> From: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> On Fri, May 29, 2015 at 12:14:46PM -0500, wdavis@xxxxxxxxxx wrote:
> > From: Will Davis <wdavis@xxxxxxxxxx>
> >
> > Lookup the bus address of the resource by finding the parent host bridge,
> > which may be different than the parent host bridge of the target device.
> >
> > Signed-off-by: Will Davis <wdavis@xxxxxxxxxx>
> > ---
> > arch/x86/kernel/pci-nommu.c | 32 ++++++++++++++++++++++++++++++++
> > 1 file changed, 32 insertions(+)
> >
> > diff --git a/arch/x86/kernel/pci-nommu.c b/arch/x86/kernel/pci-nommu.c
> > index da15918..6384482 100644
> > --- a/arch/x86/kernel/pci-nommu.c
> > +++ b/arch/x86/kernel/pci-nommu.c
> > @@ -38,6 +38,37 @@ static dma_addr_t nommu_map_page(struct device *dev, struct page *page,
> > return bus;
> > }
> >
> > +static dma_addr_t nommu_map_resource(struct device *dev, struct resource *res,
> > + unsigned long offset, size_t size,
> > + enum dma_data_direction dir,
> > + struct dma_attrs *attrs)
> > +{
> > + struct pci_bus *bus;
> > + struct pci_host_bridge *bridge;
> > + struct resource_entry *window;
> > + resource_size_t bus_offset = 0;
> > + dma_addr_t dma_address;
> > +
> > + /* Find the parent host bridge of the resource, and determine the
> > + * relative offset.
> > + */
> > + list_for_each_entry(bus, &pci_root_buses, node) {
> > + bridge = to_pci_host_bridge(bus->bridge);
> > + resource_list_for_each_entry(window, &bridge->windows) {
> > + if (resource_contains(window->res, res))
> > + bus_offset = window->offset;
> > + }
> > + }
>
> I don't think this is safe. Assume we have the following topology, and
> we want to set it up so 0000:00:00.0 can perform peer-to-peer DMA to
> 0001:00:01.0:
>
> pci_bus 0000:00: root bus resource [mem 0x80000000-0xffffffff] (bus address [0x80000000-0xffffffff])
> pci 0000:00:00.0: ...
> pci_bus 0001:00: root bus resource [mem 0x180000000-0x1ffffffff] (bus address [0x80000000-0xffffffff])
> pci 0001:00:01.0: reg 0x10: [mem 0x180000000-0x1803fffff 64bit]
>
> I assume the way this works is that the driver for 0000:00:00.0 would call
> this function with 0001:00:01.0 and [mem 0x180000000-0x1803fffff 64bit].
>
The intention is that pci_map_resource() would be called with the device to
map the region to, and the resource to map. So in this example, we would
call pci_map_resource(0000:00:00.0, [mem 0x180000000-0x1803fffff 64bit]).
The driver for 0000:00:00.0 needs to pass some information to
pci_map_resource() indicating that the mapping is for device 0000:00:00.0.
> We'll figure out that the resource belongs to 0001:00, so we return a
> dma_addr of 0x80000000, which is the bus address as seen by 0001:00:01.0.
> But if 0000:00:00.0 uses that address, it refers to something in the
> 0000:00 hierarchy, not the 0001:00 hierarchy.
>
If the bus addresses are organized as described, is peer-to-peer DMA even
possible with this nommu topology? Is there any way in which device
0000:00:00.0 can address resources under the 0001:00: root bus, since the
bus address range is identical?
> We talked about pci_bus_address() and pcibios_resource_to_bus() earlier.
> What's the subtlety that makes them unusable here? I'd rather not add more
> uses of the pci_root_buses list if we can avoid it.
>
I ran into the challenge I mentioned above, where the only struct pci_dev*
we have is for the target device, and not that of the device that owns the
resource.
> > + dma_address = (res->start - bus_offset) + offset;
> > + WARN_ON(size == 0);
> > + if (!check_addr("map_resource", dev, dma_address, size))
> > + return DMA_ERROR_CODE;
> > + flush_write_buffers();
> > + return dma_address;
> > +}
> > +
> > +
>
> You added an extra blank line here (there was already an extra one before
> nommu_sync_sg_for_device(), which is probably what you copied).
>
Thanks, I'll remove the extra line.
> > /* Map a set of buffers described by scatterlist in streaming
> > * mode for DMA. This is the scatter-gather version of the
> > * above pci_map_single interface. Here the scatter gather list
> > @@ -93,6 +124,7 @@ struct dma_map_ops nommu_dma_ops = {
> > .free = dma_generic_free_coherent,
> > .map_sg = nommu_map_sg,
> > .map_page = nommu_map_page,
> > + .map_resource = nommu_map_resource,
> > .sync_single_for_device = nommu_sync_single_for_device,
> > .sync_sg_for_device = nommu_sync_sg_for_device,
> > .is_phys = 1,
> > --
> > 2.4.0
> >
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