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Date: Fri, 18 Apr 2014 13:29:56 +0200
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
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Subject: Re: [Xenomai] Disable L2 write allocate on iMX6q
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To: Mauro Salvini <mauro.salvini@smigroup.net>
Cc: xenomai@xenomai.org

On 04/18/2014 12:00 PM, Mauro Salvini wrote:
> Hello,
> I'm testing an iMX6q Sabrelite board from Element14 with Xenomai 2.6.3.
> 
> First of all I tried kernel 3.0.43 from Xenomai git branch
> ipipe-3.0-imx6q. This kernel works well, a 15 hours running latency test
> (with manually generated system load like dd, netcat, ssh) shows an
> average latency of 5.690us and a worst case latency of 42.186us.
> Then I tried 3.4.6 vanilla kernel, but in this case latency test average
> result for is a bit worse, with a value around 18us. Worst case is about
> the same (<50us).
> I read in mailing list that L2 write allocate is disabled in 3.0.43
> kernel (in facts log says: l2x0: 16 ways, CACHE_ID 0x410000c7, AUX_CTRL
> 0x02870000, Cache size: 1048576 B).
> This feature is enabled in 3.4.6 kernel ( log says: l2x0: 16 ways,
> CACHE_ID 0x410000c7, AUX_CTRL 0x02070000, Cache size: 1048576 B). Could
> this be the cause of latency degradation observed in 3.4.6 kernel? I
> also tried 3.8.13 with same results.
> Kernel configurations are the default ones for each kernel, with hw_perf
> disabled.

Yes, L2 write allocate on imx6 has disastrous effects on latency.
L2 write allocate is disabled starting from the 3.10 kernel for the 
mainline I-pipe patches. See branch ipipe-3.10 in ipipe.git.

The patch which disables L2 write allocate is:
http://git.xenomai.org/ipipe.git/commit/?h=ipipe-3.10&id=05d6bcc014821c286248f6f4c1c953fcbde67f27

and is probably easy to backport to earlier branches (but why would you 
want to use a 3.4 kernel anyway?).

-- 
                                                                Gilles.