Guidelines to make RT patch for driver

Guidelines to make RT patch for driver

[Mikhail Lodigin]

Hello,
on my AT91SAM9263 based board RT patched kernel works great! Thanks to 
all involved. But when ethernet is in use - latency, measured by 
cyclictest, reach 7000000 and beyond.
The ethernet driver for this processor is macb.c. (in 3.2 it is in 
drivers/net/ethernet/cadence, in 3.0 it is in drivers/net/).
I'm ready to find out how to RT-patch it and of course share the 
resulted patch, but I need some starting point for patching drivers 
besides general RT-stuff.  I've noticed that some network drivers 
patched in one way and others a in another. Maybe some discussion 
existed which I didn't found. So any help would be greatly appreciated.
Thanks in advance,
Mikhail

Re: Guidelines to make RT patch for driver

[Thomas Gleixner]

On Fri, 30 Mar 2012, Mikhail Lodigin wrote:

> Hello,
> on my AT91SAM9263 based board RT patched kernel works great! Thanks to all
> involved. But when ethernet is in use - latency, measured by cyclictest, reach
> 7000000 and beyond.
> The ethernet driver for this processor is macb.c. (in 3.2 it is in
> drivers/net/ethernet/cadence, in 3.0 it is in drivers/net/).
> I'm ready to find out how to RT-patch it and of course share the resulted
> patch, but I need some starting point for patching drivers besides general
> RT-stuff.  I've noticed that some network drivers patched in one way and
> others a in another. Maybe some discussion existed which I didn't found. So
> any help would be greatly appreciated.

There is no general recipe. The best way to do it is to use the
tracer. cyclictest has a commandline option for stopping the trace
when the latency exceeds a given treshold.

     cyclictest -p80 -n -f -b 1000

will start the function tracer and stop it once the latency exceeds
1000us.

Thanks,

	tglx