
>> +		st->vref_mv = (pos_voltage_uv + neg_voltage_uv) / 1000;
>> +		st->vref_neg_mv = neg_voltage_uv / 1000;
>> +	} else if (pdata) {
>> +		st->vref_mv = pdata->vref_pos_mv + pdata->vref_neg_mv;
>> +		st->vref_neg_mv = pdata->vref_neg_mv;
>> +	} else {
>>  		dev_warn(&spi->dev, "reference voltage unspecified\n");
>> +	}
>>  
>>  	ret = ad5791_spi_write(spi, AD5791_ADDR_SW_CTRL, AD5791_SWCTRL_RESET);
>>  	if (ret)
>> diff --git a/drivers/staging/iio/dac/ad5791.h b/drivers/staging/iio/dac/ad5791.h
>> index 2a50a11..fd7edbd 100644
>> --- a/drivers/staging/iio/dac/ad5791.h
>> +++ b/drivers/staging/iio/dac/ad5791.h
>> @@ -82,6 +82,7 @@ struct ad5791_chip_info {
>>   * @reg_vss:		negative supply regulator
>>   * @chip_info:		chip model specific constants
>>   * @vref_mv:		actual reference voltage used
>> + * @vref_neg_mv:	voltage of the negative supply
>>   * @pwr_down_mode	current power down mode
>>   */
>>  
>> @@ -91,6 +92,7 @@ struct ad5791_state {
>>  	struct regulator		*reg_vss;
>>  	const struct ad5791_chip_info	*chip_info;
>>  	unsigned short			vref_mv;
>> +	unsigned int			vref_neg_mv;
>>  	unsigned			ctrl;
>>  	unsigned			pwr_down_mode;
>>  	bool				pwr_down;
>