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. _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
