Disable Caching for mmap() address

[Jonathan Haws]

> 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...


Thanks for the response, Ben.  I am hoping that by passing a mem= argument to the kernel at boot time, the memory that I am setting aside for my DMA will be kept hidden from the kernel and the MMU.  I am then mapping that memory in user space with mmap() on /dev/mem and that descriptor is being opened with the O_SYNC flag.  I just wanted to make sure I was covering all my bases.

Thanks,

Jonathan