Re: [v3 07/13] iio: imu: add Bosch Sensortec BNO055 core driver

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Sun, Mar 27, 2022 at 05:11:04PM +0100, Jonathan Cameron wrote:
> On Tue, 22 Mar 2022 11:27:14 +0100
> Andrea Merello <andrea.merello@gmail.com> wrote:
> 
> > Il giorno sab 19 feb 2022 alle ore 18:34 Jonathan Cameron
> > <jic23@kernel.org> ha scritto:
> > >
> > > On Thu, 17 Feb 2022 22:58:14 +0100 (CET)
> > > Peter Meerwald-Stadler <pmeerw@pmeerw.net> wrote:
> > >  
> > > > On Thu, 17 Feb 2022, Andrea Merello wrote:
> > > >
> > > > nice work, minor comments below  
> > >
> > > I'll review on top of Peter to save on duplication.
> > >
> > > Mostly really minor stuff.  
> 
> +CC Greg for binary attribute questions.
> 
> > 
> > :)
> > 
> > As usual, comments inline; OK for all the rest.
> > 
> > > Given this has crossed with the introduction of namespaces to quite
> > > a few IIO drivers (I have another series to do on that once I get
> > > caught up with reviews) I'd prefer it if you would move this into
> > > a symbol namespace (EXPORT_SYMBOL_NS_GPL() and appropriate namespace
> > > statements in the two bus modules.
> > >
> > > Save it being done as a follow up series.  If you prefer not to then
> > > that's fine too as it'll be a trivial follow up patch.  
> > 
> > I'll include it in V4 directly.
> > 
> > [...]
> > 
> > > > > +   case IIO_CHAN_INFO_SCALE:
> > > > > +           /* Table 3-31: 1 quaternion = 2^14 LSB */
> > > > > +           if (size < 2)
> > > > > +                   return -EINVAL;
> > > > > +           vals[0] = 1;
> > > > > +           vals[1] = 1 << 14;
> > > > > +           return IIO_VAL_FRACTIONAL_LOG2;  
> > >
> > > This doesn't look right.  Not vals[1] = 14 given FRACTIONAL_LOG2?  
> > 
> > Hum.. maybe just IIO_VAL_FRACTIONAL ?
> 
> That works as well, though I'd argue FRACTIONAL_LOG2 is the
> better option as it makes it clear the divisor is a power of 2
> and the precision might potentially be better as a result (I've not
> checked!)
> 
> > 
> > > > > +   default:
> > > > > +           return -EINVAL;
> > > > > +   }
> > > > > +}
> > > > > +  
> > 
> > [...]
> > 
> > > > > +static IIO_DEVICE_ATTR_RO(sys_calibration_auto_status, 0);
> > > > > +static IIO_DEVICE_ATTR_RO(in_accel_calibration_auto_status, 0);
> > > > > +static IIO_DEVICE_ATTR_RO(in_gyro_calibration_auto_status, 0);
> > > > > +static IIO_DEVICE_ATTR_RO(in_magn_calibration_auto_status, 0);
> > > > > +static IIO_DEVICE_ATTR_RO(calibration_data, 0);  
> > >
> > > This is documented as providing binary data but it's not using
> > > a binary attribute and that rather surprised me.
> > >
> > > Off the top of my head I can't recall why it matters though, so please
> > > take a look at whether a bin_attribute makes more sense for this.  
> > 
> > As far as I can see, it seems that a non-binary attributes only
> > support to be read at once while the binary attributes read()
> > operation supports random access i.e. it has the file position
> > parameter.
> > 
> > The calibration data is "dynamic", it's read from the HW every time,
> > and I'm not sure it makes any sense to read it in several chunks (what
> > if we read a chunk and the calibration data is updated by the HW
> > before reading the second chunk?). So, despide the fitting "binary"
> > name I'm tempted to stick with regular attribute. However I'm not sure
> > this is the only difference related to binary attributes.
> 
> +Cc Greg.  Valid choice to use a normal attribute for this?

binary attributes are to ONLY be used for data that flows to/from a
device without the kernel ever modifying the data at all.  The kerneln
is just a pass-through here.

There are a few minor exceptions, but they were exceptions, please don't
use them as a valid reason to use a binary attribute.

does that help?

greg k-h