how to avoid copies without O_DIRECT?

[Miles <bulkmail@xxxxxxxxx>]

http://kerneltrap.org/node/7563

So Linus really hates O_DIRECT on files and can see no good use for it. 
 In the referenced white paper Andrea suggests the use of O_DIRECT to 
eliminate CPU and memory bandwidth consumption during the process of 
copying to/from user space.

Indeed, lets say I have an embedded system (CPU bandwidth is not 
abundant).  I have a device which is providing an almost never-ending 
stream of data.  This data is DMA'ed into a large physically contiguous 
buffer which is mapped into a process's address space.

I want to write the data in this buffer out to a file without the CPU 
actually having to copy the data.  O_DIRECT seems like the perfect 
mechanism -- the buffer will ultimately get DMA'ed straight out of my 
user-space buffer and out to disk (assuming I meet alignment 
requirements, etc...).

If O_DIRECT is frowned upon, what is the prescribed method for 
accomplishing a zero-copy data movement from kernel->user->kernel?

I looked at the madvise() and posix_fadvise() calls but they only seem 
to alter the cache behavior -- not eliminate it.  And if I don't 
eliminate it, a copy is still taking place (unless there paging magic at 
play).

This question may have already been asked a whole bunch and I apologize 
in advance if it has (there is a lot of noise surrounding a search of 
"O_DIRECT").

Thanks!
-Miles

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