Re: [PATCH 1/3] iio: core: add IIO_VAL_INT_MICRO

[Matti Vaittinen]

Hi Jonathan,

On 4/12/23 23:32, Jonathan Cameron wrote:
> On Wed, 12 Apr 2023 15:27:14 +0300
> Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote:
>
> > There are a few cases like light sensor integration times, where values
> > returned from *_available() and read_raw() are smaller than 1 and often
> > in the units of micro. (Like micro second scale integration times,
> > always smaller than 1 second). Currently those are often handled using
> > IIO_VAL_INT_PLUS_MICRO, which requires drivers to initialize the integer
> > part to zero. Furthermore, using IIO_VAL_INT_PLUS_MICRO in iio lists
> > requires one to always allocate the 'dummy' integer part too.
> >
> > Introduce IIO_VAL_INT_MICRO which allows omitting the always zero integer.
> >
> > Signed-off-by: Matti Vaittinen <mazziesaccount@xxxxxxxxx>
> Hi Matti,
>
> I'm not keen on adding yet another case just to avoid having to
> have the integer part for IIO_VAL_INT_PLUS_MICRO.
> Seems like the wrong trade off of maintainability vs ease of use.

I see your point. I would still argue that adding the IIO_VAL_INT_MICRO 
was not really an intrusive change and I'd expect the maintenance effort 
should not be increased that much.

While the inconvenience for users in read_raw (initializing the *val = 
0) is minor (meaning the benefit of adding IIO_VAL_INT_MICRO is also 
minor in this regard), iio_lists are stronger reason to consider this. 
With IIO_VAL_INT_MICRO the iio-list memory footprint will be halved. In 
my opinion, this benefit would exceed the cost of maintenance effort 
increase - sure thing it's easy for me to say as I am not the maintainer 
;) (And as I wrote, this series was cooked in a hurry - I had no time to 
go through existing drivers to see how many could benefit from the new 
IIO_VAL_INT_MICRO. I may do this later when I get some pretty urgent 
things off my shoulders - assuming you're not opposing this change so 
strongly that this is out of the question no matter how many existing 
users could benefit from IIO_VAL_INT_MICRO).

Anyways, if this is your final stance, then I need to rework the 
integration time list allocations in the gts helper, but I am most 
likely not able to do this until a week or two from now - meaning it 
might be better to revert the bu27034 and iio-gts-helpers until this 
gets fixed. (I reserve the right to do this during some night if I can't 
get sleep though.)

Yours,
	-- Matti

> Jonathan
>
> > ---
> >   drivers/iio/industrialio-core.c | 4 ++++
> >   include/linux/iio/types.h       | 1 +
> >   2 files changed, 5 insertions(+)
> >
> > diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> > index c117f50d0cf3..c5ae965e9961 100644
> > --- a/drivers/iio/industrialio-core.c
> > +++ b/drivers/iio/industrialio-core.c
> > @@ -628,6 +628,8 @@ static ssize_t __iio_format_value(char *buf, size_t offset, unsigned int type,
> >   	switch (type) {
> >   	case IIO_VAL_INT:
> >   		return sysfs_emit_at(buf, offset, "%d", vals[0]);
> > +	case IIO_VAL_INT_MICRO:
> > +		return sysfs_emit_at(buf, offset, "0.%06u", vals[0]);
> >   	case IIO_VAL_INT_PLUS_MICRO_DB:
> >   		scale_db = true;
> >   		fallthrough;
> > @@ -758,6 +760,7 @@ static ssize_t iio_format_list(char *buf, const int *vals, int type, int length,
> >   
> >   	switch (type) {
> >   	case IIO_VAL_INT:
> > +	case IIO_VAL_INT_MICRO:
> >   		stride = 1;
> >   		break;
> >   	default:
> > @@ -952,6 +955,7 @@ static ssize_t iio_write_channel_info(struct device *dev,
> >   		case IIO_VAL_INT_PLUS_MICRO_DB:
> >   			scale_db = true;
> >   			fallthrough;
> > +		case IIO_VAL_INT_MICRO:
> >   		case IIO_VAL_INT_PLUS_MICRO:
> >   			fract_mult = 100000;
> >   			break;
> > diff --git a/include/linux/iio/types.h b/include/linux/iio/types.h
> > index 82faa98c719a..b4e316172c7f 100644
> > --- a/include/linux/iio/types.h
> > +++ b/include/linux/iio/types.h
> > @@ -30,6 +30,7 @@ enum iio_event_info {
> >   #define IIO_VAL_FRACTIONAL 10
> >   #define IIO_VAL_FRACTIONAL_LOG2 11
> >   #define IIO_VAL_CHAR 12
> > +#define IIO_VAL_INT_MICRO 13 /* val is micro <units>. Integer part is 0 */
> >   
> >   enum iio_available_type {
> >   	IIO_AVAIL_LIST,

--
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~