Re: at91 driver lost CAN messages

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

On Thursday 16 March 2017 09:34:38, Marc Kleine-Budde wrote:
> On 03/16/2017 09:01 AM, Oliver Hartkopp wrote:
> > On 13.03.2017 10:47, Marc Kleine-Budde wrote:
> >> On 03/13/2017 07:54 AM, Alexander Stein wrote:
> >>>> @Marc: What was the reason to implement NAPI that days? Has it ever
> >>>> been
> >>>> proved that NAPI had a remarkable positive effect in opposite to a
> >>>> direct CAN frame processing in the interrupt context?
> >>> 
> >>> One reason to use NAPI is out-of-order issues on SMP systems :-( You
> >>> know the problem: http://marc.info/?l=linux-can&m=143642135416355&w=2
> >>> If this is not resolved, things get even worse.
> >>> Well, now that there is generic FIFO-offload feature (drivers may need
> >>> changes to use it) there is an easy way to read CAN frames with low
> >>> latency while processing them in non-IRQ context.
> >>> IMHO reading CAN frames in softirq (NAPI) may be too late in some cases,
> >>> in RT kernels this may be even worse.
> >> 
> >> ACK.
> > 
> > Would then implementing CAN rx-offload on all(!) CAN drivers fix the
> > NAPI-too-late and the out-of-order issue?
> 
> Implementing rx-offload on the drivers using NAPI, will fix the
> NAPI-too-late issue (and turn it into a IRQ too late issue).
> 
> Implementing rx-offload on the drivers passing skbs in the IRQ handler
> will probably fix the out-of-order issue.
> 
> I'm not sure if the USB and SPI drivers are affected by the out-of-order
> issue.

IMO all drivers using netif_rx (IRQ context) rather han netif_receive_skb 
(NAPI poll context) are suffering from out-of-order issue. And I guess every 
USB CAN driver is using netif_rx in the URB callback function, which is 
essentially IRQ context. MCP2512 uses netif_rx_ni instead, dunno what is the 
actual difference to netif_rx.

Let's see if I get time to cook rx-offload on a USB driver.

Best regards,
Alexander
-- 
Dipl.-Inf. Alexander Stein
SYS TEC electronic GmbH
alexander.stein@systec-electronic.com

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