Re: can, m_can, tcan4x5x : big jitter between the packets by sending

[Oliver Hartkopp <socketcan@hartkopp.net>]

Hi Michael,

On 19.05.22 17:52, Michael Anochin wrote:

> my application continuously sends 64 bytes CANFD packets with 1MBit/s at 
> the 10ms interval.

How does this application implement this 10ms interval?

Can you check whether

     cangen -g 10 -f -I 123 -L 64 can0

has the same problems?

With

     candump -td can0

you should be able to see some timestamp gaps around 10ms.

> I use tcan4450 on the RPI4 with 5.10.103 Kernel and 
> raspbian. No other significant processes load the CPU.
> 
> When I monitor the traffic with a PCAN adapter on a Windows PC, I notice 
> that the packets sometimes arrive with a delay of 5-9ms. But the next 
> following packet arrive faster as 10ms. My desired interval of 10ms is 
> kept at the jitter of +/- 9ms.
> 
> Running the App on only one CPU core using tasksel improve the jitter 
> somewhat.

What is the use-case for this 10ms cyclic transmission? Maybe the 
CAN_BCM (which uses in-kernel highres timers) could bring an improvement 
for you.

> Am I the only one who observes such large jitter or is the m_can 
> implementation at Perepherie (spi) not so fast from the throughput and 
> is completely normal.
> 
> Maybe I should switch to 5.17 kernel? On 5.17 there are bulk read/write 
> function for spi regmap.

I don't answer to this as I don't have the required SPI knowledge ... 
but if you could upgrade to a newer kernel this is always a good 
approach ;-)

Best regards,
Oliver