Re: rt application question with rs232 communication

[Cliff Brake <cliff.brake@gmail.com>]

Clarification, I don't have the RT patch applied, just CONFIG_PREEMPT
from Linus's kernel.  Reviewing my notes from several years ago, I ran
into a similar issue and the RT patch fixed it, so now to figure out
the best way to get a recent RT patched ARM kernel ...

Cliff

On Mon, Mar 9, 2009 at 5:34 PM, Cliff Brake <cliff.brake@gmail.com> wrote:
> Hello,
>
> I have an application where I do the following:
>
> - PXA270 CPU
> - communicating over serial ports on 40ms intervals
> - communication scheme is very simple in that the linux system sends a
> packet every 40ms and expects a response back within the 40ms window
> - currently using 2.6.27 with CONFIG_PREEMPT
>
> If I set the application comm thread to real time priority, the
> sending works very well.  There is very little jitter in the 40ms send
> timing.  However, it seems that the system has trouble receiving the
> response in a timely fashion.  I often observe that it takes at least
> 10ms for the application to receive the response data after the data
> has appeared on the rs232 bus.  Is the RT patch something that would
> help speed up the serial receive response time, or is there something
> else going on that I am missing?  The system is fairly busy with
> graphical processing, but the sending is always right on schedule.
>
> It seems that interrupts may be involved with this problem as when
> sending, the send data is smaller than the FIFO size, and is probably
> put into the fifo during the write, where with the read, an interrupt
> needs to fire, etc.  Does all the kernel code that handles the serial
> data automatically run at the calling application's RT priority, or is
> there additional work required to make everything that touches the
> data RT?
>
> I'm also trying to figure out how well RT will work for CAN
> communications.  Once again we have the same problem -- a RT user
> space application needs to send and receive data with strict timing.
> Which, I can make a RT thread in user space run well (toggle a gpio
> with very little jitter), how does this translate to all the kernel
> components involved in the socketcan stack (networking, etc) that need
> to touch the data?
>
> Thanks,
> Cliff
>
> --
> =======================
> Cliff Brake
> http://bec-systems.com
>



-- 
=======================
Cliff Brake
http://bec-systems.com
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