On 11/14/2017 01:02 PM, Mark Brown wrote: > On Mon, Nov 13, 2017 at 01:17:42PM -0800, Tim Harvey wrote: > >> 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. > > No, that will simultaneously transmit and recieve three bytes. That's what the driver supposed to do. > If you want to transmit two bytes and then recieve one byte you need > two xfers, one with a len of 2 and a tx_buf, the other with a len of > 1 and a rx_buf. To read a register (mcp251x_read_reg()) the mcp251x does a 3 byte full duplex transfer. The first byte send is the command (read register) the second byte the register number the third byte is a dummy. The first 2 bytes received are ignored the 3rd byte is the register contents. Marc -- Pengutronix e.K. | Marc Kleine-Budde | Industrial Linux Solutions | Phone: +49-231-2826-924 | Vertretung West/Dortmund | Fax: +49-5121-206917-5555 | Amtsgericht Hildesheim, HRA 2686 | http://www.pengutronix.de |
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