Michal is trying hard to get the 7i80/hm2_eth.c driver working for RT-PREEMPT 
using normal socket I/O from an RT thread.

The hopes with RT-PREEMPT are obviously pegged on the assumption: 'the kernel 
is hardened, so I'm free to use any system calls from an RT thread and still 
get decent latency, so we can leverage of the stock linux driver universe'. 

This might not be a valid assumption.

I just had a long offline discussion with Nicholas Mc Guire from the rt-preempt 
effort on a related issue (signaling a non-RT thread from an RT thread; the 
method I proposed uses a write(2) system call on an eventfd(2) file descriptor; 
one of the most efficient ways to signal a poll(2) compatible event between 
threads).

Nicholas pointed me to the fact that almost all system system calls might be 
spoilers for RT threads; _including write(2)_ , hardened kernel or not. While I 
didnt fully understand every detail he said, the message was clear: the above 
assumption might not hold. 

He also pointed at that hoping for low latency when using the kernel IP stack 
may be a lost cause to start with. He hinted towards a UIO-based userland stack 
being worked on for exactly this purpose. I am still searching for details on 
this.

My recommendation is: 

  peer-review your assumptions to avoid a time waster here. 
  Get in touch with linux-rt-us...@vger.kernel.org, describe what you intend, 
get advice straight from the people who make it happen.  
  

- Michael


--

ps: IMV the search for a low-latency network I/O method is still on. Note that 
we already have a userland PCI framework thanks to Charles, which might be 
a startng point.

See also: 
  http://static.mah.priv.at/public/rtlws-proceedings/rtlws-2012/proc/Yang.pdf
  http://os.inf.tu-dresden.de/ddekit/dde_rtlws11.pdf.

semi-related: anyone looking into RT-PREEMPT on ARM CPU's should read: 
http://lwn.net/images/conf/rtlws11/papers/proc/p11.pdf
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