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MCP251x SPI CAN controller on Cavium ThunderX


I have been unsuccessful getting a MCP251x SPI based CAN controller
working on a CN80xx using Linux mainline.

When a register is read from the mcp251x driver the
octeon_spi_do_transfer() gets a spi_message with a single spi_xfer of
len=3, a tx_buf, and an rx_buf which I believe is supposed to shift
out 3 bytes out MOSI and shift in 3 bytes from MISO where the last
byte shifted in would be the response.

The cavium CN80xx MPI_TX register has fields for 'Number of bytes to
transmit' (TXNUM) and 'Total number of bytes to shift (transmit and
receive)' (TOTNUM) and these are both getting set to 3 by
octeon_spi_do_transfer() but I find that this causes unexpected data
in the shifted in response unless I make TOTNUM = TXNUM + 1.

I should also note that Cavium has a software suite called the 'BDK'
which provides a CLI to SPI transfers which allows you to set the
TXNUM and TOTNUM fields uniquely and if I send a 2-byte command
(TXNUM=2) to read a register (READ command followed by the register)
and a 1 byte read (thus TOTNUM=3) then I get the response from the
mcp251x I expect.

Is there something I'm misunderstanding about the Linux SPI API and
perhaps is there a bug in the cavium SPI controller driver or do I
perhaps have it configured wrong with respect to CPHA and/or CPOL?