Re: [PATCH] MIPS: Fixups of ALSA memory maps

[Ralf Baechle <ralf@linux-mips.org>]

On Mon, Nov 16, 2009 at 06:12:22PM +0100, Takashi Iwai wrote:

> Actually, this has been a looong-standing problem.
> I have a series of patches to fix these issues, but it's more
> intensively involved with dma_*() functions.
> 
> The patches can be found in test/dma-fix branch of sound GIT tree.
>   git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound-2.6.git test/dma-fix
> 
> This basically adds dma_mmap_coherent() function to feasible
> architectures, which is already implemented for ARM, so far.

Cool - but needs a little further tweaking to work right.  That's a
solution which will use uncached accesses on all MIPS systems.

IP27/IP35-family machines will explode when you try that.  Eventually the
cache coherency logic will notice that cache, directory caches and memory
have become inconsistent and bombard the CPU with a bunch of nasty
exceptions.

For cache-coherent machines otoh it's a big waste of performance.

int dma_mmap_coherent(struct device *dev, struct vm_area_struct *vma,
                      void *cpu_addr, dma_addr_t handle, size_t size)
{
        struct page *pg;

	if (!plat_device_is_coherent(dev))
		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
        cpu_addr = (void *)dma_addr_to_virt(handle);
        pg = virt_to_page(cpu_addr);

        return remap_pfn_range(vma, vma->vm_start,
                               page_to_pfn(pg) + vma->vm_pgoff,
                               size, vma->vm_page_prot);
}
EXPORT_SYMBOL(dma_mmap_coherent);

Thomas - you're the IP28 specialist.  Would the plat_device_is_coherent()
above have to become a cpu_is_noncoherent_r10000() call?  Any further
nasties?

  Ralf