linux-kernel - different data rate in IIO ?

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Message-ID: <4f675c805256ce92bcc9d694ab89e411@mail.gmail.com>
Date:	Mon, 30 Apr 2012 13:08:54 -0700
From:	Ge Gao <ggao@...ensense.com>
To:	Jonathan Cameron <jic23@....ac.uk>
Cc:	linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
	linux-kernel@...r.kernel.org
Subject: different data rate in IIO ?

Dear all,
I am currently developing a driver for a chip that has gyro, accelerometer
and compass sensor together and these sensor data could come at different
rate. There could be more data coming from this chip because this chip has
on-chip CPU to do some data processing. The IIO subsystem is in some sense
"fixed" once "enable" is 1. "Fixed" means the element and sequence inside
ring buffer is fixed. For example, if MPU9150, which is a 9-axis chip,
containing gyro, accelerometer and compass, is developed, the ring buffer
would have byte_per_datum of 32 bytes(6 + 6 + 6  = 18; 18 rounding up to
24; and 24 plus timestamp) if all sensors and all axis are enabled . So
every data packet should contain this amount of data no matter what. If I
have gyro running at 200 HZ, accelerometer running at 100Hz and compass
running at 50 Hz, this will have problems. Because I can't provide
accelerometer data and compass data for each packet. Some packets could
miss data. I have to fake data for these packets, either by repeating or
other non-standard ways.  Is this supposed to be? Because we could have
other data item which is even slower(10HZ quaternion data, for example).
This way, it will be more trouble. Because each data element has different
rate, while IIO needs them at the same rate.
                The best way is to have a header for each packet to
indicate what packet it is. But this way seems to violate the design goal
of IIO.  That would be more like input subsystem because input subsystem
uses different code type to distinguish different type of data thus
allowing different data type mixed together. If such driver is written,
all files under "scan_element" would be meaningless and useless.
	I got some suggestions about using multiple IIO devices in one
driver because one IIO device can only has one ring buffer. It could be OK
to handle this. However, since IIO device allocation is to allocate the
private data directly along with IIO device, it seems one IIO driver can
only have one IIO device. Could IIO kernel accept such practice that one
IIO driver has more than one IIO device? Or could there be some changes in
the IIO code such that such scenario is taken care of in the future?
Thanks.

Ge
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