From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756723AbcA2SDr (ORCPT ); Fri, 29 Jan 2016 13:03:47 -0500 Received: from mail-lf0-f48.google.com ([209.85.215.48]:35059 "EHLO mail-lf0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750868AbcA2SDp (ORCPT ); Fri, 29 Jan 2016 13:03:45 -0500 Subject: Re: [PATCH 3/7] usb: gadget: pxa25x_udc: use readl/writel for mmio To: =?UTF-8?Q?Krzysztof_Ha=c5=82asa?= , Arnd Bergmann References: <1453997722-3489596-1-git-send-email-arnd@arndb.de> <1453997843-3489728-1-git-send-email-arnd@arndb.de> <1453997843-3489728-3-git-send-email-arnd@arndb.de> Cc: Felipe Balbi , linux-arm-kernel@lists.infradead.org, Felipe Balbi , linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, Robert Jarzmik , Haojian Zhuang , Daniel Mack , Imre Kaloz , Greg Kroah-Hartman From: Sergei Shtylyov Organization: Cogent Embedded Message-ID: <56ABA97C.1000501@cogentembedded.com> Date: Fri, 29 Jan 2016 21:03:40 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello. On 01/29/2016 07:18 PM, Krzysztof Hałasa wrote: >> The unclear part here is for IXP4xx, which supports both big-endian >> and little-endian configurations. So far, the driver has done >> no byteswap in either case. I suspect that is wrong and it would >> actually need to swap in one or the other case, but I don't know >> which. > > If at all, I guess it should swap in LE mode. But it's far from certain. > >> It's also possible that there is some magic setting in >> the chip that makes the endianess of the MMIO register match the >> CPU, and in that case, the code actually does the right thing >> for all configurations, both before and after this patch. > > This is IMHO most probable. > > Actually, the IXP4xx is "natural" in BE mode (except for PCI) and > normally in LE mode it's order-coherent, meaning 32-bit "integer" > accesses need no swapping, but 8-bit and (mostly unused) 16-bit > transfers need swapping. > > Anyway, I think readl()/writel() do the right thing: in BE mode they > swap PCI accesses and don't swap normal registers, Alas, readl()/writel() don't know what registers you are calling them for, they were designed for PCI which is little-endian, so they will swap in BE mode. MBR, Sergei