Re: [Xenomai-core] Xenomai on Freescale i.MX21 (ARM926J)

[Philippe Gerum <rpm@xenomai.org>]

ROSSIER Daniel wrote:
> 
> Dear all,
> 
> We finally succeeded in the port of Xenomai on our Freescale i.MX21 board (ARM-926J);
> (The board used is the CSB535FS issued from Cogent with the BSP from Microcross)
> 
> To have further technical references, please see there: http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=i.MX_LITEKIT&parentCode=i.MX21&nodeId=0162468rH31143ZrDR
> 
> So, you will find two patches: the first one (patch-linux-2.6.14.imx21_1.0.0) is used to patch the Linux 2.6.14 for supporting the board. Then, the second one (adeos-ipipe-2.6.14-arm-imx21-0.1.00.patch) can be used against the imx21-enabled kernel for Xenomai, simply using the prepare-kernel.sh script. The patch was originally inspired from the Integrator/ARM11 patch; thanks to its author :-)
> 
> We will publish soon some results regarding the different latencies, but so far we measured about 2us between the IRQ and the timer reprogramming (at the ipipe level). The problem now is the jitter which is pretty high: about 1-2us, with some occasional peaks up to 5us.
> Having quite a few experience with this kind of board, we don't know if it comes from the code itself, or purely from the hardware. Any idea/suggestions would be welcome. For periodic tasks around 20us, everything works perfecly.
>

Those results are pretty impressive actually. Assuming that you compare 
those figures with those obtained from traditional standalone RTOS e.g. 
VxWorks on the same board, the critical difference with Xenomai is that 
Linux is competing for the hw resources, and for instance, happily 
trashes the cache under Xenomai's feet when scheduling its own tasks 
during Xeno's idle time. Additionally, Adeos adds a small overhead due 
to the interrupt virtualization, in exchange of real-time predictability 
for their delivery. Therefore, in this respect, 5 us does not look that 
bad already.

Are 20 us a measure of the worst-case interrupt latency (i.e. running 
latency -t2), or scheduling latency in user-space (i.e. running latency 
-t0)?

> I don't know to what extent this (1 or 2) patch(es) can be integrated in the official distribution;
but of course we are willing to make any adaptations to fulfill the 
requirements for that.

For the technical part, I guess that anyone interested in the ARM 
support for Adeos/Xenomai will review this code; I'll be one of these 
people, for sure. The usefulness of such contribution looks obvious to me.

For the non-technical part, a pre-requisite for adding code to the Adeos 
or Xenomai codebases is to properly identify it as coming from a 
legitimate source, so if you, as a submitter, can confirm that you may 
freely contribute it on behalf of the HEIG-VD (I guess?) or yourself, 
that's fine with us. Btw, at first glance, I did not find any additional 
GPL copyrights coming with your changes in the patches, it would be 
better to have them, so that we always know who contributed to what, as 
much as possible.

> 
> It's a first step; I hope it will help some other ARM9 developers and look forward to reading some feedbacks.
> 
> I profit to thank a lot the Xenomai team (Philippe, Jan, Gilles and the others) for their excellent work and support).
> 

Xenomai exists because people participate in the Sisyphean task of 
making it better, like you just did. In other words, welcome to the 
band. Let's roll the rock.

> Kind regards
> 
> Daniel
> 
> 
> 
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-- 

Philippe.