Message-ID: <20120516130507.GA10774@localhost>
Date:	Wed, 16 May 2012 15:05:07 +0200
From:	Johan Hovold <jhovold@...il.com>
To:	Jonathan Cameron <jic23@...nel.org>
Cc:	Johan Hovold <jhovold@...il.com>,
	Jonathan Cameron <jic23@....ac.uk>,
	Rob Landley <rob@...dley.net>,
	Richard Purdie <rpurdie@...ys.net>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Florian Tobias Schandinat <FlorianSchandinat@....de>,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
	linux-fbdev@...r.kernel.org
Subject: Re: [PATCH v2 2/4] iio: add LM3533 ambient light sensor driver

On Tue, May 15, 2012 at 09:00:46PM +0100, Jonathan Cameron wrote:
> On 05/15/2012 05:44 PM, Johan Hovold wrote:
> > On Tue, May 08, 2012 at 02:47:19PM +0100, Jonathan Cameron wrote:
> >> On 5/3/2012 5:36 PM, Johan Hovold wrote:
> >>> On Thu, May 03, 2012 at 12:40:10PM +0100, Jonathan Cameron wrote:
> >>>> On 5/3/2012 11:26 AM, Johan Hovold wrote:
> >>>>> 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.
> >>> [...]
> >>>
> >>>>> 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@...il.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 ;)
> >>
> >>>> 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)."

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.)

> > [...]
> > 
> >>>>> +What:		/sys/bus/iio/devices/iio:deviceX/target[m]_[n]
> >>>>> +Date:		April 2012
> >>>>> +KernelVersion:	3.5
> >>>>> +Contact:	Johan Hovold<jhovold@...il.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?

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.

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.

> 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

> >>> To simplify somewhat (by ignoring some of the rules): If the average
> >>> adc input drops below boundary0_low, the zone register reads 0; if it
> >>> drops below boundary1_low, it reads 1, and so on. If the input it
> >>> increases over boundary3_high, the zone register return 4; if it
> >>> increases passed boundary2_high, it returns zone 3, etc.
> >>>
> >>> That is, roughly something like (we get 8-bits of input from the ADC):
> >>>
> >>> 	zone 0
> >>>
> >>> boundary0_low	51
> >>> boundary0_high	53
> >>>
> >>> 	zone 1
> >>>
> >>> boundary1_low	102
> >>> boundary1_high	106
> >>>
> >>> 	zone 2
> >>>
> >>> boundary2_low	153
> >>> boundary2_high	161
> >>>
> >>> 	zone 3
> >>>
> >>> boundary3_low	204
> >>> boundary3_high	220
> >>>
> >>> 	zone 4
> >>>
> >>> [ Figure 6 on page 20 in the datasheets should make it clear. ]
> >>>
> >>> The ALS interface and it's zone concept can then be used to control the
> >>> LEDs and backlights of the chip, by determining the target brightness for
> >>> each zone, e.g., set brightness to 52 when in zone 0.
> >>>
> >>> To complicate things further (and it is complicated), there are three
> >>> such sets of target brightness values: ALSM1, ALSM2, ALSM3.
> >>>
> >>> So for each LED or backlight you can set ALS-input control mode, by
> >>> saying that the device should get it's brightness levels from target set
> >>> 1, 2, or 3.
> >>>
> >>> [ And it gets even more complicated, as ALSM1 can only control
> >>>    backlight0, where as ALSM2 and ALSM3 can control any of the remaining
> >>>    devices, but that's irrelevant here. ]
> >>>
> >>> Initially, I thought this interface to be too esoteric to be worth
> >>> generalising, but it sort of fits with event thresholds so I gave it a
> >>> try.
> >> Glad you did and it pretty much fits, be it with a few extensions being 
> >> necessary.
> >>> The biggest conceptual problem, I think, is that the zone
> >>> boundaries can be used to control the other devices, even when the event
> >>> is not enabled (or even an irq line not configured). That is, I find it
> >>> a bit awkward that the event thresholds also defines the zones (a sort of
> >>> discrete scaling factor).
> >> That is indeed awkward. I'm not sure how we handle this either.  If we 
> >> need to control these from the other devices (e.g. the back light
> >> driver) then we'll have to get them into chan_spec and use the
> >> inkernel interfaces to do it.  Not infeasible but I was hoping to
> >> avoid that until we have had a few months to see what similar devices
> >> show up (on basis nothing in this world is a one off for long ;)
> > 
> > I don't think the control bits can or should be generalised at this
> > point. The same ALS-target values may be used to control more than one
> > device, so they need to be set from the als rather from the controlled
> > device (otherwise, changing the target value of led1 could change that
> > of the other three leds without the user realising that this can be a
> > side effect).
> Good point.  Nasty little device to write an interface for :)

Indeed. Thanks for appreciating that. ;)
 
> >>> Perhaps simply keeping the attributes outside of events (e.g. named
> >>> boundary[n]_{low,high}) and having a custom event enabled (e.g.
> >>> in_illuminance_zone_change_en) is the best solution?
> >> Maybe, but it's ugly and as you have said, they do correspond pretty well to
> >> thresholds so I'd rather you went with that.
> >> The core stuff for registering events clearly needs a rethink.... For 
> >> now doing it as you describe above (with the addition fo hysteresis
> >> attributes) should be fine.  Just document the 'quirks'.
> > 
> > Ok, I'll keep the event/zone interface as it stands for now and we'll
> > see if it can be generalised later. [ See my comment on the hysteresis
> > above: there are only the rising/falling thresholds (low/high
> > boundaries) and no boundary or hysteresis settings. ]
> On that, just to reiterate, to have anything generalizable, userspace
> needs to know that hysterisis exists on the individual thresholds
> (though it is clearly a function of the neighbouring one).

See my comments above.

Thanks,
Johan
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