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

[Takashi Iwai]

At Mon, 16 Nov 2009 18:43:24 +0100,
Ralf Baechle wrote:
> 
> 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.

OK, that's really bad.

> 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?

Thanks for checking!


thanks,

Takashi
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