From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [RESEND PATCH 2/4] asm-generic: io: don't perform swab during {in,out} string functions Date: Fri, 19 Oct 2012 13:53:06 +0100 Message-ID: <20121019125306.GJ4582@mudshark.cambridge.arm.com> References: <1350488704-3711-1-git-send-email-will.deacon@arm.com> <1350488704-3711-3-git-send-email-will.deacon@arm.com> <1350518655.4678.120.camel@pasglop> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:49516 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758416Ab2JSMxT (ORCPT ); Fri, 19 Oct 2012 08:53:19 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Geert Uytterhoeven Cc: Benjamin Herrenschmidt , "linux-kernel@vger.kernel.org" , "linux-arch@vger.kernel.org" , Mike Frysinger On Thu, Oct 18, 2012 at 06:48:16AM +0100, Geert Uytterhoeven wrote: > On Thu, Oct 18, 2012 at 2:04 AM, Benjamin Herrenschmidt > wrote: > > The sort story is that endianness is not a property of the IO port but > > of the information that transit through it. If you're just going to copy > > it into memory, you want to preserve it's format and so do not byteswap. > > > > The byteswap we do on standard accessors is a "helper" because we assume > > that underneath those IO ports are registers that are Little Endian. But > > when using one as a window to a byte stream, we must not arbitrarily > > swap the byte stream. We copy it as-is to memory, and then one can work > > at interpreting the various fields that might or might not be present in > > that stream with the appropriate accessors for memory accesses. > > So assume you have the bytestream "Hello, world!\n" in memory on the > PCI device.I.e. > > 00000000 48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 0a |Hello, world!.| > > You want to copy it to system RAM using readsl(), which does: > > u32 *buf = buffer; > do { > u32 x = __raw_readl(addr + PCI_IOBASE); > *buf++ = x; > } while (--count); > > On little endian, the first __raw_readl() should return "0x6c6c6548", so > it is stored correctly by "*buf = x ". > On big endian, the first __raw_readl() should return "0x48656c6c" instead, > else it's stored incorrectly by "*buf = x ". So far so good... > But the PCI bus is little endian, so I expect __raw_readl() would return > "0x6c6c6548", and thus needs swapping? I think this would only happen if your busses are wired swapped, in which case you'll have to handle this in your arch code because reading from a device and then writing to memory will end up with the data in the wrong order (the data stream won't be affected by passing through the CPU). Will