Re: CAN messages being lost on i.MX25 with flexcan - continued (was CAN messages being lost on i.MX25 with flexcan - 2012-04-19)

[Wolfgang Grandegger <wg@grandegger.com>]

On 10/30/2013 09:43 AM, "Martin Kožuský [KK micro s.r.o.]" wrote:
> -------- Original Message  --------
> Subject: Re: CAN messages being lost on i.MX25 with flexcan - continued
> (was CAN messages being lost on i.MX25 with flexcan - 2012-04-19)
> From: Wolfgang Grandegger
> To: Martin Kozusky, linux-can@vger.kernel.org
> Date: 29. Říjen 2013 15:30:08
> 
>> On 10/29/2013 01:40 PM, Martin Kozusky wrote:
>>> Dne 29.10.2013 13:03, Wolfgang Grandegger napsal(a):
>>>> On 10/29/2013 11:46 AM, Martin Kozusky wrote:
>> ...
>>>>> Hello Wolfgang,
>>>>> it seems that my architecture (arm/mx25 on 2.6.35 kernel) is missing
>>>>> HAVE_FUNCTION_GRAPH_TRACER, HAVE_DYNAMIC_FTRACE options so it won't be
>>>>> that easy, will be?
>>>>> Timestamps that ftrace is showing me are in 10 miliseconds resolution,
>>>>> that won't help me much :(
>>
>> Are high resolution timers enabled in the kernel? Still, event tracing
>> could would already be useful.
>>
>>>> Probably that version is to old for proper ftrace support. The 100us
>>>> you
>>>> measured for alloc_can_skb() is worst case, right? What is the mean
>>>> value?
>>>
>>> Now I checked again and logged every call (don't know if realy
>>> everything was logged but something was :) and I see that it is not
>>> 100usec, but only around 20usec (mean value - checked by eye). There
>>> were some very long calls (around 2ms!) that were puttings errors in my
>>> sum/count formula (may be I should filter out calls longer that
>>> 200usec), with this error it was not 100usec, but almost 1ms (my value
>>> was only 32bit and was overflowing when I wrote it first time). So
>>> normally around 20usec, but with very long calls around 2-3 ms (looks
>>> like those long are periodic - each 6th - 8th call is much longer, but
>>> not all the time)
>>
>> Are these long latencies related to the SDcard accesses? I think the
>> problem is the rather long latencies caused by other kernel activity. In
>> your case caused by the MMC (SDcard) driver, I assume. The Flexcan
>> controller does buffer up to 5 messages before loosing packets.
> I think it is not primary related to SD card, those tests I was doing
> lately were done when system was idle, no special processes were
> running. But when I do access SD card, problems get bigger.
> 
>>> But still with those 20usec call, there are many RX overflows, if I
>>> disable the call alloc_can_skb, there are no overflows and all is
>>> received (but still not processed further, because I don't have skb
>>> ... )
>>
>> Could you run this test for a longer time? The probability is just lower
>> that RX gets interrupted for a longer time, I think.
> I have run it for a few minutes, problem is still the same :(
>  
>> I see a few approaches to overcome this problem:
>>
>> - fix the MMC driver to cause less latency. If you are lucky this is
>>    already the case in more recent versions of the kernel.
> Hard to say if this helps when it's also doing in idle.

OK, then I misinterpreted your error description.

>> - Use the "-rt" extension (CONFIG_PREEPMT_RT). It will then allow to
>>    adjust priorities of soft and hard irq threads.
> I don't know if there is a patch for this :(
>  
>> - Do the RX processing in the interrupt context, which would require
>>    some custom modifications.
> I was thinking about writing data directly to fifo file, my program
> would read from it

Well, before hacking something you should try to find out what is
provoking the long latencies (> 1ms). FTrace is your friend. Therefore I
would try to get a more recent version of the kernel running somehow.
2.6.39 should already be much better.

Wolfgang.