PCI DMA to user mem on mpc83xx

[Ira W. Snyder]

On Mon, May 23, 2011 at 11:12:41AM +0200, Andre Schwarz wrote:
> Ira,
> 
> we have a pretty old PCI device driver here that needs some basic rework 
> running on 2.6.27 on several MPC83xx.
> It's a simple char-device with "give me some data" implemented using 
> read() resulting in zero-copy DMA to user mem.
> 
> There's get_user_pages() working under the hood along with 
> SetPageDirty() and page_cache_release().
> 
> Main goal is to prepare a sg-list that gets fed into a DMA controller.
> 
> I wonder if there's a more up-to-date/efficient and future proof scheme 
> of creating the mapping.
> 
> 
> Could you provide some pointers or would you stick to the current scheme ?
> 

This scheme is the best you'll come up with for zero-copy IO. I used
get_user_pages_fast(), but otherwise my implementation was the same.
These interfaces should be fairly future proof.

In the end, I realized that most of my transfers were 4 bytes in length,
and zero copy IO was a waste of effort. I decided to use mmap instead.

Ira