From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with ESMTP id D50D46B0044 for ; Tue, 23 Dec 2008 16:33:18 -0500 (EST) Message-ID: <495158F4.5090904@qualcomm.com> Date: Tue, 23 Dec 2008 13:32:36 -0800 From: Max Krasnyansky MIME-Version: 1.0 Subject: Re: [PATCH 1/1] Userspace I/O (UIO): Add support for userspace DMA References: <43FC624C55D8C746A914570B66D642610367F29B@cos-us-mb03.cos.agilent.com> <1228379942.5092.14.camel@twins> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Hugh Dickins Cc: Peter Zijlstra , "edward_estabrook@agilent.com" , "linux-kernel@vger.kernel.org" , "hjk@linutronix.de" , "gregkh@suse.de" , "edward.estabrook@gmail.com" , linux-mm , Thomas Gleixner List-ID: Hugh Dickins wrote: > On Thu, 4 Dec 2008, Peter Zijlstra wrote: >> On Wed, 2008-12-03 at 14:39 -0700, edward_estabrook@agilent.com wrote: >>> The gist of this implementation is to overload uio's mmap >>> functionality to allocate and map a new DMA region on demand. The >>> bus-specific DMA address as returned by dma_alloc_coherent is made >>> available to userspace in the 1st long word of the newly created >>> region (as well as through the conventional 'addr' file in sysfs). >>> >>> To allocate a DMA region you use the following: >>> /* Pass this magic number to mmap as offset to dynamically allocate a >>> chunk of memory */ #define DMA_MEM_ALLOCATE_MMAP_OFFSET 0xFFFFF000UL >>> ... >>> Comments appreciated! >> Yuck! >> >> Why not create another special device that will give you DMA memory when >> you mmap it? That would also allow you to obtain the physical address >> without this utter horrid hack of writing it in the mmap'ed memory. >> >> /dev/uioN-dma would seem like a fine name for that. > > I couldn't agree more. It sounds fine as a local hack for Edward to > try out some functionality he needed in a hurry; but as something > that should enter the mainline kernel in that form - no. Agree with Peter and Hugh here. Also I have a use case where I need to share DMA buffers between two or more devices. So I think we need a generic DMA device that does operations like alloc, mmap, etc. Mmapped regions can then be used with UIO devices. I'll put together a prototype of that some time early next year. Max -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org