MPC7455 DMA buffer strangeness

Adrian Cox adrian at
Tue Jul 6 18:34:04 EST 2004

On Mon, 2004-07-05 at 09:31, Oliver Korpilla wrote:
> I'm mapping a DMA buffer allocated with pci_alloc_consistent() into user
> space with a mmap hook like this:
> int vme_mmap(struct file *file_ptr, struct vm_area_struct *vma)
> {
>         DPRINTF("Attempting to map %#lx bytes of memory at "
>                 "physical address %#lx\n", vma->vm_end - vma->vm_start,
>                 vma->vm_pgoff << PAGE_SHIFT);
>         vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);

The memory allocated by pci_alloc_consistent will be cacheable memory,
as PowerPC 60x is a cache coherent architecture. You've set up a
non-cacheable userspace mapping to the same address by using
pgprot_noncached(). Two mappings of the same physical memory must have
the same cache settings.

> On the MPC7455, though, write accesses seem to be applied or not applied
> in a somewhat random fashion. Sometimes an offsetted write into the buffer
> is there, and sometimes not. Writing at the begin of the buffer seems to
> disapper always.

This probably only shows on the 7455 because the caches of the 7455 are
much larger.

- Adrian Cox
Humboldt Solutions Ltd.

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