Re: [PATCH 9/9] x86/iommu: use dma_ops_list in get_dma_ops

[Joerg Roedel]

On Fri, Sep 26, 2008 at 01:26:19PM +0530, Amit Shah wrote:
> * On Monday 22 Sep 2008 23:51:21 Joerg Roedel wrote:
> > This patch enables stackable dma_ops on x86. To do this, it also enables
> > the per-device dma_ops on i386.
> >
> > Signed-off-by: Joerg Roedel <joerg.roedel@xxxxxxx>
> > ---
> >  arch/x86/kernel/pci-dma.c     |   26 ++++++++++++++++++++++++++
> >  include/asm-x86/device.h      |    6 +++---
> >  include/asm-x86/dma-mapping.h |   14 +++++++-------
> >  3 files changed, 36 insertions(+), 10 deletions(-)
> >
> > diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> > index b990fb6..2e517c2 100644
> > --- a/arch/x86/kernel/pci-dma.c
> > +++ b/arch/x86/kernel/pci-dma.c
> > @@ -82,6 +82,32 @@ void x86_register_dma_ops(struct dma_mapping_ops *ops,
> >  	write_unlock_irqrestore(&dma_ops_list_lock, flags);
> >  }
> >
> > +struct dma_mapping_ops *find_dma_ops_for_device(struct device *dev)
> > +{
> > +	int i;
> > +	unsigned long flags;
> > +	struct dma_mapping_ops *entry, *ops = NULL;
> > +
> > +	read_lock_irqsave(&dma_ops_list_lock, flags);
> > +
> > +	for (i = 0; i < DMA_OPS_TYPE_MAX; ++i)
> > +		list_for_each_entry(entry, &dma_ops_list[i], list) {
> > +			if (!entry->device_supported)
> > +				continue;
> > +			if (entry->device_supported(dev)) {
> > +				ops = entry;
> > +				goto out;
> > +			}
> > +		}
> > +out:
> > +	read_unlock_irqrestore(&dma_ops_list_lock, flags);
> 
> For PVDMA, we want the "native" dma_ops to succeed first, eg, nommu, and then 
> do our "PV DMA", which is just translating gpa  to hpa and then program the 
> hardware. This isn't being done here. This can be done by extending the 
> return type:
> 
> DMA_DEV_NOT_SUPPORTED
> DMA_DEV_HANDLED
> DMA_DEV_PASS
> 
> Where NOT_SUPPORTED means we should look for the next one in the chain 
> (current return value 0), DEV_HANDLED means the dma operation has been 
> handled successfully (current return value 1) and DEV_PASS means fall back to 
> the next layer and then return back.

I am not sure I fully understand what you mean? Why do we need to call
nommu handlers first for PVDMA devices?
I think that PVDMA devices must always be handled by a pv-dma_ops
implementation. So it makes more sense for me to assign the the dma_ops
of this implementation to the per-device dma_ops structure when we do
the first call to the dma api. So we pay this overhead of finding out
who is responsible only once and not at every call to the dma api.

Joerg

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