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

[Jonathan Cameron]

On Wed, 12 Oct 2022 10:40:38 +0300
Matti Vaittinen <mazziesaccount@xxxxxxxxx> 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).

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 :)

> 
> 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).

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.

J

> 
> >> And it does nothing with it. Maybe Jonathan can shed a light what is it for
> >> (I mean the field)?
> >>  
> 
> I agree. It'd be great to listen to someone who actually knows what he 
> is talking about and is not just guessing as I am ^_^;
> 
> Yours,
> 	-- Matti
>