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

From: Hugh Dickins
Date: Thu Dec 04 2008 - 05:27:26 EST

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

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