firmware: arm_scmi: Add atomic mode support to virtio transport

Add support for .mark_txdone and .poll_done transport operations to SCMI
VirtIO transport as pre-requisites to enable atomic operations.

Add a Kernel configuration option to enable SCMI VirtIO transport polling
and atomic mode for selected SCMI transactions while leaving it default
disabled.

Link: https://lore.kernel.org/r/20220217131234.50328-4-cristian.marussi@arm.com
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
Cc: Peter Hilber <peter.hilber@opensynergy.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/Kconfig

@@ -118,6 +118,21 @@ config ARM_SCMI_TRANSPORT_VIRTIO_VERSION1_COMPLIANCE
 	  the ones implemented by kvmtool) and let the core Kernel VirtIO layer
 	  take care of the needed conversions, say N.
 
+config ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+	bool "Enable atomic mode for SCMI VirtIO transport"
+	depends on ARM_SCMI_TRANSPORT_VIRTIO
+	help
+	  Enable support of atomic operation for SCMI VirtIO based transport.
+
+	  If you want the SCMI VirtIO based transport to operate in atomic
+	  mode, avoiding any kind of sleeping behaviour for selected
+	  transactions on the TX path, answer Y.
+
+	  Enabling atomic mode operations allows any SCMI driver using this
+	  transport to optionally ask for atomic SCMI transactions and operate
+	  in atomic context too, at the price of using a number of busy-waiting
+	  primitives all over instead. If unsure say N.
+
 endif #ARM_SCMI_PROTOCOL
 
 config ARM_SCMI_POWER_DOMAIN

drivers/firmware/arm_scmi/driver.c

@@ -648,7 +648,8 @@ static void scmi_handle_notification(struct scmi_chan_info *cinfo,
 
 	unpack_scmi_header(msg_hdr, &xfer->hdr);
 	if (priv)
-		xfer->priv = priv;
+		/* Ensure order between xfer->priv store and following ops */
+		smp_store_mb(xfer->priv, priv);
 	info->desc->ops->fetch_notification(cinfo, info->desc->max_msg_size,
 					    xfer);
 	scmi_notify(cinfo->handle, xfer->hdr.protocol_id,
@@ -680,7 +681,8 @@ static void scmi_handle_response(struct scmi_chan_info *cinfo,
 		xfer->rx.len = info->desc->max_msg_size;
 
 	if (priv)
-		xfer->priv = priv;
+		/* Ensure order between xfer->priv store and following ops */
+		smp_store_mb(xfer->priv, priv);
 	info->desc->ops->fetch_response(cinfo, xfer);
 
 	trace_scmi_rx_done(xfer->transfer_id, xfer->hdr.id,