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

[Max Krasnyansky <maxk@qualcomm.com>]

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