linux-kernel - Re: HID Sensor support for True/Magnetic North usage attributes

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Message-ID: <53764EDE.8090008@linux.intel.com>
Date:	Fri, 16 May 2014 10:46:06 -0700
From:	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:	Reyad Attiyat <reyad.attiyat@...il.com>
CC:	jic23@...nel.org, linux-kernel@...r.kernel.org,
	linux-iio@...r.kernel.org
Subject: Re: HID Sensor support for True/Magnetic North usage attributes

On 05/13/2014 07:14 PM, Reyad Attiyat wrote:
> Dear IIO/HID maintainers,
>
> I have a device, Surface Pro, that has the hid-sensor-hub and many
> sensors attached. With the help of Srinivas I was able to get them all
> working except for the magnometer. It uses the hid-magn-3d driver as
> it should but it does not contain an axis (X, Y, Z) usage attributes.
> Instead it only has a True North usage attribute. I see two solutions
> to this problem and was inquiring which one would work best?
>
> 1) Modify the hid-magn-3d driver to handle True North attribute. I
> realize there might not be many devices that have this so not sure if
> appropriate. I think this could be done; by passing a variable amount
> of IIO Channels when setting up the hid-magn-3d driver, depending on
> how many axis and/or if it find True/Magnetic North usage attribute.
I prefer this approach. The spec doesn't say whether magnetic flux for 
x,y or z are mandatory.
Ultimately they are used to calculate the true north. So if the hub is 
trying to expose true north,
we should add this attribute.

"Heading True North SV – Indicates compass true north heading is not 
compensated.
                     Default unit of measure is degrees; can be 
overridden using
                    explicit Unit and/or Unit Exponent."

Thanks,
Srinivas
> 2) Create a whole new driver that handles True/Magnetic North. This
> would not work on my device as it is set to Compass 3D Usage
> Attribute. This could be resolved by adding another quirk for the
> Surface to ensure it used the new driver.
>
> For both options I think we'd need a new IIO_MOD_NORTH for the
> iio_chan_spec, as the current ones don't really apply. I like the
> first solution as it could allow for handling of devices with only one
> or two axis present. I do realize if the hid-mangn-3d driver was
> changed it's name would not make sense anymore and the pattern I'm
> notcing is a driver for each HID Usage.
>
> Here's a link to the hid report description with some labels for my device:
> Bug 73321 Comment 7
> https://bugzilla.kernel.org/show_bug.cgi?id=73321#c7
>
> I'd be willing to work on this. Just wanted to know what would work best
>
> Thank You,
> Reyad Attiyat
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