Re: [Gumstix-users] User space or kernel driver for ADC SPI?

[John L. <joh...@gm...>]

The 19us requirement comes from an ADC sampling at 52kSample/s.

I think I should throw the userspace spidev approach out the window.

Do you think it would be likely to read every data point by writing a custom
kernel protocol driver that uses the mcspi protocol driver?

Thanks,
John

On Wed, Jun 17, 2009 at 11:17 AM, Hugo Vincent <hug...@gm...>wrote:

> On 17/06/2009, at 8:41 PM, John Lists wrote:
>
> 19usec is very very short. Even if you were using PREEMPT_RT, it's
> unlikely (uh, very *very* unlikely based on my benchmarks) you'd be
> able to meet this deadline 100% of the time from userspace, even on an
> otherwise idle system. Without the RT patch, I would guess you would
> have 0% chance of meeting this deadline consistently using events and
> spidev from userspace.


> I'd suggest you repost your question to the linux-omap list (
> lin...@vg...
> ) for a reply from a McSPI driver expert. And try to be more specific
> with how you would handle this device from user space - "GPIO event in
> userspace" is meaninglessly vague.
>
> Other than in very particular circumstances, it is usually necessary,
> and always preferable, to handle interrupts in the kernel.
>
> If you wanted to quickly try an experiment, hook up the uio-driver to
> a spare GPIO pin in board-overo.c, configure it's interrupt, and
> externally make the pin pulse every 19 usec (e.g. with signal
> generator). Then try reading from /dev/uioX and see if you're missing
> interrupts. Try under different types of load (IO, cache thrashing,
> heavy memory traffic etc).
>
> See the uio docs here:
>
> http://www.free-electrons.com/kerneldoc/latest/DocBook/uio-howto/ch03s02.html
>
> http://www.free-electrons.com/kerneldoc/latest/DocBook/uio-howto/ch04s03.html
>
> Regards,
> Hugo Vincent
>
>
>
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