CAN required CLK_CANx_APB and CLK_CANx_APB to be running and will not
request any clock by itself. We can ensure both clocks to be running
by preventing the MCU from entering IDLE state.
The SAMD5x/SAME5x Family Data Sheet says in Section
"39.6.9 Sleep Mode Operation" says:
> The CAN can be configured to operate in any idle sleep mode. The CAN
> cannot operate in Standby sleep mode.
>
> [...]
>
> To leave low power mode, CLK_CANx_APB and GCLK_CANx must be active
> before writing CCCR.CSR to '0'. The CAN will acknowledge this by
> resetting CCCR.CSA = 0. Afterwards, the application can restart CAN
> communication by resetting bit CCCR.INIT.
tl;dr: At most SAM0_PM_IDLE is allowed while not shutting down the CAN
controller, but even that will pause communication (including RX).
Apparently, the CAN controller was never tested without also using the
USB peripheral, which kept the clocks running as side effect.
Setting up a DMA transfer can take longer than sending out a buffer
byte by byte if the buffer is small.
DMA only shows advantages for large buffers, using it for every transfer
will cause a net slowdown.
Since we did not come up with a good way to determine the treshold based
on the SPI frequency, just use a fixed buffer for now so that DMA can be
used without slowing things down overall.
`pm_set()` gets called by the idle thread whose stack is too small
for normal DEBUG()/printf().
Use DEBUG_PUTS() instead to print the static debug strings.
This adds supoprt for the Atmel SAMD51 & SAME54 SoC.
The SAME5x/SAMD5x is a line of Cortex-M4F MCUs that share peripherals
with the samd2x Cortex-M0+ and saml1x Cortex-M23 parts.
