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
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