[PATCH v12 08/10] dt-bindings: document stall property for IOMMU masters

[Auger Eric]

Hi Jean-Philippe,

On 1/27/21 4:43 PM, Jean-Philippe Brucker wrote:
> On ARM systems, some platform devices behind an IOMMU may support stall,
> which is the ability to recover from page faults. Let the firmware tell us
> when a device supports stall.
> 
> Reviewed-by: Rob Herring <robh at kernel.org>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe at linaro.org>
Reviewed-by: Eric Auger <eric.auger at redhat.com>

Eric
> ---
>  .../devicetree/bindings/iommu/iommu.txt        | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/iommu/iommu.txt b/Documentation/devicetree/bindings/iommu/iommu.txt
> index 3c36334e4f94..26ba9e530f13 100644
> --- a/Documentation/devicetree/bindings/iommu/iommu.txt
> +++ b/Documentation/devicetree/bindings/iommu/iommu.txt
> @@ -92,6 +92,24 @@ Optional properties:
>    tagging DMA transactions with an address space identifier. By default,
>    this is 0, which means that the device only has one address space.
>  
> +- 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.
> +
>  
>  Notes:
>  ======
>