Disable Caching for mmap() address

[Benjamin Herrenschmidt]

On Mon, 2009-11-09 at 16:21 -0700, Jonathan Haws wrote:
> All,
> 
> I would like to disable caching for an address that was returned from a call to mmap().  I am using this address for DMA operations in user space and want to make sure that the data cache is turned off for that buffer.
> 
> The way this works is the driver simply takes an address I provide and begins a DMA operation to that location in RAM (I have ensured that this is a physical address I am passing already).  When the DMA is complete, an interrupt fires and the ISR gives a semaphore that the user space application is pending on (RT_SEM from Xenomai).  I have tried simply calling a cache invalidate routine in the ISR before I give the semaphore, but the kernel crashes when I try to call that routine - my guess it because the kernel does not have direct access to that location in memory (only my application does, according to the MMU).
> 
> Anyway, all I want to do is make sure that the buffer is never stored in the cache and that I always fetch it from RAM.  How can I specify that using mmap() on the /dev/mem device, or is there a better way to accomplish this?

There is no "proper" way to do this, in large part because it's not
always legal to map memory non-cached for various reasons I don't
have time to explain right now...

You may be able to get it working though but using a specific driver
with an mmap function that tweaks the attributes or using mmap
of /dev/mem after opening it with O_SYNC (off the top of my mind)

But it's a bit fishy as the kernel has a cacheable mapping of most
of memory and so you may end up with cache aliases...

Cheers,
Ben.