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CAN (HMX)

Overview⚓︎

6 CAN-FD channels, typically named can0 to can5

HMX CAN performance tuning⚓︎

The HMX platform has 4 of its 6 CAN interfaces on two SPI buses. Depending on the use case, the interrupt handling CAN be tuned for performance or CPU load.

Use Ethtool to limit the number of interrupts⚓︎

Wait until 6 frames have been received or 1000 µs passed: 

for i in 0 3 4 5; do 
    ethtool -C can${i} tx-usecs-irq 1000 rx-usecs-irq 1000 rx-frames-irq 6 tx-frames-irq 6
done

Turn off DMA for SPI⚓︎

The driver uses DMA for transfers >64 bytes, but the DMA overhead is larger than the gain. As CAN-FD frames (metadata+date) are bigger than 64 bytes, DMA is used quite a lot. Disable DMA with:

echo N>/sys/module/spi_imx/parameters/use_dma

Polling of transmit complete⚓︎

If the driver doesn't use DMA, it uses PIO + transmit complete IRQ. The IRQ also comes with a certain overhead, so polling might be cheaper. This parameter sets the limit where polling is still used. The default is 30 µs. This value depends on your hardware and can be tuned.

Set up a typical CAN bus load and measure the CPU usage with htop. Tweak the polling limit with:

echo 50> /sys/module/spi_imx/parameters/polling_limit_us

Sleep and wake up⚓︎

To enable wake on CAN you will need to set up the channel and go bus on, unless disabled. Wake on individual CAN frame is supported theoretically for CAN1 and CAN2 but has not been tested by us. Current driver does not support it.