Message-ID: <4833EE22.80502@cam.ac.uk>
Date:	Wed, 21 May 2008 10:40:50 +0100
From:	Jonathan Cameron <jic23@....ac.uk>
To:	mgross@...ux.intel.com
CC:	Jonathan Cameron <Jonathan.Cameron@...il.com>,
	spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org,
	LM Sensors <lm-sensors@...sensors.org>
Subject: Re: [spi-devel-general] Accelerometer,	Gyros and ADC's etc within
 the kernel.

Hi Mark,

>> ST Micro LIS3L02DQ 3D accelerometer.  SPI device, no buffering, interrupt 
>> pin
>> raised high on new data being available. Currently the driver assumes, if
>> interrupts are enabled, that this is connected to a specified gpio.
>> (http://www.st.com/stonline/books/pdf/docs/10175.pdf)
> 
> FWIW: I have this device talking to a PIC-18 and pushing the results
> over the serial port.

That's certainly interesting and I guess one of the easiest ways of getting data 
from spi to a desktop machine. I'll admit my interest is more with embedded 
machines.

> WRT linux support, I can't think of a generalized way to create a driver
> that would be able to be used with this device in Linux.  You need to
> know witch IRQ line the DR line is connected too, and if you are
> bit-banging the SPI data off the thing, then you need to know which
> GPIO's of the host CPU you'll be using. 
Yes, and that means we would use the delights of platform data and the generic 
gpio subsystem.  Although I believe generic gpio isn't available on every 
platform as yet, it is getting there and is certainly in place for ARM (and 
cleans up an awful lot of hardware interfaces!)
> If you have SPI hardware then
> you need to know where to pull the data from.  The problem doesn't seem to
> generalize well.
Yes, the intention would not be to generalize the hardware comms, but rather the
interface on the user side.  This is similar to a number of other subsystems 
such as hwmon, where you share functionality as much as possible, but allow very 
different approaches to things like actual hardware register reads as and when 
they are necessary.

> Also, If you are playing with accelerometer data, you likely need some
> real time support or at lest a reliable time stamping of the data to do
> anything interesting.
Definitely. Obviously the accuracy of this time stamp is going to be limited by
the variable nature of when an interrupt handler picks it up, but knowledge of 
the device specs can allow filtering of this timing data.  This is certainly 
something we would want to consider at a later date although it may well be more 
sensible to leave this to userspace applications.

> Another problem area is around SPI itself.  There are variations of
> device implementations around chip select polarity, clock biasing
> (rising,falling, or midpoint) sampling from one SPI part to the next.
Indeed.  This is well handled by the SPI subsystem. I've spent far too much time 
with a scope recently fiddling with these parameters for badly documented chips!

>> VTI SCA3000 E05 3D accelerometer equiped with substantial ring buffer. SPI
>> device.  Can operate either in buffered or direct mode.  In buffered mode, 
>> interrupts
>> can be used to indicate when the ring buffer is 3/4 full triggering a 
>> download to
>> a larger ring buffer in the kernel if necessary.
>> (http://www.vti.fi/en/products-solutions/products/accelerometers/sca3000-accelerometers/)
>>
> 
> ring buffered data can make RT applications harder...
Agreed.  With this sort of device two modes would be needed (and are supported 
by the hardware).  The first gives direct data access and the second uses the 
ring buffer.  Which you use would obviously be dependent on the application. 
Some applications want the most recent data on all occasions, whilst some will 
be more interested in ensuring that all data is captured.

>>
>> Would be nice if practical to allow the framework to include RS232 devices 
>> such
>> as those from www.xsens.com, www.isense.com and others.
> 
> 
> I'm not sure what you are asking for, you started off with SPI driver
> for interfacing a handful of accelerometer devices.  Now your talking
> about the serial port.

The question here is concerned with standardising mainly the userspace 
interfaces (and hardware interfaces only when appropriate). Some of these 
devices (the ST accel above for example) support multiple hardware interfaces 
specs (I2C and SPI for that one).  So in a similar way to it making sense to 
group all TV adapters irrespective of how they are interfaced to the computer, 
in order to ensure a consistent interface to user space, it makes sense to share 
these interfaces amongst drivers talking to these devices irrespective of what 
their hardware interface is.
> 
> Besides the consumer of accelerometer data is user space applications
> (mine attempt to be somewhat RT applications) 

Thanks,

--
Jonathan Cameron

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