Re: [PATCH v2 3/5] sh/PCI: Pass GAPSPCI_DMA_BASE CPU address to dma_declare_coherent_memory()

[Bjorn Helgaas <bhelgaas@google.com>]

[+cc Magnus, linux-sh (sorry, I forgot to tell stgit to cc you)]

On Wed, May 07, 2014 at 10:15:10AM +0200, Arnd Bergmann wrote:
> On Wednesday 07 May 2014 09:55:16 Arnd Bergmann wrote:
> > On Tuesday 06 May 2014 16:48:33 Bjorn Helgaas wrote:
> > > @@ -51,8 +53,12 @@ static void gapspci_fixup_resources(struct pci_dev *dev)
> > >                 /*
> > >                  * Redirect dma memory allocations to special memory window.
> > >                  */
> > > +               region.start = GAPSPCI_DMA_BASE;
> > > +               region.end = GAPSPCI_DMA_BASE + GAPSPCI_DMA_SIZE - 1;
> > > +               res.flags = IORESOURCE_MEM;
> > > +               pcibios_bus_to_resource(dev->bus, &res, &region);
> > >                 BUG_ON(!dma_declare_coherent_memory(&dev->dev,
> > > -                                               GAPSPCI_DMA_BASE,
> > > +                                               res->start,
> > >                                                 GAPSPCI_DMA_BASE,
> > >                                                 GAPSPCI_DMA_SIZE,
> > >                                                 DMA_MEMORY_MAP |
> > 
> > Not sure if this is an improvement. I understand the intention, but
> > it's actually possible for the offset to be different both ways:
> > 
> > Your patch applies the outbound mem_offset that was provided when
> > registering the MMIO resource for the PCI host bridge. What the driver
> > needs instead is the inbound DMA offset that gets applied by some
> > host bridges that don't have a 1:1 mapping but also don't have an
> > IOMMU.

I don't understand where the inbound DMA offset comes into play.  As I
understand it, we're talking about a region of memory on the PCI bus,
not in system RAM, and there are two ways to access it: (1) the CPU
makes MMIO accesses to it, and (2) the device makes DMA accesses to
it.

The CPU accesses go through the host bridge MMIO aperture and will
have the outbound mem_offset applied to them.  The DMA accesses are
handled by the device itself and there's no host bridge or IOMMU or
inbound DMA offset involved.

I think there are two reasonable ways a PCI driver could use
dma_declare_coherent_memory():

  1) The bus address of the region might be in a BAR.  In that case,
  it should pass pci_resource_start() (the CPU phys_addr_t) as the
  first address, and pci_bus_address(pci_resource_start()) (the bus
  dma_addr_t) as the second.

  2) The bus address of the region might be discovered from the device
  in some non-standard way.  In this case, the driver reads the bus
  address dma_addr_t directly from the device, and it should use
  something like the pcibios_bus_to_resource() code I proposed to find
  the CPU phys_addr_t.

For this platform, all the addresses are hard-coded and there is no
outbound MMIO offset, so we can't tell which to use and in this
system, it doesn't matter if we do anything at all.

The only reason I would make a change here is because the current code
cannot be safely copied (another driver might be used in systems where
the host bridge does have an outbound MMIO offset).  If we make this
code use either strategy, it would be a clue to future users that they
should not assume the physical address is the same as the bus address.

I changed the patch (below) to add a comment and to use the first
strategy (though there isn't an actual BAR, so we can't do it exactly
that way).

> > We know that on this particular platform, both are zero, so
> > the original code works and it will keep working with your change,
> > but I think it's a mistake anyway. I have seen both kinds of offsets
> > in the past on real machines, but I am not aware of any where they
> > are both nonzero and have the same value.
> 
> Hmm, looking at it again, it seem even weirder: the address we register
> for DMA allocations is the same that gets passed into the PCI host
> for MMIO resources. Something is very strange here. Still, I'd rather
> not touch it at all, whatever it does.

It's definitely strange.  It looks to me like the memory on the device
takes up the entire 32KB host bridge MMIO aperture.  I don't know what
driver uses this, but the device must be programmed via I/O ports,
which sort of makes sense given the previous lines in this quirk that
set the BAR1 resource to be in the I/O aperture.

Bjorn



sh/PCI: Pass GAPSPCI_DMA_BASE CPU & bus address to dma_declare_coherent_memory()

From: Bjorn Helgaas <bhelgaas@google.com>

dma_declare_coherent_memory() needs both the CPU physical address and the
bus address of the device memory.  They are the same on this platform, but
in general we should use pcibios_resource_to_bus() to account for any
address translation done by the PCI host bridge.

This makes no difference on Dreamcast, but is safer if the usage is copied
to future drivers.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Magnus Damm <damm@opensource.se>
CC: linux-sh@vger.kernel.org
---
 arch/sh/drivers/pci/fixups-dreamcast.c |   18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/arch/sh/drivers/pci/fixups-dreamcast.c b/arch/sh/drivers/pci/fixups-dreamcast.c
index d6cde700e316..f22127f966ce 100644
--- a/arch/sh/drivers/pci/fixups-dreamcast.c
+++ b/arch/sh/drivers/pci/fixups-dreamcast.c
@@ -31,6 +31,8 @@
 static void gapspci_fixup_resources(struct pci_dev *dev)
 {
 	struct pci_channel *p = dev->sysdata;
+	struct resource res;
+	struct pci_bus_region region;
 
 	printk(KERN_NOTICE "PCI: Fixing up device %s\n", pci_name(dev));
 
@@ -50,11 +52,21 @@ static void gapspci_fixup_resources(struct pci_dev *dev)
 
 		/*
 		 * Redirect dma memory allocations to special memory window.
+		 *
+		 * If this GAPSPCI region were mapped by a BAR, the CPU
+		 * phys_addr_t would be pci_resource_start(), and the bus
+		 * address would be pci_bus_address(pci_resource_start()).
+		 * But apparently there's no BAR mapping it, so we just
+		 * "know" its CPU address is GAPSPCI_DMA_BASE.
 		 */
+		res.start = GAPSPCI_DMA_BASE;
+		res.end = GAPSPCI_DMA_BASE + GAPSPCI_DMA_SIZE - 1;
+		res.flags = IORESOURCE_MEM;
+		pcibios_resource_to_bus(dev, &region, &resource);
 		BUG_ON(!dma_declare_coherent_memory(&dev->dev,
-						GAPSPCI_DMA_BASE,
-						GAPSPCI_DMA_BASE,
-						GAPSPCI_DMA_SIZE,
+						res.start,
+						region.start,
+						resource_size(&res),
 						DMA_MEMORY_MAP |
 						DMA_MEMORY_EXCLUSIVE));
 		break;