Re: [PATCH 3/3] x86: restore old GART alloc_coherent behavior

FUJITA Tomonori
2008-09-24T12:46:33Z

On Wed, 24 Sep 2008 14:11:27 +0200
Joerg Roedel <joerg.roedel@amd.com> wrote:

> On Wed, Sep 24, 2008 at 08:48:37PM +0900, FUJITA Tomonori wrote:
> > Currently, GART alloc_coherent tries to allocate pages with GFP_DMA32
> > for a device having dma_masks > 24bit < 32bits. If GART gets an
> > address that a device can't access to, GART try to map the address to
> > a virtual I/O address that the device can access to.
> >
> > But Andi pointed out, "The GART is somewhere in the 4GB range so you
> > cannot use it to map anything < 4GB. Also GART is pretty small."
> >
> > http://lkml.org/lkml/2008/9/12/43
> >
> > That is, it's possible that GART doesn't have virtual I/O address
> > space that a device can access to. The above behavior doesn't work for
> > a device having dma_masks > 24bit < 32bits.
> >
> > This patch restores old GART alloc_coherent behavior (before the
> > alloc_coherent rewrite).
>
> Patchset looks good in principle. But I have a small question, see
> below.
>
> >
> > Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
> > ---
> > arch/x86/kernel/pci-gart_64.c | 43 +++++++++++++++++++---------------------
> > 1 files changed, 20 insertions(+), 23 deletions(-)
> >
> > diff --git a/arch/x86/kernel/pci-gart_64.c b/arch/x86/kernel/pci-gart_64.c
> > index 7e08e46..25c94fb 100644
> > --- a/arch/x86/kernel/pci-gart_64.c
> > +++ b/arch/x86/kernel/pci-gart_64.c
> > @@ -487,31 +487,28 @@ static void *
> > gart_alloc_coherent(struct device *dev, size_t size, dma_addr_t *dma_addr,
> > gfp_t flag)
> > {
> > - void *vaddr;
> > dma_addr_t paddr;
> > unsigned long align_mask;
> > - u64 dma_mask = dma_alloc_coherent_mask(dev, flag);
> > -
> > - vaddr = (void *)__get_free_pages(flag | __GFP_ZERO, get_order(size));
> > - if (!vaddr)
> > - return NULL;
> > -
> > - paddr = virt_to_phys(vaddr);
> > - if (is_buffer_dma_capable(dma_mask, paddr, size)) {
> > - *dma_addr = paddr;
> > - return vaddr;
> > - }
> > -
> > - align_mask = (1UL << get_order(size)) - 1;
> > -
> > - *dma_addr = dma_map_area(dev, paddr, size, DMA_BIDIRECTIONAL,
> > - align_mask);
> > - flush_gart();
> > -
> > - if (*dma_addr != bad_dma_address)
> > - return vaddr;
> > -
> > - free_pages((unsigned long)vaddr, get_order(size));
> > + struct page *page;
> > +
> > + if (force_iommu && !(flag & GFP_DMA)) {
> > + flag &= ~(__GFP_DMA | __GFP_HIGHMEM | __GFP_DMA32);
> > + page = alloc_pages(flag | __GFP_ZERO, get_order(size));
> > + if (!page)
> > + return NULL;
> > +
> > + align_mask = (1UL << get_order(size)) - 1;
> > + paddr = dma_map_area(dev, page_to_phys(page), size,
> > + DMA_BIDIRECTIONAL, align_mask);
>
> Can't we check if a mapping is required before calling dma_map_area?
> Your recently introduced IOMMU helper functions should make that easy
> and GART address space is a rare resource. AFAIR this is also the
> behaviour of the old generic dma_alloc_coherent function.

I think that the behavior of the old generic dma_alloc_coherent
function and GART is different.

The old GART code does virtual mappings only with force_iommu option
enabled. The old GART code always do virtual mappings with force_iommu
option enabled (unless GFP_DMA is set).

> > +
> > + flush_gart();
>
> This should depend on need_flush.

Theoretically, yes, I think. But this patch restores the old GART code
behavior.