From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756303AbaEIWiu (ORCPT ); Fri, 9 May 2014 18:38:50 -0400 Received: from relay5-d.mail.gandi.net ([217.70.183.197]:60267 "EHLO relay5-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751306AbaEIWit (ORCPT ); Fri, 9 May 2014 18:38:49 -0400 X-Originating-IP: 50.43.32.211 Date: Fri, 9 May 2014 15:38:39 -0700 From: Josh Triplett To: "H. Peter Anvin" Cc: Arnd Bergmann , Greg Kroah-Hartman , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org Subject: Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit and 32-bit ports Message-ID: <20140509223837.GB5725@thin> References: <20140509191914.GA7286@jtriplet-mobl1> <9233735.5FfZoZovqP@wuerfel> <536D406D.2080508@zytor.com> <4366326.1D6xUnlac7@wuerfel> <536D46AD.3070608@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <536D46AD.3070608@zytor.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 09, 2014 at 02:20:45PM -0700, H. Peter Anvin wrote: > On 05/09/2014 02:12 PM, Arnd Bergmann wrote: > > > >> However, if we're going to have these devices I'm wondering if having > >> /dev/portw and /dev/portl (or something like that) might not make sense, > >> rather than requiring a system call per transaction. > > > > Actually the behavior of /dev/port for >1 byte writes seems questionable > > already: There are very few devices on which writing to consecutive > > port numbers makes sense. Normally you just want to write a series > > of bytes (or 16/32 bit words) into the same port number instead, > > as the outsb()/outsw()/outsl() functions do. > > > > Indeed. I missed the detail that it increments the port index; it is > virtually guaranteed to be bogus. Exactly. It might make sense to have ioport8/ioport16/ioport32 devices that accept arbitrary-length reads and writes (divisible by the size) and do the equivalent of the string I/O instructions outs/ins, but for the moment I'd like to add the single device that people always seem to want and can't get from /dev/port. If someone's doing enough writes that doing a syscall per in/out instruction seems like too much overhead, they can write a real device driver or use ioperm/iopl. - Josh triplett