Re: [PATCH v5] can: flexcan: Re-write receive path to use MB queue instead of FIFO

[Alexander Stein <alexander.stein@systec-electronic.com>]

Hello David,

On Tuesday 30 September 2014 09:13:55, David Jander wrote:
> > > Nevertheless, emptying the FIFO in the IRQ handler will still be a big
> > > improvement, since the only thing that could still kill the driver and
> > > cause message loss is interrupt latency, which normally should not be so
> > > high. NAPI scheduling latency is probably much worse, and this is the
> > > biggest issue with the current driver.
> > > 
> > > Any suggestion on what to do?
> > 
> > Get rid of NAPI and use RT-preempt with proper priorities :) But joke aside,
> > which workload does increase the NAPI latency so much, an overrun occurs? I
> > tested CAN bursts on i.MX35 without any loss.
> 
> I have seen overruns on an i.MX6 at only 250kbaud receiving back-to-back
> messages of 1 byte long. I usually test bursts of 10000 messages or more.

Mh, that's odd. I have run several tests a 1MBaud on an i.MX35 with 2 CAN nodes attached each sending bursts of 250 message every 200ms with a total message count of 250000 each. No overruns, losses or message misordering.

> Things get a lot worse if you also happen to have kernel messages output to a
> serial console and plug in an USB device (because there are printk's in the
> EHCI driver inside spin locks with interrupts disabled!!), but that's a
> different story.

Eek. Well, adding quiet to the command line avoids that. IIRC there is even a Kconfig option to disable that announce to kernel log.

Best regards,
Alexander

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Dipl.-Inf. Alexander Stein

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