On 05/20/2015 01:30 PM, William Davis wrote:
-----Original Message-----
From: Mark Hounschell [mailto:markh@xxxxxxxxxx]
Sent: Wednesday, May 20, 2015 7:11 AM
To: Bjorn Helgaas; William Davis
Cc: joro@xxxxxxxxxx; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-
pci@xxxxxxxxxxxxxxx; Terence Ripperda; John Hubbard; jglisse@xxxxxxxxxx;
konrad.wilk@xxxxxxxxxx; Jonathan Corbet; David S. Miller
Subject: Re: [PATCH v2 4/7] DMA-API: Add dma_(un)map_resource()
documentation
On 05/19/2015 07:43 PM, Bjorn Helgaas wrote:
[+cc Dave, Jonathan]
On Mon, May 18, 2015 at 01:25:01PM -0500, wdavis@xxxxxxxxxx wrote:
From: Will Davis <wdavis@xxxxxxxxxx>
Add references to both the general API documentation as well as the
HOWTO.
Signed-off-by: Will Davis <wdavis@xxxxxxxxxx>
---
Documentation/DMA-API-HOWTO.txt | 39
+++++++++++++++++++++++++++++++++++++--
Documentation/DMA-API.txt | 36 +++++++++++++++++++++++++++++++--
---
2 files changed, 68 insertions(+), 7 deletions(-)
diff --git a/Documentation/DMA-API-HOWTO.txt
b/Documentation/DMA-API-HOWTO.txt index 0f7afb2..89bd730 100644
--- a/Documentation/DMA-API-HOWTO.txt
+++ b/Documentation/DMA-API-HOWTO.txt
@@ -138,6 +138,10 @@ What about block I/O and networking buffers? The
block I/O and
networking subsystems make sure that the buffers they use are valid
for you to DMA from/to.
+In some systems, it may also be possible to DMA to and/or from a
+peer device's MMIO region, as described by a 'struct resource'. This
+is referred to as a peer-to-peer mapping.
+
DMA addressing limitations
Does your device have any DMA addressing limitations? For example,
is @@ -648,6 +652,35 @@ Every dma_map_{single,sg}() call should have its
dma_unmap_{single,sg}()
counterpart, because the bus address space is a shared resource and
you could render the machine unusable by consuming all bus addresses.
+Peer-to-peer DMA mappings can be obtained using dma_map_resource()
+to map another device's MMIO region for the given device:
+
+ struct resource *peer_mmio_res = &other_dev->resource[0];
+ dma_addr_t dma_handle = dma_map_resource(dev, peer_mmio_res,
+ offset, size, direction);
+ if (dma_handle == 0 || dma_mapping_error(dev, dma_handle))
+ {
+ /*
+ * If dma_handle == 0, dma_map_resource() is not
+ * implemented, and peer-to-peer transactions will not
+ * work.
+ */
+ goto map_error_handling;
+ }
+
+ ...
+
+ dma_unmap_resource(dev, dma_handle, size, direction);
+
+Here, "offset" means byte offset within the given resource.
+
+You should both check for a 0 return value and call
+dma_mapping_error(), as dma_map_resource() can either be not
+implemented or fail and return error as outlined under the
dma_map_single() discussion.
+
+You should call dma_unmap_resource() when DMA activity is finished,
+e.g., from the interrupt which told you that the DMA transfer is done.
+
If you need to use the same streaming DMA region multiple times and
touch
the data in between the DMA transfers, the buffer needs to be synced
properly in order for the CPU and device to see the most up-to-date
and @@ -765,8 +798,8 @@ failure can be determined by:
- checking if dma_alloc_coherent() returns NULL or dma_map_sg
returns 0
-- checking the dma_addr_t returned from dma_map_single() and
dma_map_page()
- by using dma_mapping_error():
+- checking the dma_addr_t returned from dma_map_single(),
+dma_map_resource(),
+ and dma_map_page() by using dma_mapping_error():
dma_addr_t dma_handle;
@@ -780,6 +813,8 @@ failure can be determined by:
goto map_error_handling;
}
+- checking if dma_map_resource() returns 0
+
- unmap pages that are already mapped, when mapping error occurs in
the middle
of a multiple page mapping attempt. These example are applicable to
dma_map_page() as well.
diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index 5208840..c25c549 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -283,14 +283,40 @@ and <size> parameters are provided to do partial
page mapping, it is
recommended that you never use these unless you really know what the
cache width is.
+dma_addr_t
+dma_map_resource(struct device *dev, struct resource *res,
+ unsigned long offset, size_t size,
+ enum dma_data_direction_direction)
+
+API for mapping resources. This API allows a driver to map a peer
+device's resource for DMA. All the notes and warnings for the other
+APIs apply here. Also, the success of this API does not validate or
+guarantee that peer-to-peer transactions between the device and its
+peer will be functional. They only grant access so that if such
+transactions are possible, an IOMMU will not prevent them from
+succeeding.
If the driver can't tell whether peer-to-peer accesses will actually
work, this seems like sort of a dubious API. I'm trying to imagine
how a driver would handle this. I guess whether peer-to-peer works
depends on the underlying platform (not the devices themselves)? If
we run the driver on a platform where peer-to-peer *doesn't* work,
what happens? The driver can't tell, so we just rely on the user to
say "this isn't working as expected"?
Yes, it's quite difficult to tell whether peer-to-peer will actually work,
and it usually involves some probing and heuristics on the driver's part.
I wouldn't say that this makes it a dubious API - it's a piece of the
puzzle that's absolutely necessary for a driver to set up peer-to-peer in
an IOMMU environment.
I currently just do
page = virt_to_page(__va(bus_address));
then just use the normal API. Works for writes anyway.
Most currently available hardware doesn't allow reads but will allow writes
on PCIe peer-to-peer transfers. All current AMD chipsets are this way. I'm
pretty sure all Intel chipsets are this way also.
Most != all. As an example, Mellanox offers the ability to do peer-to-peer
transfers:
http://www.mellanox.com/page/products_dyn?product_family=116
which would indicate there is at least some platform out there which allows
peer-to-peer reads. I don't think that that being a minority configuration
should preclude it from support.
What happens with reads
is they are just dropped with no indication of error other than the data
will not be as expected. Supposedly the PCIe spec does not even require any
peer-to-peer support. Regular PCI there is no problem and this API could be
useful. However I doubt seriously you will find a pure PCI motherboard that
has an IOMMU.
I don't understand the chipset manufactures reasoning for disabling PCIe
peer-to-peer reads. We would like to make PCIe versions of our cards but
their application requires peer-to-peer reads and writes. So we cannot
develop PCIe versions of the cards.
Again, Regular PCI there is no problem and this API could be useful.
IOMMU or not.
If we had a pure PCI with IOMMU env, how will this API handle when the 2
devices are on the same PCI bus. There will be NO IOMMU between the devices
on the same bus. Does this API address that configuration?
What is the expected behavior in this configuration? That the "mapping"
simply be the bus address (as in the nommu case)?
I suspect just using the bus address would sort of defeat one or more
purposes of the IOMMU. The bus address would certainly be what I would
want to use though.
In an IOMMU environment, the DMA ops would be one of the IOMMU
implementations, so these APIs would create a mapping for the peer device
resource, even if it's on the same bus. Would a transaction targeting that
mapping be forwarded upstream until it hits an IOMMU, which would then send
the translated request back downstream? Or is my understanding of this
configuration incorrect?
It's my understanding of the IOMMU that is lacking here. I have no idea
if that is actually what would happen. Does it?
Regards
Mark
Thanks,
Will
Mark
+If this API is not provided by the underlying implementation, 0 is
+returned and the driver must take appropriate action. Otherwise, the
+DMA address is returned, and that DMA address should be checked by
+the driver (see dma_mapping_error() below).
+
+void
+dma_unmap_resource(struct device *dev, dma_addr_t dma_address, size_t
size,
+ enum dma_data_direction direction)
+
+Unmaps the resource previously mapped. All the parameters passed in
+must be identical to those passed in to (and returned by) the
+mapping API.
+
int
dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
-In some circumstances dma_map_single() and dma_map_page() will fail
to create -a mapping. A driver can check for these errors by testing
the returned -DMA address with dma_mapping_error(). A non-zero return
value means the mapping -could not be created and the driver should take
appropriate action (e.g.
-reduce current DMA mapping usage or delay and try again later).
+In some circumstances dma_map_single(), dma_map_page() and
+dma_map_resource() will fail to create a mapping. A driver can check
+for these errors by testing the returned DMA address with
+dma_mapping_error(). A non-zero return value means the mapping could
+not be created and the driver should take appropriate action (e.g.
reduce current DMA mapping usage or delay and try again later).
int
dma_map_sg(struct device *dev, struct scatterlist *sg,
--
2.4.0
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