[11/37] dt-bindings: document stall and PASID properties for IOMMU masters

Message ID 20180212183352.22730-12-jean-philippe.brucker@arm.com
State Not Applicable, archived
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Series
  • Shared Virtual Addressing for the IOMMU
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Commit Message

Jean-Philippe Brucker Feb. 12, 2018, 6:33 p.m.
On ARM systems, some platform devices behind an IOMMU may support stall
and PASID features. Stall is the ability to recover from page faults and
PASID offers multiple process address spaces to the device. Together they
allow to do paging with a device. Let the firmware tell us when a device
supports stall and PASID.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
---
 Documentation/devicetree/bindings/iommu/iommu.txt | 24 +++++++++++++++++++++++
 1 file changed, 24 insertions(+)

Comments

Rob Herring Feb. 19, 2018, 2:51 a.m. | #1
On Mon, Feb 12, 2018 at 06:33:26PM +0000, Jean-Philippe Brucker wrote:
> On ARM systems, some platform devices behind an IOMMU may support stall
> and PASID features. Stall is the ability to recover from page faults and
> PASID offers multiple process address spaces to the device. Together they
> allow to do paging with a device. Let the firmware tell us when a device
> supports stall and PASID.
> 
> Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
> ---
>  Documentation/devicetree/bindings/iommu/iommu.txt | 24 +++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/iommu/iommu.txt b/Documentation/devicetree/bindings/iommu/iommu.txt
> index 5a8b4624defc..8066b3852110 100644
> --- a/Documentation/devicetree/bindings/iommu/iommu.txt
> +++ b/Documentation/devicetree/bindings/iommu/iommu.txt
> @@ -86,6 +86,30 @@ have a means to turn off translation. But it is invalid in such cases to
>  disable the IOMMU's device tree node in the first place because it would
>  prevent any driver from properly setting up the translations.
>  
> +Optional properties:
> +--------------------
> +- dma-can-stall: When present, the master can wait for a transaction to
> +  complete for an indefinite amount of time. Upon translation fault some
> +  IOMMUs, instead of aborting the translation immediately, may first
> +  notify the driver and keep the transaction in flight. This allows the OS
> +  to inspect the fault and, for example, make physical pages resident
> +  before updating the mappings and completing the transaction. Such IOMMU
> +  accepts a limited number of simultaneous stalled transactions before
> +  having to either put back-pressure on the master, or abort new faulting
> +  transactions.
> +
> +  Firmware has to opt-in stalling, because most buses and masters don't
> +  support it. In particular it isn't compatible with PCI, where
> +  transactions have to complete before a time limit. More generally it
> +  won't work in systems and masters that haven't been designed for
> +  stalling. For example the OS, in order to handle a stalled transaction,
> +  may attempt to retrieve pages from secondary storage in a stalled
> +  domain, leading to a deadlock.
> +
> +- pasid-bits: Some masters support multiple address spaces for DMA, by
> +  tagging DMA transactions with an address space identifier. By default,
> +  this is 0, which means that the device only has one address space.

So 3 would mean 8 address spaces?

Maybe pasid-num-bits would be a bit clearer. Either way,

Reviewed-by: Rob Herring <robh@kernel.org>

Rob
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Patch

diff --git a/Documentation/devicetree/bindings/iommu/iommu.txt b/Documentation/devicetree/bindings/iommu/iommu.txt
index 5a8b4624defc..8066b3852110 100644
--- a/Documentation/devicetree/bindings/iommu/iommu.txt
+++ b/Documentation/devicetree/bindings/iommu/iommu.txt
@@ -86,6 +86,30 @@  have a means to turn off translation. But it is invalid in such cases to
 disable the IOMMU's device tree node in the first place because it would
 prevent any driver from properly setting up the translations.
 
+Optional properties:
+--------------------
+- dma-can-stall: When present, the master can wait for a transaction to
+  complete for an indefinite amount of time. Upon translation fault some
+  IOMMUs, instead of aborting the translation immediately, may first
+  notify the driver and keep the transaction in flight. This allows the OS
+  to inspect the fault and, for example, make physical pages resident
+  before updating the mappings and completing the transaction. Such IOMMU
+  accepts a limited number of simultaneous stalled transactions before
+  having to either put back-pressure on the master, or abort new faulting
+  transactions.
+
+  Firmware has to opt-in stalling, because most buses and masters don't
+  support it. In particular it isn't compatible with PCI, where
+  transactions have to complete before a time limit. More generally it
+  won't work in systems and masters that haven't been designed for
+  stalling. For example the OS, in order to handle a stalled transaction,
+  may attempt to retrieve pages from secondary storage in a stalled
+  domain, leading to a deadlock.
+
+- pasid-bits: Some masters support multiple address spaces for DMA, by
+  tagging DMA transactions with an address space identifier. By default,
+  this is 0, which means that the device only has one address space.
+
 
 Notes:
 ======