Re: [PATCH v2 2/2] virtio: Document DMA coherency

[Will Deacon <will.deacon@arm.com>]

On Thu, Feb 02, 2017 at 04:36:21PM +0000, Robin Murphy wrote:
> Since making use of the DMA API will require the architecture code to
> have the correct notion of device cache-coherency on architectures like
> ARM, explicitly call this out in the virtio-mmio DT binding. The ship
> has sailed for legacy virtio, but let's hope that we can head off any
> future firmware mishaps.
> 
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  Documentation/devicetree/bindings/virtio/mmio.txt | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/virtio/mmio.txt b/Documentation/devicetree/bindings/virtio/mmio.txt
> index 5069c1b8e193..999a93faa67c 100644
> --- a/Documentation/devicetree/bindings/virtio/mmio.txt
> +++ b/Documentation/devicetree/bindings/virtio/mmio.txt
> @@ -7,6 +7,16 @@ Required properties:
>  - compatible:	"virtio,mmio" compatibility string
>  - reg:		control registers base address and size including configuration space
>  - interrupts:	interrupt generated by the device
> +- dma-coherent:	required if the device (or host emulation) accesses memory
> +		cache-coherently, absent otherwise
> +
> +Linux implementation note:
> +
> +virtio devices not advertising the VIRTIO_F_IOMMU_PLATFORM flag have been
> +implicitly assumed to be cache-coherent by Linux, and for legacy reasons this
> +behaviour is likely to remain.  If VIRTIO_F_IOMMU_PLATFORM is advertised, then
> +such assumptions cannot be relied upon and the "dma-coherent" property must
> +accurately reflect the coherency of the device.
>  
>  Example:
>  
> @@ -14,4 +24,5 @@ Example:
>  		compatible = "virtio,mmio";
>  		reg = <0x3000 0x100>;
>  		interrupts = <41>;
> +		dma-coherent;

I think this is a sensible update to the binding and is independent of
whatever we decide to do for IOMMUs and DMA on legacy devices.

Acked-by: Will Deacon <will.deacon@arm.com>

Will