Re: Reply to [RFC PATCH v2 0/1] Adding support for IIO SCMI based sensors

Jyoti Bhayana
2021-01-17T07:17:12Z

Hi Jonathan,

Can you clarify one thing ? If we go with option 2 which is using
IIO_AVAIL_RANGE for min,step,high using IIO_VAL_INT then how will it
ensure the possible floating value for step as we are using int type?

Thanks,
Jyoti

On Sat, Jan 16, 2021 at 11:33 AM Jonathan Cameron <jic23@kernel.org> wrote:
>
> On Mon, 11 Jan 2021 13:17:51 -0800
> Jyoti Bhayana <jbhayana@google.com> wrote:
>
> > Hi Jonathan,
> >
> > I know it is a bit confusing. Let me try to explain it with some
> > examples to hopefully clarify some things here.
> > SCMI Platform talks to the native/actual sensor, gets the raw values
> > from the native sensor and applies the scale and then sends those
> > values to the SCMI agent and the SCMI IIO driver.
> > Since the sensor readings which SCMI IIO driver gets are integer, to
> > convert them to float , we need to apply scale to these sensor values
> > which is the unit_exponent(power-of-10 multiplier in two’s-complement
> > format) specified in the SCMI specification
> >
> > Native Sensor -> SCMI platform->SCMI Agent->SCMI IIO Driver
> >
> > So if Native Sensor gets the sensor value
> > 32767 and the scale the SCMI Platform is using is 0.002392.
> > SCMI platform does the calculation of 32767 * 0.002392 = 78.378664
> > and send the sensor value as 78378664 and the scale as .000001 to the
> > SCMI agent and SCMI IIO driver
> >
> > so for SCMI IIO driver the sensor reading = 78378664 and scale = .000001
> > and the sensor value is sensor_reading * scale = 78378664 * .000001
> > = 78.378664
> > and the resolution which the SCMI Platform sends to the SCMI agent is 0.002392.
> > In the SCMI IIO driver, scale which is .000001 is applied to the min
> > range/max range and the actual sensor values.
> > sensor resolution which is 0.002392 is just passed to the userspace
> > layer so that they know the Native sensor resolution/scale
> > being applied by the SCMI platform.
>
> That was pretty much where I'd gotten to.
> Whilst the reasoning might be different it is equivalent to a sensor
> providing info on expected noise by giving a 'valid resolution'.
> In that case as well you have a sensor providing a number that looks to have
> more precision than it actually does.
>
> Anyhow, that similarity doesn't really matter here.
>
> I'll also add that a design that applies scale in two places is inherently
> less than ideal. A cleaner design would have maintained the separation
> between scale and raw value all the way up the stack. That would result
> in 0 loss of information and also be a cleaner interface.
> Ah well, we live with what we have :)
>
> >
> > Regarding your comments in the previous email, when you mentioned
> > "what we actually
> > need is non standard ABI for resolution"? Does that mean that it is ok
> > to have sensor resolution
> > as the IIO attribute shown below?
> >
> > static IIO_DEVICE_ATTR(sensor_resolution, 0444, scmi_iio_get_sensor_resolution,
> > NULL, 0);
>
> We could do something new (see later for why I don't think we need to)
> Would need to clearly reflect what it applies to and I'm not sure resolution
> is even an unambiguous name given sensor resolution is often described as 8bit
> 10bit etc. E.g. this selection table from Maxim for ADCs.
> https://www.maximintegrated.com/en/products/parametric/search.html?fam=prec_adc&tree=master&metaTitle=Precision%20ADCs%20(%20%205Msps)&hide=270
> Of course sometimes it's also used for what you want here.
>
> Hohum. So we might be still be able to do this with standard ABI but we
> are going to need to do some maths in the driver.
> So if we were to express it via
>
> in_accel_raw_avail for example we could use the [low step high] form.
>
> low and high are straight forward as those are expressed directly from
> axis_min_range and axis_max_range which I think are in the same units
> as the _raw channel itself.
>
> For resolution, we have it expressed as [res] x 10^res_exponent
> and if we just put that in as the 'step' above it would have the wrong
> exponent (as we'd expect to still have to apply your 0.00001 from above
> example).
>
> Hence we express it as [res] x 10^(res_exponent - exponent)
>
> I'm going to slightly modify your example above because the two exponents
> are the same so it's hard to tell if I have them right way around.
> Hence let res = 0.00293 = 293 x 10^(-5) (I just dropped the trailing 2)
>
> scale = 10^(-6) exponent = -6
>
> So step = 2392 x 10^(-5 + 6) = 2390
> giving us
>
> [min 2390 max] for _raw_available
>
> Hence when userspace comes along and wants this in relevant base units (here
> m/sec^2) it applies the x10^(-6) mutliplier from _scale we get out expected value
> of 0.00239 m/sec^2
>
> That should work for any case we see but the maths done in the driver will have
> to cope with potential negative exponents for step.
>
> One catch will be the 64 bit potential values for min and max :(
>
> >
> > static struct attribute *scmi_iio_attributes[] = {
> > &iio_dev_attr_sensor_resolution.dev_attr.attr,
> > NULL,
> > };
> >
> > and for the min/max range, I can use the read_avail callback?
>
> I would have said yes normally but if we are going to cope with
> a potential floating point value for step as touched on above we
> may have to do it by hand in the driver. Not ideal but may
> be only option :(
>
> >
> > Also, for the min/max range, there were two options discussed in the
> > email thread:
> > option 1) Add new IIO val Type IIO_VAL_INT_H32_L32, and modify the
> > iio_format_value to format the 64 bit int properly for the userspace
> > option 2) Ignore the H32 bits and use the existing IIO_VAL_INT as just
> > L32 bits should be sufficient for current sensor values.
>
> Ignore is a strong way of putting it. We would definitely want to
> shout about it if we do get anything set in H32.
>
> If we are fairly sure that we aren't going to anything greater than
> 32 bits than we are fine.
>
> It should be possible to work through the worst cases given
> limits of say +-20g for accelerometers for example and the relatively
> limited exponents (5 bits). + sensible resolution.
>
> If it's fairly safe I'd like to go for option 2. as it would ensure we
> can do floating point for the step (which is then used to compute the
> resolution value for android)
>
> Thanks
>
> Jonathan
>
> >
> > Let me know which option you prefer for min/max range. and also please
> > confirm if it is ok to have an IIO attribute for resolution like
> > mentioned above.
> >
> >
> > Thanks,
> > Jyoti
> >
> > Thank you so much
> >
> > Jyoti
> >
> >
> >
> > On Mon, Jan 11, 2021 at 4:34 AM Jonathan Cameron
> > <Jonathan.Cameron@huawei.com> wrote:
> > >
> > > On Sun, 10 Jan 2021 22:44:44 -0800
> > > Jyoti Bhayana <jbhayana@google.com> wrote:
> > >
> > > > Hi Jonathan,
> > > >
> > > > In section 4.7.2.5.1 of the specification, the following exponent is
> > > > the scale value
> > > >
> > > > uint32 axis_attributes_high
> > > > Bits[15:11] Exponent: The power-of-10 multiplier in two’s-complement
> > > > format that is applied to the sensor unit
> > > > specified by the SensorType field.
> > > >
> > > > and the resolution is
> > > >
> > > > uint32 axis_resolution
> > > > Bits[31:27] Exponent: The power-of-10 multiplier in two’s-complement format
> > > > that is applied to the Res field. Bits[26:0] Res: The resolution of
> > > > the sensor axis.
> > > >
> > > > From code in scmi_protocol.h
> > > > /**
> > > > * scmi_sensor_axis_info - describes one sensor axes
> > > > * @id: The axes ID.
> > > > * @type: Axes type. Chosen amongst one of @enum scmi_sensor_class.
> > > > * @scale: Power-of-10 multiplier applied to the axis unit.
> > > > * @name: NULL-terminated string representing axes name as advertised by
> > > > * SCMI platform.
> > > > * @extended_attrs: Flag to indicate the presence of additional extended
> > > > * attributes for this axes.
> > > > * @resolution: Extended attribute representing the resolution of the axes.
> > > > * Set to 0 if not reported by this axes.
> > > > * @exponent: Extended attribute representing the power-of-10 multiplier that
> > > > * is applied to the resolution field. Set to 0 if not reported by
> > > > * this axes.
> > > > * @attrs: Extended attributes representing minimum and maximum values
> > > > * measurable by this axes. Set to 0 if not reported by this sensor.
> > > > */
> > > >
> > > > struct scmi_sensor_axis_info {
> > > > unsigned int id;
> > > > unsigned int type;
> > > > int scale; //This is the scale used for min/max range
> > > > char name[SCMI_MAX_STR_SIZE];
> > > > bool extended_attrs;
> > > > unsigned int resolution;
> > > > int exponent; // This is the scale used in resolution
> > > > struct scmi_range_attrs attrs;
> > > > };
> > > >
> > > > The scale above is the Power-of-10 multiplier which is applied to the min range
> > > > and the max range value
> > > > but the resolution is equal to resolution and multiplied by
> > > > Power-of-10 multiplier
> > > > of exponent in the above struct.
> > > > So as can be seen above the value of the power of 10 multiplier used
> > > > for min/max range
> > > > can be different than the value of the power of 10 multiplier used for
> > > > the resolution.
> > > > Hence, if I have to use IIO_AVAIL_RANGE to specify min/max range and as well
> > > > as resolution, then I have to use the float format with the scale applied.
> > > >
> > > > If there is another way which you know of and prefer, please let me know.
> > > I'll confess I've gotten a bit lost here.
> > >
> > > So I think where we are is how to describe the range of the sensor and why we can't
> > > use in_accel_x_raw_available to provide the
> > >
> > > Understood that the resolution can have different scaling. That is presumably
> > > to allow for the case where a device is reporting values at a finer scale than
> > > it's real resolution. Resolution might take into account expected noise for
> > > example. So it should be decoupled from the scaling of both the actual measurements
> > > and the axis high / low limits.
> > >
> > > However, I'd read that as saying the axis high / low limits and the actual sensor
> > > readings should be scaled by the exponent in axis_attributes_high.
> > > So I think we are fine for the range, but my earlier assumption that resolution
> > > was equivalent to scale in IIO (real world value for 1LSB) may be completely wrong
> > > as resolution may be unconnected to how you convert to a real world value?
> > >
> > > If nothing else I'd like to suggest the spec needs to be tightened a bit here
> > > to say exactly how we convert a value coming in to real world units (maybe
> > > I'm just missing it).
> > >
> > > Anyhow, I suspect we've been looking at this the wrong way and what we actually
> > > need is non standard ABI for resolution.
> > >
> > > Jonathan
> > >
> > >
> > >
> > >
> > > >
> > > > Thanks,
> > > > Jyoti
> > > >
> > > >
> > > >
> > > >
> > > > Thanks,
> > > > Jyoti
> > > >
> > > > On Sat, Jan 9, 2021 at 11:01 AM Jonathan Cameron <jic23@kernel.org> wrote:
> > > > >
> > > > > On Wed, 6 Jan 2021 21:23:53 +0000
> > > > > Jyoti Bhayana <jbhayana@google.com> wrote:
> > > > >
> > > > > > Hi Jonathan,
> > > > > >
> > > > > > Instead of adding IIO_VAL_INT_H32_L32, I am thinking of adding IIO_VAL_FRACTIONAL_LONG
> > > > > > or IIO_VAL_FRACTIONAL_64 as the scale/exponent used for min/max range can be different
> > > > > > than the one used in resolution according to specification.
> > > > >
> > > > > That's somewhat 'odd'. Given min/max are inherently values the sensor is supposed to
> > > > > be able to return why give them different resolutions? Can you point me at a specific
> > > > > section of the spec? The axis_min_range_low etc fields don't seem to have units specified
> > > > > but I assumed they were in sensor units and so same scale factors?
> > > > >
> > > > > >
> > > > > > I am planning to use read_avail for IIO_CHAN_INFO_PROCESSED using IIO_AVAIL_RANGE
> > > > > > and this new IIO_VAL_FRACTIONAL_64 for min range,max range and resolution.
> > > > > > Instead of two values used in IIO_VAL_FRACTIONAL, IIO_VAL_FRACTIONAL_64 will use 4 values
> > > > > > val_high,val_low,and val2_high and val2_low.
> > > > >
> > > > > I'm not keen on the changing that internal kernel interface unless we absolutely
> > > > > have to. read_avail() is called from consumer drivers and they won't know anything
> > > > > about this new variant.
> > > > >
> > > > > >
> > > > > > Let me know if that is an acceptable solution.
> > > > >
> > > > > Hmm. It isn't a standard use of the ABI given the value in the buffer is (I assume)
> > > > > raw (needs scale applied). However, it isn't excluded by the ABI docs. Whether
> > > > > a standard userspace is going to expect it is not clear to me.
> > > > >
> > > > > I don't want to end up in a position where we end up with available being generally
> > > > > added for processed when what most people care about is what the value range they
> > > > > might get from a polled read is (rather than via a buffer).
> > > > >
> > > > > So I'm not that keen on this solution but if we can find a way to avoid it.
> > > > >
> > > > > Jonathan
> > > > >
> > > > >
> > > > > >
> > > > > >
> > > > > > Thanks,
> > > > > > Jyoti
> > > > > >
> > > > >
> > >
>