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