Re: [PATCH 3/4 RESEND] Drivers: hv: vmbus: Propagate VMbus coherence to each VMbus device

[Robin Murphy <robin.murphy@arm.com>]

On 2022-03-17 16:25, Michael Kelley via iommu wrote:
> VMbus synthetic devices are not represented in the ACPI DSDT -- only
> the top level VMbus device is represented. As a result, on ARM64
> coherence information in the _CCA method is not specified for
> synthetic devices, so they default to not hardware coherent.
> Drivers for some of these synthetic devices have been recently
> updated to use the standard DMA APIs, and they are incurring extra
> overhead of unneeded software coherence management.
> 
> Fix this by propagating coherence information from the VMbus node
> in ACPI to the individual synthetic devices. There's no effect on
> x86/x64 where devices are always hardware coherent.
> 
> Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> ---
>   drivers/hv/vmbus_drv.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 12a2b37..c0e993ad 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -904,6 +904,21 @@ static int vmbus_probe(struct device *child_device)
>   			drv_to_hv_drv(child_device->driver);
>   	struct hv_device *dev = device_to_hv_device(child_device);
>   	const struct hv_vmbus_device_id *dev_id;
> +	enum dev_dma_attr coherent;
> +
> +	/*
> +	 * On ARM64, propagate the DMA coherence setting from the top level
> +	 * VMbus ACPI device to the child VMbus device being added here.
> +	 * Older Hyper-V ARM64 versions don't set the _CCA method on the
> +	 * top level VMbus ACPI device as they should.  Treat these cases
> +	 * as DMA coherent since that's the assumption made by Hyper-V.
> +	 *
> +	 * On x86/x64 these calls assume coherence and have no effect.
> +	 */
> +	coherent = acpi_get_dma_attr(hv_acpi_dev);
> +	if (coherent == DEV_DMA_NOT_SUPPORTED)
> +		coherent = DEV_DMA_COHERENT;
> +	acpi_dma_configure(child_device, coherent);

acpi_dma_configure is for devices represented in ACPI. The commit 
message implies that these VMBus devices aren't represented in ACPI. 
What gives?

Robin.

>   
>   	dev_id = hv_vmbus_get_id(drv, dev);
>   	if (drv->probe) {