From: Andrew Miklas <public@mikl.as>
Reply-To: public@mikl.as
To: linux-kernel@vger.kernel.org
Subject: Userspace DMA
Date: Sun, 16 Nov 2003 18:42:00 -0500
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Hi,


Is there an accepted way of doing userspace DMA with Linux?

I've gone over a large number of posts, and all seem to recommend something 
different, and each method seems to involve certain drawbacks.

What I'd like to do is allow a process to be able to do operations like 
pci_alloc_consistent and pci_free_consistent.  I'm trying to do this to allow 
a driver running under the Bochs emulator to interact with its (non-emulated) 
hardware using DMA.  

I was originally going to do a pci_alloc_consistent and then mmap the space 
into the emulator's process.  The process would then get the physical address 
of the memory through an ioctl, or I would leave the physical address at the 
start of the allocated buffer for the process to later retrieve.

However, it appears that remap_page_range won't work with addresses obtained 
from pci_alloc_consistent.  I've also read that using the nopage callback can 
cause issues if the range I need to allocate is greater than one page.

Most of the approaches I've found seem to require that the hardware be capable 
of scatter-gather DMA (video4linux and bttv, for ex.).  I've also seen one 
solution that had the driver do a pci_alloc_consistent on a certain ioctl, 
and then return the physical address of the buffer.  The process would then 
mmap /dev/mem to access the buffer.  However, someone warned that this method 
could have coherency issues.

Is there some other method that I should be looking at to accomplish this?


Thanks,


Andrew
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