[Xenomai] Disable L2 write allocate on iMX6q

Gilles Chanteperdrix gilles.chanteperdrix at xenomai.org
Fri Apr 18 14:11:51 CEST 2014

On 04/18/2014 02:08 PM, Mauro Salvini wrote:
> On Fri, 2014-04-18 at 13:29 +0200, Gilles Chanteperdrix wrote:
>> 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?).
> Thank you very much, I will try the patch.
> I haven't a strict need to use 3.4 kernel, it was only a test. By the
> way, is there a general rule to follow to choose a kernel version for an
> ARM architecture (e.g. latest kernel release against older but
> most-SOC-optimized release)? For example, for Xenomai 2.6.3 there are
> three ipipe patches (for 3.4.6, 3.5.7 and 3.8.13 kernels), but as stated
> into README file iMXQ6 should use ipipe patch contained into mcx dir,
> that is for 3.0.43 kernel .

No, the README does not say that you should use the patch for the 3.0.43
kernel. The patch is there IN CASE you want to use that kernel, because
that kernel is based on the one provided by Freescale. But you are very
much encouraged to use more recent mainline releases.


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