firmware: arm_scmi: Add virtio transport

This transport enables communications with an SCMI platform through virtio;
the SCMI platform will be represented by a virtio device.

Implement an SCMI virtio driver according to the virtio SCMI device spec
[1]. Virtio device id 32 has been reserved for the SCMI device [2].

The virtio transport has one Tx channel (virtio cmdq, A2P channel) and
at most one Rx channel (virtio eventq, P2A channel).

The following feature bit defined in [1] is not implemented:
VIRTIO_SCMI_F_SHARED_MEMORY.

The number of messages which can be pending simultaneously is restricted
according to the virtqueue capacity negotiated at probing time.

As soon as Rx channel message buffers are allocated or have been read
out by the arm-scmi driver, feed them back to the virtio device.

Since some virtio devices may not have the short response time exhibited
by SCMI platforms using other transports, set a generous response
timeout.

SCMI polling mode is not supported by this virtio transport since deemed
meaningless: polling mode operation is offered by the SCMI core to those
transports that could not provide a completion interrupt on the TX path,
which is never the case for virtio whose core callbacks can easily call
into core scmi_rx_callback upon messages reception.

[1] https://github.com/oasis-tcs/virtio-spec/blob/master/virtio-scmi.tex
[2] https://www.oasis-open.org/committees/ballot.php?id=3496

Link: https://lore.kernel.org/r/20210803131024.40280-16-cristian.marussi@arm.com
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
Co-developed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
[ Peter: Adapted patch for submission to upstream. ]
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
[ Cristian: simplified driver logic, changed link_supplier and channel
	    available/setup logic, removed dummy callbacks ]
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

MAINTAINERS

@@ -17940,6 +17940,7 @@ F:	drivers/regulator/scmi-regulator.c
 F:	drivers/reset/reset-scmi.c
 F:	include/linux/sc[mp]i_protocol.h
 F:	include/trace/events/scmi.h
+F:	include/uapi/linux/virtio_scmi.h
 
 SYSTEM RESET/SHUTDOWN DRIVERS
 M:	Sebastian Reichel <sre@kernel.org>

drivers/firmware/arm_scmi/Kconfig

@@ -66,6 +66,17 @@ config ARM_SCMI_TRANSPORT_SMC
 	  If you want the ARM SCMI PROTOCOL stack to include support for a
 	  transport based on SMC, answer Y.
 
+config ARM_SCMI_TRANSPORT_VIRTIO
+	bool "SCMI transport based on VirtIO"
+	depends on VIRTIO
+	select ARM_SCMI_HAVE_TRANSPORT
+	select ARM_SCMI_HAVE_MSG
+	help
+	  This enables the virtio based transport for SCMI.
+
+	  If you want the ARM SCMI PROTOCOL stack to include support for a
+	  transport based on VirtIO, answer Y.
+
 endif #ARM_SCMI_PROTOCOL
 
 config ARM_SCMI_POWER_DOMAIN

drivers/firmware/arm_scmi/Makefile

@@ -5,6 +5,7 @@ scmi-transport-$(CONFIG_ARM_SCMI_HAVE_SHMEM) = shmem.o
 scmi-transport-$(CONFIG_ARM_SCMI_TRANSPORT_MAILBOX) += mailbox.o
 scmi-transport-$(CONFIG_ARM_SCMI_TRANSPORT_SMC) += smc.o
 scmi-transport-$(CONFIG_ARM_SCMI_HAVE_MSG) += msg.o
+scmi-transport-$(CONFIG_ARM_SCMI_TRANSPORT_VIRTIO) += virtio.o
 scmi-protocols-y = base.o clock.o perf.o power.o reset.o sensors.o system.o voltage.o
 scmi-module-objs := $(scmi-bus-y) $(scmi-driver-y) $(scmi-protocols-y) \
 		    $(scmi-transport-y)

drivers/firmware/arm_scmi/common.h

@@ -418,6 +418,9 @@ extern const struct scmi_desc scmi_mailbox_desc;
 #ifdef CONFIG_ARM_SCMI_TRANSPORT_SMC
 extern const struct scmi_desc scmi_smc_desc;
 #endif
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO
+extern const struct scmi_desc scmi_virtio_desc;
+#endif
 
 void scmi_rx_callback(struct scmi_chan_info *cinfo, u32 msg_hdr, void *priv);
 void scmi_free_channel(struct scmi_chan_info *cinfo, struct idr *idr, int id);

drivers/firmware/arm_scmi/driver.c

@@ -1983,6 +1983,9 @@ static const struct of_device_id scmi_of_match[] = {
 #endif
 #ifdef CONFIG_ARM_SCMI_TRANSPORT_SMC
 	{ .compatible = "arm,scmi-smc", .data = &scmi_smc_desc},
+#endif
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO
+	{ .compatible = "arm,scmi-virtio", .data = &scmi_virtio_desc},
 #endif
 	{ /* Sentinel */ },
 };

include/uapi/linux/virtio_ids.h

@@ -55,6 +55,7 @@
 #define VIRTIO_ID_FS			26 /* virtio filesystem */
 #define VIRTIO_ID_PMEM			27 /* virtio pmem */
 #define VIRTIO_ID_MAC80211_HWSIM	29 /* virtio mac80211-hwsim */
+#define VIRTIO_ID_SCMI			32 /* virtio SCMI */
 #define VIRTIO_ID_BT			40 /* virtio bluetooth */
 
 /*

include/uapi/linux/virtio_scmi.h

+/* SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause) */
+/*
+ * Copyright (C) 2020-2021 OpenSynergy GmbH
+ * Copyright (C) 2021 ARM Ltd.
+ */
+
+#ifndef _UAPI_LINUX_VIRTIO_SCMI_H
+#define _UAPI_LINUX_VIRTIO_SCMI_H
+
+#include <linux/virtio_types.h>
+
+/* Device implements some SCMI notifications, or delayed responses. */
+#define VIRTIO_SCMI_F_P2A_CHANNELS 0
+
+/* Device implements any SCMI statistics shared memory region */
+#define VIRTIO_SCMI_F_SHARED_MEMORY 1
+
+/* Virtqueues */
+
+#define VIRTIO_SCMI_VQ_TX 0 /* cmdq */
+#define VIRTIO_SCMI_VQ_RX 1 /* eventq */
+#define VIRTIO_SCMI_VQ_MAX_CNT 2
+
+#endif /* _UAPI_LINUX_VIRTIO_SCMI_H */