Re: c_can: (newbie) high system load when frame not acked?

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 01/13/2015 04:10 PM, Viktor Babrian wrote:
>> Maybe you can turn the error interrupts off and still have it
>> function. Maybe it won't (may be required for bus off recovery).
> 
> Thanks for the detailed answer. I read the pages and took a look into
> the code.
> 
> Regarding state-change behavior, DCAN controller is similar to the one
> in i.MX6. (It has explicit interrupt source for Act->Warn and Pass->BOFF
> transitions).
> Although as I understand CAN, you have to have a successful RX or TX
> event to have the counters decrease. That  means that you will have an
> immediate report (i.e. interrupt) on Warn->Active, Pass->Warn and
> BOFF->Active. As far as I see, this latter is explicitly handled by
> upper layers so driver will have to know it anyway.
> This means that if I turn off bus error monitoring, the only event I
> will not get immediate report of is Warn->Pass. (Pass->Warn can be
> reported on successful message events if the driver is implemented
> accordingly. Currently it isn't). However it could be reported later
> upon the first successful message event. I beleive missing an accurate
> report of active->passive state change is far better than having a
> non-responding system in the field in scenarios that do happen.
> 
> Anyway I modified the code so that state change interrupt is not enabled
> when user sets berr-reporting off (as you kindly suggested). It solves
> the load issue. Bus-off recovery behavior did not change (I made a few
> tests only). Btw the driver disables all interrupts in bus off so I
> guess it does not matter what exact flags were set.
> 
>> The consequence (depending on the CPU and Linux build options) is
>> 10%-100% CPU utilisation when there's nothing to ACK the packet, and a
>> higher figure again when the bus isn't terminated.
>>
>> CAN is meant to be used in cars or factories. Running with only one
>> node or a disconnected bus is not a "normal" condition, so the
>> hardware and drivers don't handle those conditions well.
> Well error states are defined by the spec so they ARE normal. What is
> not normal is to bring down system performace when they happen.
> 
> 
> I alose added a line to put the controller in init mode in c_can_stop()
> so that bringing the bus down will actually cause the controller stop
> transmitting. I hope it's the way to do this, it would be nice if some
> expert could verify this.

Can you send patches?

Marc

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