Re: [RFC PATCH v2 4/5] iio: accel: Support Kionix/ROHM KX022A accelerometer

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Tue, 18 Oct 2022 14:10:59 +0300
Matti Vaittinen <mazziesaccount@gmail.com> wrote:

> On 10/14/22 16:42, Jonathan Cameron wrote:
> > On Wed, 12 Oct 2022 10:40:38 +0300
> > Matti Vaittinen <mazziesaccount@gmail.com> wrote:
> >   
> >> On 10/10/22 16:20, Vaittinen, Matti wrote:  
> >>> On 10/10/22 14:58, Andy Shevchenko wrote:  
> >>>> On Mon, Oct 10, 2022 at 12:12:34PM +0300, Matti Vaittinen wrote:
> >>>> ...
> >>>>     
> >>>>>>> +	ret = regmap_bulk_read(data->regmap, chan->address, &data->buffer,
> >>>>>>> +			       sizeof(s16));  
> >>>>     
> >>>>>> No endianess awareness (sizeof __le16 / __be16)  
> >>>>     
> >>>>>>> +	if (ret)
> >>>>>>> +		return ret;
> >>>>>>> +
> >>>>>>> +	*val = data->buffer[0];  
> >>>>>>
> >>>>>> Ditto (get_unaligned_be16/le16 / le16/be16_to_cpup()).  
> >>>>>
> >>>>> I have probably misunderstood something but I don't see why we should use
> >>>>> 'endianess awareness' in drivers? I thought the IIO framework code takes
> >>>>> care of the endianes conversions based on scan_type so each individual
> >>>>> driver does not need to do that. That however has been just my assumption. I
> >>>>> will need to check this. Thanks for pointing it out.  
> >>>>
> >>>> The IIO core uses endianness field only once in iio_show_fixed_type() AFAICS.  
> >>
> >> Following is some hand waving and speculation after my quick code read.
> >> So, I may be utterly wrong in which case please do correct me...
> >>
> >> Anyways, it seems to me that you're correct. The endianness field is
> >> only used by the IIO to build the channel information for user-space so
> >> that applications reading data can parse it. As far as I understand, the
> >> driver does not need to do the conversions for user-space, but the
> >> user-space tools should inspect the type information and do the
> >> conversion. I think it makes sense as user-space applications may be
> >> better equipped to do some maths. It also may be some applications do
> >> not want to spend cycles doing the conversion but the conversions can be
> >> done later "offline" for the captured raw data. So omitting conversion
> >> in the IIO driver kind of makes sense to me.  
> > 
> > That was indeed the original reasonining for buffered data path
> > (note the endian marker is for scans only which only apply in buffered
> >   / chardev case).  
> 
> So, in a case where we "push_to_buffers" the data, we can leave the data 
> to use the endianess we advertise via endianess info field?

Exactly.

> 
> > It's less obvious for the sysfs path as that's inherently slow.
> > We could have made this a problem for the IIO core, but we didn't :)  
> 
> But again, as far as I understood, the user-space is still expected to 
> read the sysfs field for "scan_elements/in_accel_<channel>_type"? I 
> guess it would be confusing to say "le:s16/16>>0" there while returning 
> CPU native endianess values from sysfs files?

Agreed that it is probably less than ideal but that's what the interface
is.  scan_elements refers to the "scan elements"  channels read via sysfs
files are not scan elements - scan's are only relevant to buffered readback.

> 
> >> I haven't thoroughly looked (and I have never used) the in-kernel IIO
> >> APIs for getting the data. A quick look at the
> >> include/linux/iio/consumer.h allows me to assume the iio_chan_spec can
> >> be obtained by the consumer drivers. This should make the endianess
> >> information available for the consumer drivers as well. So, again,
> >> consumer drivers can parse the raw-format data themself.  
> > 
> > yes consumers should be be endian aware if they are using the
> > callback buffer route to get the data.  Now you mention it, we
> > may well have cases where that isn't handled correctly.
> > There are few enough users of that interface that it might well work
> > by coincidence rather than design. oops.
> >   
> >>
> >> I have this far only used the sysfs and iio_generic_buffer on a
> >> little-endian machine so I have had no issues with the little-endian
> >> data and I have only observed the code. Hence I can not really say if my
> >> reasoning is correct - or if it is how IIO has been designed to operate.
> >> But based on my quick study I don't see a need for the IIO driver to do
> >> endianess conversion to any other format but what is indicated by
> >> scan_type. Specifically for KX022A, the data is already 16B LE when read
> >> from the sensor. This is also advertised by scan_type so no conversion
> >> should be needed (unless, of course, I am mistaken :]).  
> > 
> > Ah. I'd missed that. Data storage should reflect the read back endianness
> > and for the read_raw path you need to perform the conversion in driver
> > (but not the high perf push to buffers path).  
> 
> Oh, really? I think it might be confusing to say "le:s16/16>>0" in 
> "scan_elements/in_accel_<channel>_type" but return something else from 
> the in_accel_<channel>_raw. Especially the "raw" word at the end of the 
> file signals the data is in non converted raw format.
> 
> I take your word for that if you say this is what the user-space 
> expects, it just is not what I did expect. Well, I do very little work 
> on the user-space these days ;) Still just to be on safe side - do you 
> mean I should convert the data returned from read_raw to the CPU endianess?

yes.

> 
> > Sure we could probably have handled read_raw in tree as well but we didn't
> > and probably too late to sensibly fix that now.  One of many things we'd
> > probably do differently if we were starting again.  
> 
> Well, this is pretty usual story :) Predicting the future is hard. My 
> crystal ball ran out of batteries a long ago ;)

:)

> 
> Best Regards
> 	-- Matti
>