Re: [PATCH v2 2/4] iio: add LM3533 ambient light sensor driver

On Wed, May 16, 2012 at 03:21:14PM +0100, Jonathan Cameron wrote:

On 5/16/2012 2:05 PM, Johan Hovold wrote:

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On Tue, May 15, 2012 at 09:00:46PM +0100, Jonathan Cameron wrote:

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On 05/15/2012 05:44 PM, Johan Hovold wrote:

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On Tue, May 08, 2012 at 02:47:19PM +0100, Jonathan Cameron wrote:

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On 5/3/2012 5:36 PM, Johan Hovold wrote:

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On Thu, May 03, 2012 at 12:40:10PM +0100, Jonathan Cameron wrote:

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On 5/3/2012 11:26 AM, Johan Hovold wrote:

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Add sub-driver for the ambient light sensor interface on National
Semiconductor / TI LM3533 lighting power chips.

The sensor interface can be used to control the LEDs and backlights of
the chip through defining five light zones and three sets of
corresponding brightness target levels.

The driver provides raw and mean adc readings along with the current
light zone through sysfs. A threshold event can be generated on zone
changes.

Code is fine.  Pretty much all my comments are to do with the interface.

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diff --git a/drivers/staging/iio/Documentation/sysfs-bus-iio-light-lm3533-als b/drivers/staging/iio/Documentation/sysfs-bus-iio-light-lm3533-als
new file mode 100644
index 0000000..9849d14
--- /dev/null
+++ b/drivers/staging/iio/Documentation/sysfs-bus-iio-light-lm3533-als

@@ -0,0 +1,62 @@
+What:		/sys/bus/iio/devices/iio:deviceX/gain
+Date:		April 2012
+KernelVersion:	3.5
+Contact:	Johan Hovold<jhovold@gmail.com>
+Description:
+		Set the ALS gain-resistor setting (0..127) for analog input
+		mode, where
+
+		0000000 - ALS input is high impedance
+		0000001 - 200kOhm (10uA at 2V full-scale)
+		0000010 - 100kOhm (20uA at 2V full-scale)
+		...
+		1111110 - 1.587kOhm (1.26mA at 2V full-scale)
+		1111111 - 1.575kOhm (1.27mA at 2V full-scale)
+
+		R_als = 2V / (10uA * gain)	(gain>    0)

Firstly, no magic numbers.  These are definitely magic.

Not that magic as they're clearly documented (in code and public
datasheets), right? What would you prefer instead?

The numbers on the right of the - look good to me though then this isn't
a gain.  (200kohm) and the infinite element is annoying.  Why not
compute the actual gains?
Gain = (Rals*10e-6)/2  and use those values?  Yes you will have to do
a bit of fixed point maths in the driver but the advantage is you'll
have real values that are standardizable across multiple devices
and hence allow your device to be operated by generic userspace
code.  Welcome to standardising interfaces - my favourite occupation ;)

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Secondly see in_illuminance0_scale for a suitable existing attribute.

I didn't consider scale to be appropriate given the following
documentation (e.g, for in_voltageY_scale):

sorry  I just did this to someone else in another review (so I'm
consistently wrong!)

in_voltageY_calibscale is what I should have said.  That one applies a
scaling before the raw reading is generated (so in hardware).

Ok, then calibscale is the appropriate attribute for the resistor
setting. But as this is a device-specific hardware-calibration setting
I would suggest using the following interface:

What:		/sys/bus/iio/devices/iio:deviceX/in_illuminance_calibscale
Description:
		Set the ALS calibration scale (internal resistors) for
		analog input mode, where the scale factor is the current in uA
		at 2V full-scale (10..1270, 10uA step), that is,

		R_als = 2V / in_illuminance_calibscale

		This setting is ignored in PWM mode.

This is a generic element that really ought to just fit in with the
equivalent in sysfs-bus-iio for calibscan.  It's a ratio, so it should
be unit free for starters.

I'm starting to doubt that calibscale is really appropriate in this case.

For starters, the description in sysfs-bus-iio doesn't really apply:

	"Hardware applied calibration scale factor. (assumed to fix
	 production inaccuracies)."

Hmm.. if you really don't like this, Michael Hennerich had a case
where this made even less sense, so we now have hardwaregain.
Use that if you like...

I really think that this should remain a device specific attribute as I
originally suggested. It's an integration parameter that needs to be set
precisely depending on the actual hardware setup (which analog light
sensor and other external components). 

The lm3533 also supports two types of light sensors: pwm- and analog-
output ones. The resistor select settings only applies when in analog
mode as the input is always high impedance otherwise. Thus a generic
attribute (such as calibscale or hardware gain) shouldn't be used as it
will have no effect whatsoever in PWM-mode.

I'm thus back at my original proposal, albeit with a different name (I
think a lot of this discussion could have been avoided had I not
misnamed the parameter "gain"): 

What:		/sys/bus/iio/devices/iio:deviceX/r_select
Description:
		Set the ALS internal pull-down resistor for analog input mode
		(1..127), such that,

		R_als = 200000 / r_select	(ohm)

		This setting is ignored in PWM-mode (input is always high
		impedance in PWM-mode).

I don't think much is gained from using ohm as the unit: it just adds
complexity and the selected resistor setting will likely not match the
input value anyway. It's better that the chip integrators have full
control over which resistor setting is actually used so that it matches
external components.

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The resistor setting of the lm3533 is about fitting an external analog
light sensor to the lm3533 als interface (which is basically just an adc
with some extra logic), that is, it is used to match the output current
of the chosen sensor so that the ADC measures 2V at full LUX.

It's not a setting to calibrate "inaccuracies", but rather an
integration parameter that is set once when the characteristics of the
light sensor is known. (Sure, it could be used later to increase
sensitivity as well, but the main purpose is to fit a new light sensor
to a generic input interface.)

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+What:		/sys/bus/iio/devices/iio:deviceX/target[m]_[n]
+Date:		April 2012
+KernelVersion:	3.5
+Contact:	Johan Hovold<jhovold@gmail.com>
+Description:
+		Set the target brightness for ALS-mapper m in light zone n
+		(0..255), where m in 1..3 and n in 0..4.

Don't suppose you could do a quick summary of what these zones are
and why there are 3 ALS-mappers?  I'm not getting terribly far on a
quick look at the datasheet!

Of course. The average adc readings are mapped to five light zones using
eight zone boundary registers (4 boundaries with hysteresis) and a set
of rules.

This is going to be fun.  We'll need the boundaries and attached
hysteresis attributes to fully specify these (nothing would indicate
that hysterisis is involved otherwise).

You can't define the hysteresis explicitly with the lm3533 register
interface, rather it's is defined implicitly in case threshY_falling is
less than threshY_rasising.

So the raising/falling attributes should be enough, right?

Nope, because they don't tell a general userspace application what is
going on.  Without hysterisis attributes it has no way of knowing there
is hysterisis present.

Well an application could simply look at the difference between raising
and falling to determine the hysteresis?

Only if it knows it has your sensor.  For other sensors it could be 
completely separate or not present.  If the parameter is missing 
assumption is that there is no hysterisis.

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It gets more complicated as the lm3533 allow the raising threshold to
be lower than the falling. It appears the device is using whichever
register is lower for the falling threshold. I guess I should compensate
for this in the driver.

That's nasty.

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Furthermore, you can define threshold 1 to be lower than threshold 0,
effectively preventing zone 1 to be reached. In this case, dropping
below thres1_falling gives zone 0, and raising above thres1_raising gives
zone 2. In particular, no threshold event is generated when
thres0_{falling/raising} is passed in either direction. But perhaps this
should just be documented as a feature/quirk of the device.

Seems sensible...

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Feel free to make them read only though.

So you're suggesting something like:

	events/in_illuminance0_threshY_falling_value
	events/in_illuminance0_threshY_raising_value
	events/in_illuminance0_threshY_hysteresis

where hysteresis is a read-only attribute whose value is
	
	threshY_raising_value - threshY_falling_value

yes.  Annoying it may be but it matches existing interface.

I'm posting a v4 which includes the above proposal for resistor select.
I've also added the hysteresis attributes as requested and fixed the
device threshold quirkiness mentioned above (the device is using
whichever register value is smaller as the falling threshold).

Thanks,
Johan