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

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Date:	Thu, 04 Dec 2008 09:39:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	edward_estabrook@...lent.com
Cc:	linux-kernel@...r.kernel.org, hjk@...utronix.de, gregkh@...e.de,
	edward.estabrook@...il.com, hugh <hugh@...itas.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 Wed, 2008-12-03 at 14:39 -0700, edward_estabrook@...lent.com wrote:
> From: Edward Estabrook <Edward_Estabrook@...lent.com>
> 
> Here is a patch that adds the ability to dynamically allocate (and
> use) coherent DMA from userspace by extending the userspace IO driver.
> This patch applies against 2.6.28-rc6.
> 
> 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
> 
> void* memory = mmap (NULL, size, PROT_READ | PROT_WRITE , MAP_SHARED,
> fd, DMA_MEM_ALLOCATE_MMAP_OFFSET); u_int64_t *addr = *(u_int64_t *)
> memory;
> 
> where 'size' is the size in bytes of the region you want and fd is the
> opened /dev/uioN file.
> 
> Allocation occurs in page sized pieces by design to ensure that
> buffers are page-aligned.
> 
> Memory is released when uio_unregister_device() is called.
>
> I have used this extensively on a 2.6.21-based kernel and ported it to
> 2.6.28-rc6 for review / submission here.
> 
> 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.
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