Re: mcp251xfd on RPi 5.4 downstream

[Patrick Menschel <menschel.p@posteo.de>]

>> Its just a bread board install and I had to reduce the Spi-Max-Frequency
>> due to 10cm long cables. It least I believe it's the cable length.
> 
> What happens if you don't reduce the SPI frequency?

CRC Errors above 16Mhz spi frequency.

## No limit
$ dmesg | grep -Ei can\|spi
[   30.006596] CAN device driver interface
[   30.276213] spi_master spi0: will run message pump with realtime priority
[   30.436807] mcp251xfd spi0.0 (unnamed net_device) (uninitialized):
CRC read error at address 0x0000 (length=4, data=00 07 18 04, CRC=0xb29d).
[   30.436835] mcp251xfd spi0.0 (unnamed net_device) (uninitialized):
Controller failed to enter mode Configuration Mode (4) and stays in
Internal Loopback Mode (2).
[   30.437036] mcp251xfd spi0.0 (unnamed net_device) (uninitialized):
CRC read error at address 0x0000 (length=4, data=00 07 18 04, CRC=0xb29d).
[   30.437261] mcp251xfd: probe of spi0.0 failed with error -74

## 18Mhz
$ dmesg | grep -Ei can\|spi
[   30.345967] CAN device driver interface
[   30.503372] spi_master spi0: will run message pump with realtime priority
[   30.542854] mcp251xfd spi0.0 (unnamed net_device) (uninitialized):
CRC read error at address 0x0000 (length=4, data=60 07 18 04, CRC=0xb28c).
[   30.543057] mcp251xfd spi0.0 (unnamed net_device) (uninitialized):
CRC read error at address 0x0000 (length=4, data=60 07 18 04, CRC=0xb28c).
[   30.543286] mcp251xfd: probe of spi0.0 failed with error -74

## 16Mhz
$ dmesg | grep -Ei can\|spi
[   29.496863] CAN device driver interface
[   30.028215] spi_master spi0: will run message pump with realtime priority
[   30.171532] mcp251xfd spi0.0 can0: MCP2517FD rev0.0 (-RX_INT
+MAB_NO_WARN +CRC_REG +CRC_RX +CRC_TX +ECC -HD c:40.00MHz m:16.00MHz
r:16.00MHz e:0.00MHz) successfully initialized.


--
Patrick