Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752711AbbHWPof (ORCPT ); Sun, 23 Aug 2015 11:44:35 -0400 Received: from saturn.retrosnub.co.uk ([178.18.118.26]:43868 "EHLO saturn.retrosnub.co.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751444AbbHWPod (ORCPT ); Sun, 23 Aug 2015 11:44:33 -0400 Subject: Re: [PATCH 2/3] iio: temperature: add max6675 thermocouple converter driver To: Matt Porter References: <1438635410-3757-1-git-send-email-mporter@konsulko.com> <1438635410-3757-3-git-send-email-mporter@konsulko.com> <20150806173844.GI23173@beef> <55C5EA7C.70906@kernel.org> <20150820002307.GD6276@beef> Cc: Peter Meerwald , Linux IIO List , Linux Kernel Mailing List From: Jonathan Cameron Message-ID: <55D9EA5F.4040206@kernel.org> Date: Sun, 23 Aug 2015 16:44:31 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <20150820002307.GD6276@beef> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1934 Lines: 51 On 20/08/15 01:23, Matt Porter wrote: > 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. > Thanks, I'd never picked up on that before! Jonathan > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/