linux-kernel - Re: [PATCH 3/3] x86: restore old GART alloc

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Message-ID: <20080924125740.GT24392@amd.com>
Date:	Wed, 24 Sep 2008 14:57:40 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	mingo@...e.hu, linux-kernel@...r.kernel.org, andi@...stfloor.org
Subject: Re: [PATCH 3/3] x86: restore old GART alloc_coherent behavior

On Wed, Sep 24, 2008 at 09:43:59PM +0900, FUJITA Tomonori wrote:
> On Wed, 24 Sep 2008 14:11:27 +0200
> Joerg Roedel <joerg.roedel@....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@....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).

Hmm true, the old code called map_simple and not map_single so it mapped
always.

> 
> 
> > > +
> > > +		flush_gart();
> > 
> > This should depend on need_flush.
> 
> Theoretically, yes, I think. But this patch restores the old GART code
> behavior.

Ok. The map_simple function of GART did it unconditionally too so this
is the old behavior. But its safe here to flush conditionally. I'll send
a follow-up patch.

Joerg

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