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

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Date:	Thu, 4 Dec 2008 10:27:22 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	edward_estabrook@...lent.com, linux-kernel@...r.kernel.org,
	hjk@...utronix.de, gregkh@...e.de, 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

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.

Hugh
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