Messages in this thread | | | Date | Wed, 19 Aug 2015 20:23:07 -0400 | From | Matt Porter <> | Subject | Re: [PATCH 2/3] iio: temperature: add max6675 thermocouple converter driver |
| |
On Sat, Aug 08, 2015 at 12:39:40PM +0100, Jonathan Cameron wrote: > On 06/08/15 18:38, Matt Porter wrote: > > On Mon, Aug 03, 2015 at 11:26:12PM +0200, Peter Meerwald wrote: > >> On Mon, 3 Aug 2015, Matt Porter wrote: > > > > ... > > > >>> +static int max6675_read(struct max6675_state *st, int *val) > >>> +{ > >>> + int ret; > >>> + > >>> + ret = spi_read(st->spi, val, 2); > >>> + if (ret < 0) > >>> + return ret; > >>> + > >>> + /* Temperature is bits 14..3 */ > >>> + *val = (*val >> 3) & 0xfff; > >> > >> what about endianness conversion? > >> use be16_to_cpu() > > > > Apologies, I spoke before engaging the brain on my first reply to this > > As specified by the SPI subsystem docs, SPI buffers are always stored > > in native endian order. There is no need for endianness conversion here. > First of all, which doc say this? > Secondly how does SPI know the endianness of the sensor which is what > actually matters here? I2C can in theory make these guarantees as there > is an expected byte order on the wire (even if quite a few drivers don't > conform to the spec anyway). No such guarantee can exist for SPI.
include/linux/spi/spi.h:
* In-memory data values are always in native CPU byte order, translated * from the wire byte order (big-endian except with SPI_LSB_FIRST). So * for example when bits_per_word is sixteen, buffers are 2N bytes long * (@len = 2N) and hold N sixteen bit words in CPU byte order.
So, as you mention, there's no standardized byte order but it's controlled with the per transfer flag and big endian by default.
-Matt
| |