linux-kernel - Re: [PATCH 1/1] Userspace I/O (UIO): Add support for userspace DMA

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Date:	Tue, 23 Dec 2008 13:32:36 -0800
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Hugh Dickins <hugh@...itas.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"edward_estabrook@...lent.com" <edward_estabrook@...lent.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hjk@...utronix.de" <hjk@...utronix.de>,
	"gregkh@...e.de" <gregkh@...e.de>,
	"edward.estabrook@...il.com" <edward.estabrook@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/1] Userspace I/O (UIO): Add support for userspace DMA

Hugh Dickins wrote:
> On Thu, 4 Dec 2008, Peter Zijlstra wrote:
>> On Wed, 2008-12-03 at 14:39 -0700, edward_estabrook@...lent.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




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