PCI: Add DMA alias quirk for Microsemi Switchtec NTB
Add a quirk for the Microsemi Switchtec parts to allow DMA access via
non-transparent bridging to work when the IOMMU is turned on.
This exclusively addresses the ability of a remote NT endpoint to perform
DMA accesses through the locally enumerated NT endpoint. Other aspects of
the Switchtec NTB functionality, such as interrupts for doorbells and
messages are independent of this quirk, and will work whether the IOMMU is
on or off.
When a requestor on one NT endpoint accesses memory on another NT endpoint,
it does this via a devfn proxy ID. Proxy IDs are statically assigned to
each NT endpoint by the NTB hardware as part of the release-from-reset
sequence prior to PCI enumeration. These proxy IDs cannot be modified
dynamically, and are not visible to the host during enumeration.
When the Switchtec NTB driver loads it will map local requestor IDs, such
as the root complex and transparent bridge DMA engines, to proxy IDs by
populating those requestor IDs in hardware mapping table table entries.
This establishes a fixed relationship between a requestor ID and a proxy
ID.
When a peer on a remote NT endpoint performs an access within a particular
translation window in it's NT endpoint BAR address space, that access is
translated to a DMA request on the local endpoint's bus. As part of the
translation process, the original requestor ID has its devfn replaced with
the proxy ID, and the bus portion of the BDF is replaced with the bus of
the local NT endpoint. Thus, the DMA access from a remote NT endpoint will
appear on the local bus to have come from the unknown devfn which the IOMMU
will reject.
Interrogate NTB hardware registers for each remote NT endpoint to obtain
the proxy IDs that have been assigned to it and alias them to the local
(enumerated) NT endpoint's device. The IOMMU then accepts the remote proxy
IDs as if they were requests coming directly from the enumerated endpoint,
giving remote requestors access to memory resources which the local host
has made available.
Note that the aliasing of the proxy IDs cannot be performed at the driver
level given the current IOMMU architecture. Superficially this is because
pci_add_dma_alias() symbol is not exported. Functionally, the current
IOMMU design requires the aliasing to be performed prior to the creation of
IOMMU groups. If a driver were to attempt to use pci_add_dma_alias() in
its probe routine it would fail since the IOMMU groups have been set up by
that time. If the Switchtec hardware supported dynamic proxy ID
(re-)assignment this would be an issue, but it does not.
To further clarify static proxy ID assignment: While the requester ID to
proxy ID mapping can be dynamically changed, the number and value of proxy
IDs given to an NT EP cannot, even for dynamic reconfiguration such as
hot-add. Therefore, the chip configuration must account a priori for the
proxy IDs needs, considering both static and dynamic system configurations.
For example, a port on the chip may not having anything plugged into it at
start of day; but it must have a sufficient number of proxy IDs assigned to
accommodate the supported devices which may be hot-added.
Switchtec NTB functionality with the IOMMU off is unchanged by this quirk.
Signed-off-by: Doug Meyer <dmeyer@gigaio.com>
[bhelgaas: use hard-coded Device IDs instead of adding #defines for each]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
1 file changed