Re: RFC: Light sensors, unifying current options?

[Zhang Rui <rui.zhang@intel.com>]

On Mon, 2009-09-07 at 19:42 +0800, Jonathan Cameron wrote:
> Zhang Rui wrote:
> > Hi, Jonathan,
> > 
> > On Thu, 2009-09-03 at 21:51 +0800, Jonathan Cameron wrote:
> >> Dear All,
> >>
> >> This thread is a follow up to (amongst others)
> >>
> >> [lm-sensors] Ambient Light sensor for Intersil-ISL29020 device
> >>
> >> Currently there are a number of light sensor drivers either in the
> >> mainline kernel, posted to various mailing lists or sitting in various
> >> testing trees.
> >>
> >> For example.
> >>
> >> Intersil ISL29020
> >> http://www.intersil.com/products/deviceinfo.asp?pn=ISL29020 driver
> >> posted by Kalan Trisal to lm-sensors (as hwmon device rejected for
> >> being out of subsystem scope)
> >> http://lists.lm-sensors.org/pipermail/lm-sensors/2009-September/026575.html
> >>
> >> ALS_sysfs class and als_acpi driver (V6 posted to lkml earlier this week).
> >> http://lkml.org/lkml/2009/9/1/38
> >>
> >> TSL2561 under the industrial I/O Framework. (in Greg KH's tree, will
> >> being in staging after merge window - there due to lack of review
> >> more than any known problems.)
> >> http://www.farnell.com/datasheets/49661.pdf
> >> http://lkml.org/lkml/2009/7/2/189
> >>
> >> TSL2550 under i2c/chips/ which as a location is going away.
> >> http://www.farnell.com/datasheets/48715.pdf
> >>
> >> (any others people know of?)
> >>
> >> Two big questions:
> >>
> >> * Are there sufficient shared characteristics between these devices to
> >> all for a unified framework? (would certainly be nice!)
> >>
> >> * What applications are they used for? This will drive the question
> >> of what functionality is desirable. (particularly do we need an event
> >> infrastructure or not).
> >>
> >> To sumarize the functionality currently provided by the above drivers
> >> (or that should probably be added)
> >>
> >> ISL29020:
> >> * sensing_range
> >> * lux_level
> >> * power state (should probably move over to the new power management
> >>   framework)
> >>
> >> ALS:
> >> * illuminance (equivalent of lux_level)
> >> * adjustment (I don't follow the purpose of this, but then I don't know
> >> anything about how this is being used!)
> >>
> > adjustment is a percentage value used by userspace to adjust the display
> > backlight.
> > 
> > According to the ACPI spec, ACPI ALS device has "ambient light
> > illuminance to display luminance" mappings that can be used by an OS to
> > calibrate its ambient light policy for a given sensor configuration. The
> > OS can use this information to extrapolate an ALS response curve. i.e.
> > ACPI ALS knows what to do when ambient light illuminance changes, but it
> > won't change the backlight. Instead, it exports this info to user space
> > via the "adjustment" attribute.
> > user space applications can get this value and change the display
> > brightness via the backlight sysfs I/F.
> Is this conversion entirely independent of the physical configuration of
> the sensor?

yes.

>   I can sort of imagine cases where some direct pickup from
> the backlight occurs alongside the ambient and some where none does.
> Fair enough if not.
> 
do you mean that on some platforms, als may change the backlight
directly when ambient light changes?

> > IMO, the ALS device should do the following work:
> > 1. catch the ambient light illuminance change.
> Sometimes this is more complex with the ability to separately read light
> levels in different frequency ranges (e.g. infrared and visible)  Still
> this value can usually be derived.

so, "illuminance" should be a required attribute, right?

> > 2. tell the userspace what to do with this change.
> > isn't this true for the other ALS devices in this thread?
> None of the others (other than the additional asus one mentioned
> later in this thread - which I haven't looked at) have any concept of what userspace
> should do with the value. They simply measure it (and supply appropriate
> threshold interrupts etc)

then what does OS do upon these interrupts? nothing?
who is responsible for changing the backlight, BIOS?

> > 
> >> TSL2561
> >> * infrared (raw value)
> >> * broad spectrum (raw value)
> >> (I'm of the view any derived values should probably be done in userspace)
> >> This one is under IIO at the moment for two reasons.
> >> 1) I hit the same issue of no suitable subsystem, but for a much larger
> >> class of sensor devices. Light sensors are just one example (that's not
> >> to say I mind hiving this lot off to a system of their own).
> >> 2) To provide an event interface (which I haven't yet done)
> >> Driver should also include:
> >> * integration time
> >> * gain control 
> >>
> >> TSL2550
> >> * power state
> >> * operating mode
> >> * lux (actually calculated from two separate readings as
> >> per the tsl2561 but the are not available to userspace)
> >>
> >> Applications:
> >>
> >> 1) Backlight intensity type apps (guessing that covers most people)
> >> 2) Environmental monitoring apps (the crossbow imb400 imote2 daughter
> >> board I'm using doesn't have any screen or other direct interface, its
> >> simply a lightweight sensor platform).
> >> 3) High speed apps (all current sensors are pretty slow so this isn't
> >> yet relevant).
> >>
> >> My personal feelings is that the IIO is overkill for these types
> >> of sensors (slow update rate, tsl2550 takes 400ms, tsl2561 12-400ms)
> >> unless we want the event handling infrastructure. I'm inclined to
> >> say it is unecessary given the same result could be obtained by
> >> polling only a few times a second.
> >>
> > agree.
> > this is not a problem to ACPI ALS device.
> > ACPI sends a notification to the ACPI ALS device when illuminance is
> > changed.
> > 
> >> My comments on ALS may be wrong or misleading as they are based on a
> >> brief read of the code (please correct me!)  A lot of the
> >> infrastructure is only necessary if we have in kernel users
> >>  (and at
> >> the moment the functionality doesn't appear to be there for any such
> >> users to acquire access to these sensors in the first place.  For
> >> example, the approach used by hwmon of letting drivers define their
> >> own attributes seems to me to be more easily extendable than ALS' use
> >> of an ops structure.
> > 
> > I agree that the ops structure is unnecessary.
> > To make the als_sys class more generic, we just need to
> > 1. defines the generic attributes that the native ALS driver must
> > follow.
> > 2. registers an als device in the als_sys class.
> > and the native driver should be responsible for the sysfs attributes.
> Yup. 
> > Because my approach is made by reading the ACPI spec, I'm not sure what
> > should be done in the native driver and what should be done in the
> > generic driver at the beginning.
> For now I'd be tempted to keep as little as possible in the generic
> driver and start moving stuff in only once a particular element
> is verified to be relevant to almost every device.
> > 
> > thanks for pointing this out.
> > 
> >>  For example, I'm not convinced it makes sense for
> >> drivers to have to have a get_adjustment attribute or indeed even
> >> necessarily have a direct illuminance attribute (deriving the relevant
> >> value may be a case of userspace combining several associated
> >> readings).
> > 
> > what these associated readings are?
> > I think we can define some optional attributes besides the required
> > attributes.
> Agreed.
> > but we should make clear what is necessary for an ALS device, and what
> > optional features it may support.
> Yes,
> 
> For now I'd be inclined to stick to the ability to read illuminance 
> in some specified unit.  Perhaps some other flag to specify something
> about the frequency range of the sensor?
> 
ACPI ALS doesn't support this.
what's this frequency range used for?
is there some reason that userspace should know this?

> Maybe similar to hwmon approach allowing for multiple readings of a given
> type?
> 
> illumiance[n]
> illuminmance_type[n]

what's the value in illuminance_type, infrared/visible/ultraviolet?

> 
> Everything else optional.  Actually I'm personally of the view everything
> should be optional as long as any close matches in functionality are given
> the same names.  It's up to userspace to figure out of the device supports
> what it wants to use.  Things that I can envisioned not meeting the above
> but still being appropriately placed within als:
> 
> * Device that simply tells you whether ambient is greater or less than
> backlight value + some offset (perhaps with controllable offset).
> 
sorry, I don't understand this.
IMO, ambient and backlight are two different concepts.
that's why ACPI spec defines "ambient light illuminance to display
luminance" mappings.

> * Weird and wonderful sensor types we can't even envision at this point in
> time!
> 
> Perhaps the trick is to document the 'required' parameters as being those
> required without consulting the maintainer / mailing list then they can be
> adjusted over time as more device drivers are written.
> 
> This is definitely the sort of driver
> where the fine grained power management stuff should be encouraged.  After
> all not a lot of point in having them powered up if the screen is off!
> As ever whether people put this stuff in is up to them.  Others can always
> submit patches adding it drivers at a later date.
> 
agree.

thanks,
rui