[RFC PATCH 0/7] firmware: arm_scmi: Add virtio transport

[RFC PATCH 0/7] firmware: arm_scmi: Add virtio transport

[Peter Hilber]

This series implements an SCMI virtio driver according to the virtio
SCMI device spec patch v5 [1], after simple preparatory changes to the
existing arm-scmi driver.

The virtio transport differs in some respects from the existing
shared-memory based SCMI transports.

Message timeouts can be a problem if the virtio device (SCMI platform)
does not have real-time properties. I set a high message rx timeout
value which should work for non real-time virtio devices as well. There
are other timeouts for delayed response and polling which were not
addressed yet. Delayed responses are not really needed since, with the
virtio transport, message responses may be transmitted out of order.
Polling doesn't make sense at least for virtio devices without real-time
behavior, in my understanding.

There are some known issues which will be resolved before removing the
RFC tag:

- Further work is needed on the scmi_xfer management. Unlike shmem based
  transports, the virtio transport is actually exchanging messages with
  the SCMI agent through the scmi_xfer tx and rx buffers. In case of a
  message rx timeout, the arm-scmi driver could try to re-use the
  scmi_xfer, while that might still be used by the virtio device. I
  think part of the scmi_xfers_info bookkeeping could be optionally
  outsourced to the transport to remediate this.

- After arm-scmi driver probe failure, or after remove, the scmi-virtio
  driver may still try to process and forward message responses from the
  virtio device.

- We must be sure that the virtio transport option (such as virtio over
  MMIO) is available when the virtio SCMI device is probed.

The series is based on for-next/scmi [2], on top of

commit 66d90f6ecee7 ("firmware: arm_scmi: Enable building as a single module")

The series was actually tested with a v5.4 based kernel, with the Base
protocol and Sensor management protocol. The virtio SCMI device used was
a proprietary implementation by OpenSynergy. Delayed responses were not
tested.

Any comments are very welcome.

[1] https://lists.oasis-open.org/archives/virtio-comment/202005/msg00096.html
[2] https://git.kernel.org/pub/scm/linux/kernel/git/sudeep.holla/linux.git


Igor Skalkin (7):
  firmware: arm_scmi, smccc, mailbox: Make shmem based transports
    optional
  firmware: arm_scmi: Document that max_msg is a per channel type limit
  firmware: arm_scmi: Add op to override max message #
  firmware: arm_scmi: Add per message transport data
  firmware: arm_scmi: Add xfer_init_buffers transport op
  dt-bindings: arm: Add virtio transport for SCMI
  firmware: arm_scmi: Add virtio transport

 .../devicetree/bindings/arm/arm,scmi.txt      |  38 +-
 MAINTAINERS                                   |   1 +
 drivers/firmware/Kconfig                      |  19 +-
 drivers/firmware/arm_scmi/Makefile            |   3 +-
 drivers/firmware/arm_scmi/common.h            |  31 +-
 drivers/firmware/arm_scmi/driver.c            |  83 +++-
 drivers/firmware/arm_scmi/virtio.c            | 470 ++++++++++++++++++
 drivers/firmware/smccc/Kconfig                |   1 +
 drivers/mailbox/Kconfig                       |   1 +
 include/uapi/linux/virtio_ids.h               |   1 +
 include/uapi/linux/virtio_scmi.h              |  41 ++
 11 files changed, 664 insertions(+), 25 deletions(-)
 create mode 100644 drivers/firmware/arm_scmi/virtio.c
 create mode 100644 include/uapi/linux/virtio_scmi.h


base-commit: 66d90f6ecee755e9c19a119c9255e80091165498
-- 
2.25.1

[RFC PATCH 1/7] firmware: arm_scmi, smccc, mailbox: Make shmem based transports optional

[Peter Hilber]

From: Igor Skalkin <igor.skalkin@opensynergy.com>

Upon adding the virtio transport in this patch series, SCMI will also
work without shared memory based transports. Also, the mailbox transport
may not be needed if the smc transport is used.

- Compile shmem.c only if a shmem based transport is available.

- Remove hard dependency of SCMI on mailbox.

Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
---
 drivers/firmware/Kconfig           | 9 ++++++++-
 drivers/firmware/arm_scmi/Makefile | 2 +-
 drivers/firmware/arm_scmi/common.h | 2 ++
 drivers/firmware/arm_scmi/driver.c | 2 ++
 drivers/firmware/smccc/Kconfig     | 1 +
 drivers/mailbox/Kconfig            | 1 +
 6 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/Kconfig b/drivers/firmware/Kconfig
index afdbebba628a..bdde51adb267 100644
--- a/drivers/firmware/Kconfig
+++ b/drivers/firmware/Kconfig
@@ -9,7 +9,7 @@ menu "Firmware Drivers"
 config ARM_SCMI_PROTOCOL
 	tristate "ARM System Control and Management Interface (SCMI) Message Protocol"
 	depends on ARM || ARM64 || COMPILE_TEST
-	depends on MAILBOX
+	depends on ARM_SCMI_HAVE_SHMEM
 	help
 	  ARM System Control and Management Interface (SCMI) protocol is a
 	  set of operating system-independent software interfaces that are
@@ -27,6 +27,13 @@ config ARM_SCMI_PROTOCOL
 	  This protocol library provides interface for all the client drivers
 	  making use of the features offered by the SCMI.
 
+config ARM_SCMI_HAVE_SHMEM
+	bool
+	default n
+	help
+	  This declares whether a shared memory based transport for SCMI is
+	  available.
+
 config ARM_SCMI_POWER_DOMAIN
 	tristate "SCMI power domain driver"
 	depends on ARM_SCMI_PROTOCOL || (COMPILE_TEST && OF)
diff --git a/drivers/firmware/arm_scmi/Makefile b/drivers/firmware/arm_scmi/Makefile
index bc0d54f8e861..3cc7fa40a464 100644
--- a/drivers/firmware/arm_scmi/Makefile
+++ b/drivers/firmware/arm_scmi/Makefile
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-only
 scmi-bus-y = bus.o
 scmi-driver-y = driver.o notify.o
-scmi-transport-y = shmem.o
+scmi-transport-$(CONFIG_ARM_SCMI_HAVE_SHMEM) = shmem.o
 scmi-transport-$(CONFIG_MAILBOX) += mailbox.o
 scmi-transport-$(CONFIG_HAVE_ARM_SMCCC_DISCOVERY) += smc.o
 scmi-protocols-y = base.o clock.o perf.o power.o reset.o sensors.o system.o
diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
index 37fb583f1bf5..36c38334a045 100644
--- a/drivers/firmware/arm_scmi/common.h
+++ b/drivers/firmware/arm_scmi/common.h
@@ -240,7 +240,9 @@ struct scmi_desc {
 	int max_msg_size;
 };
 
+#ifdef CONFIG_MAILBOX
 extern const struct scmi_desc scmi_mailbox_desc;
+#endif
 #ifdef CONFIG_HAVE_ARM_SMCCC
 extern const struct scmi_desc scmi_smc_desc;
 #endif
diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
index c5dea87edf8f..d070687cf2f6 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -910,7 +910,9 @@ ATTRIBUTE_GROUPS(versions);
 
 /* Each compatible listed below must have descriptor associated with it */
 static const struct of_device_id scmi_of_match[] = {
+#ifdef CONFIG_MAILBOX
 	{ .compatible = "arm,scmi", .data = &scmi_mailbox_desc },
+#endif
 #ifdef CONFIG_HAVE_ARM_SMCCC_DISCOVERY
 	{ .compatible = "arm,scmi-smc", .data = &scmi_smc_desc},
 #endif
diff --git a/drivers/firmware/smccc/Kconfig b/drivers/firmware/smccc/Kconfig
index 15e7466179a6..69c4d6cabf62 100644
--- a/drivers/firmware/smccc/Kconfig
+++ b/drivers/firmware/smccc/Kconfig
@@ -9,6 +9,7 @@ config HAVE_ARM_SMCCC_DISCOVERY
 	bool
 	depends on ARM_PSCI_FW
 	default y
+	select ARM_SCMI_HAVE_SHMEM
 	help
 	 SMCCC v1.0 lacked discoverability and hence PSCI v1.0 was updated
 	 to add SMCCC discovery mechanism though the PSCI firmware
diff --git a/drivers/mailbox/Kconfig b/drivers/mailbox/Kconfig
index 05b1009e2820..5ffe1ab0c869 100644
--- a/drivers/mailbox/Kconfig
+++ b/drivers/mailbox/Kconfig
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-only
 menuconfig MAILBOX
 	bool "Mailbox Hardware Support"
+	select ARM_SCMI_HAVE_SHMEM
 	help
 	  Mailbox is a framework to control hardware communication between
 	  on-chip processors through queued messages and interrupt driven
-- 
2.25.1

[RFC PATCH 2/7] firmware: arm_scmi: Document that max_msg is a per channel type limit

[Peter Hilber]

From: Igor Skalkin <igor.skalkin@opensynergy.com>

struct scmi_desc.max_msg specifies a limit for the pending messages.
This limit is a per SCMI channel type (tx, rx) limit. State that
explicitly in the inline documentation. The following patch will add an
op to override the limit per channel type.

Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
---
 drivers/firmware/arm_scmi/common.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
index 36c38334a045..4cc78eb27f14 100644
--- a/drivers/firmware/arm_scmi/common.h
+++ b/drivers/firmware/arm_scmi/common.h
@@ -229,8 +229,8 @@ struct scmi_transport_ops {
  *
  * @ops: Pointer to the transport specific ops structure
  * @max_rx_timeout_ms: Timeout for communication with SoC (in Milliseconds)
- * @max_msg: Maximum number of messages that can be pending
- *	simultaneously in the system
+ * @max_msg: Maximum number of messages for a channel type (tx or rx) that can
+ *	be pending simultaneously in the system
  * @max_msg_size: Maximum size of data per message that can be handled.
  */
 struct scmi_desc {
-- 
2.25.1

[RFC PATCH 3/7] firmware: arm_scmi: Add op to override max message #

[Peter Hilber]

From: Igor Skalkin <igor.skalkin@opensynergy.com>

The number of messages that the upcoming scmi-virtio transport can
support depends on the virtio device (SCMI platform) and can differ for
each channel. (The scmi-virtio transport does only have one tx and at
most 1 rx channel.)

Add an optional transport op so that scmi-virtio can report the actual
max message # for each channel type. Respect these new limits.

Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
---
 drivers/firmware/arm_scmi/common.h |  8 ++++-
 drivers/firmware/arm_scmi/driver.c | 49 ++++++++++++++++++++++--------
 2 files changed, 43 insertions(+), 14 deletions(-)

diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
index 4cc78eb27f14..0f1540cb2d84 100644
--- a/drivers/firmware/arm_scmi/common.h
+++ b/drivers/firmware/arm_scmi/common.h
@@ -201,6 +201,9 @@ struct scmi_chan_info {
  * @chan_available: Callback to check if channel is available or not
  * @chan_setup: Callback to allocate and setup a channel
  * @chan_free: Callback to free a channel
+ * @get_max_msg: Optional callback to provide max_msg dynamically
+ * 	@max_msg: Maximum number of messages for the channel type (tx or rx)
+ * 		that can be pending simultaneously in the system
  * @send_message: Callback to send a message
  * @mark_txdone: Callback to mark tx as done
  * @fetch_response: Callback to fetch response
@@ -213,6 +216,8 @@ struct scmi_transport_ops {
 	int (*chan_setup)(struct scmi_chan_info *cinfo, struct device *dev,
 			  bool tx);
 	int (*chan_free)(int id, void *p, void *data);
+	int (*get_max_msg)(bool tx, struct scmi_chan_info *base_cinfo,
+			   int *max_msg);
 	int (*send_message)(struct scmi_chan_info *cinfo,
 			    struct scmi_xfer *xfer);
 	void (*mark_txdone)(struct scmi_chan_info *cinfo, int ret);
@@ -230,7 +235,8 @@ struct scmi_transport_ops {
  * @ops: Pointer to the transport specific ops structure
  * @max_rx_timeout_ms: Timeout for communication with SoC (in Milliseconds)
  * @max_msg: Maximum number of messages for a channel type (tx or rx) that can
- *	be pending simultaneously in the system
+ *	be pending simultaneously in the system. May be overridden by the
+ *	get_max_msg op.
  * @max_msg_size: Maximum size of data per message that can be handled.
  */
 struct scmi_desc {
diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
index d070687cf2f6..d66f19b27c44 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -61,11 +61,13 @@ static atomic_t transfer_last_id;
  *	Index of this bitmap table is also used for message
  *	sequence identifier.
  * @xfer_lock: Protection for message allocation
+ * @max_msg: Maximum number of messages that can be pending
  */
 struct scmi_xfers_info {
 	struct scmi_xfer *xfer_block;
 	unsigned long *xfer_alloc_table;
 	spinlock_t xfer_lock;
+	int max_msg;
 };
 
 /**
@@ -157,13 +159,11 @@ static struct scmi_xfer *scmi_xfer_get(const struct scmi_handle *handle,
 	u16 xfer_id;
 	struct scmi_xfer *xfer;
 	unsigned long flags, bit_pos;
-	struct scmi_info *info = handle_to_scmi_info(handle);
 
 	/* Keep the locked section as small as possible */
 	spin_lock_irqsave(&minfo->xfer_lock, flags);
-	bit_pos = find_first_zero_bit(minfo->xfer_alloc_table,
-				      info->desc->max_msg);
-	if (bit_pos == info->desc->max_msg) {
+	bit_pos = find_first_zero_bit(minfo->xfer_alloc_table, minfo->max_msg);
+	if (bit_pos == minfo->max_msg) {
 		spin_unlock_irqrestore(&minfo->xfer_lock, flags);
 		return ERR_PTR(-ENOMEM);
 	}
@@ -594,32 +594,44 @@ int scmi_handle_put(const struct scmi_handle *handle)
 }
 
 static int __scmi_xfer_info_init(struct scmi_info *sinfo,
-				 struct scmi_xfers_info *info)
+				 struct scmi_xfers_info *info,
+				 bool tx,
+				 struct scmi_chan_info *base_cinfo)
 {
 	int i;
 	struct scmi_xfer *xfer;
 	struct device *dev = sinfo->dev;
 	const struct scmi_desc *desc = sinfo->desc;
 
+	info->max_msg = desc->max_msg;
+
+	if (desc->ops->get_max_msg) {
+		int ret =
+			desc->ops->get_max_msg(tx, base_cinfo, &info->max_msg);
+
+		if (ret)
+			return ret;
+	}
+
 	/* Pre-allocated messages, no more than what hdr.seq can support */
-	if (WARN_ON(desc->max_msg >= MSG_TOKEN_MAX)) {
+	if (WARN_ON(info->max_msg >= MSG_TOKEN_MAX)) {
 		dev_err(dev, "Maximum message of %d exceeds supported %ld\n",
-			desc->max_msg, MSG_TOKEN_MAX);
+			info->max_msg, MSG_TOKEN_MAX);
 		return -EINVAL;
 	}
 
-	info->xfer_block = devm_kcalloc(dev, desc->max_msg,
+	info->xfer_block = devm_kcalloc(dev, info->max_msg,
 					sizeof(*info->xfer_block), GFP_KERNEL);
 	if (!info->xfer_block)
 		return -ENOMEM;
 
-	info->xfer_alloc_table = devm_kcalloc(dev, BITS_TO_LONGS(desc->max_msg),
+	info->xfer_alloc_table = devm_kcalloc(dev, BITS_TO_LONGS(info->max_msg),
 					      sizeof(long), GFP_KERNEL);
 	if (!info->xfer_alloc_table)
 		return -ENOMEM;
 
 	/* Pre-initialize the buffer pointer to pre-allocated buffers */
-	for (i = 0, xfer = info->xfer_block; i < desc->max_msg; i++, xfer++) {
+	for (i = 0, xfer = info->xfer_block; i < info->max_msg; i++, xfer++) {
 		xfer->rx.buf = devm_kcalloc(dev, sizeof(u8), desc->max_msg_size,
 					    GFP_KERNEL);
 		if (!xfer->rx.buf)
@@ -636,10 +648,21 @@ static int __scmi_xfer_info_init(struct scmi_info *sinfo,
 
 static int scmi_xfer_info_init(struct scmi_info *sinfo)
 {
-	int ret = __scmi_xfer_info_init(sinfo, &sinfo->tx_minfo);
+	int ret;
+	struct scmi_chan_info *base_tx_cinfo;
+	struct scmi_chan_info *base_rx_cinfo;
+
+	base_tx_cinfo = idr_find(&sinfo->tx_idr, SCMI_PROTOCOL_BASE);
+	if (unlikely(!base_tx_cinfo))
+		return -EINVAL;
+
+	ret = __scmi_xfer_info_init(sinfo, &sinfo->tx_minfo, true,
+				    base_tx_cinfo);
 
-	if (!ret && idr_find(&sinfo->rx_idr, SCMI_PROTOCOL_BASE))
-		ret = __scmi_xfer_info_init(sinfo, &sinfo->rx_minfo);
+	base_rx_cinfo = idr_find(&sinfo->rx_idr, SCMI_PROTOCOL_BASE);
+	if (!ret && base_rx_cinfo)
+		ret = __scmi_xfer_info_init(sinfo, &sinfo->rx_minfo, false,
+					    base_rx_cinfo);
 
 	return ret;
 }
-- 
2.25.1

[RFC PATCH 4/7] firmware: arm_scmi: Add per message transport data

[Peter Hilber]

From: Igor Skalkin <igor.skalkin@opensynergy.com>

The virtio transport in this patch series can be simplified by using the
scmi_xfer tx/rx buffers for data exchange with the virtio device, and
for saving the message state. But the virtio transport requires
prepending a transport-specific header. Also, for data exchange using
virtqueues, the tx and rx buffers should not overlap.

The first step to solve the aforementioned issues is to add a
transport-specific data pointer to scmi_xfer.

Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
---
 drivers/firmware/arm_scmi/common.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
index 0f1540cb2d84..e3ccec954e93 100644
--- a/drivers/firmware/arm_scmi/common.h
+++ b/drivers/firmware/arm_scmi/common.h
@@ -131,6 +131,7 @@ struct scmi_msg {
  *	buffer for the rx path as we use for the tx path.
  * @done: command message transmit completion event
  * @async_done: pointer to delayed response message received event completion
+ * @extra_data: Transport-specific private data pointer
  */
 struct scmi_xfer {
 	int transfer_id;
@@ -139,6 +140,7 @@ struct scmi_xfer {
 	struct scmi_msg rx;
 	struct completion done;
 	struct completion *async_done;
+	void *extra_data;
 };
 
 void scmi_xfer_put(const struct scmi_handle *h, struct scmi_xfer *xfer);
-- 
2.25.1

[RFC PATCH 5/7] firmware: arm_scmi: Add xfer_init_buffers transport op

[Peter Hilber]

From: Igor Skalkin <igor.skalkin@opensynergy.com>

The virtio transport in this patch series can be simplified by using the
scmi_xfer tx/rx buffers for data exchange with the virtio device, and
for saving the message state. But the virtio transport requires
prepending a transport-specific header. Also, for data exchange using
virtqueues, the tx and rx buffers should not overlap.

After the previous patch, this is the second and final step to enable
the virtio transport to use the scmi_xfer buffers for data exchange.

Add an optional op through which the transport can allocate the tx/rx
buffers along with room for the prepended transport-specific headers.

Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
---
 drivers/firmware/arm_scmi/common.h |  3 +++
 drivers/firmware/arm_scmi/driver.c | 21 +++++++++++++++------
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
index e3ccec954e93..c6071ffe4346 100644
--- a/drivers/firmware/arm_scmi/common.h
+++ b/drivers/firmware/arm_scmi/common.h
@@ -206,6 +206,7 @@ struct scmi_chan_info {
  * @get_max_msg: Optional callback to provide max_msg dynamically
  * 	@max_msg: Maximum number of messages for the channel type (tx or rx)
  * 		that can be pending simultaneously in the system
+ * @xfer_init_buffers: Callback to initialize buffers for scmi_xfer
  * @send_message: Callback to send a message
  * @mark_txdone: Callback to mark tx as done
  * @fetch_response: Callback to fetch response
@@ -220,6 +221,8 @@ struct scmi_transport_ops {
 	int (*chan_free)(int id, void *p, void *data);
 	int (*get_max_msg)(bool tx, struct scmi_chan_info *base_cinfo,
 			   int *max_msg);
+	int (*xfer_init_buffers)(struct scmi_chan_info *cinfo,
+				 struct scmi_xfer *xfer, int max_msg_size);
 	int (*send_message)(struct scmi_chan_info *cinfo,
 			    struct scmi_xfer *xfer);
 	void (*mark_txdone)(struct scmi_chan_info *cinfo, int ret);
diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
index d66f19b27c44..2e25f6f068f5 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -632,12 +632,21 @@ static int __scmi_xfer_info_init(struct scmi_info *sinfo,
 
 	/* Pre-initialize the buffer pointer to pre-allocated buffers */
 	for (i = 0, xfer = info->xfer_block; i < info->max_msg; i++, xfer++) {
-		xfer->rx.buf = devm_kcalloc(dev, sizeof(u8), desc->max_msg_size,
-					    GFP_KERNEL);
-		if (!xfer->rx.buf)
-			return -ENOMEM;
-
-		xfer->tx.buf = xfer->rx.buf;
+		if (desc->ops->xfer_init_buffers) {
+			int ret = desc->ops->xfer_init_buffers(
+				base_cinfo, xfer, desc->max_msg_size);
+
+			if (ret)
+				return ret;
+		} else {
+			xfer->rx.buf = devm_kcalloc(dev, sizeof(u8),
+						    desc->max_msg_size,
+						    GFP_KERNEL);
+			if (!xfer->rx.buf)
+				return -ENOMEM;
+
+			xfer->tx.buf = xfer->rx.buf;
+		}
 		init_completion(&xfer->done);
 	}
 
-- 
2.25.1

[RFC PATCH 6/7] dt-bindings: arm: Add virtio transport for SCMI

[Peter Hilber]

From: Igor Skalkin <igor.skalkin@opensynergy.com>

Document the properties for arm,scmi-virtio compatible nodes. The
backing virtio SCMI device is described in patch [1].

[1] https://lists.oasis-open.org/archives/virtio-comment/202005/msg00096.html

Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
---
 .../devicetree/bindings/arm/arm,scmi.txt      | 38 ++++++++++++++++++-
 1 file changed, 36 insertions(+), 2 deletions(-)

diff --git a/Documentation/devicetree/bindings/arm/arm,scmi.txt b/Documentation/devicetree/bindings/arm/arm,scmi.txt
index 55deb68230eb..844ff3c40a49 100644
--- a/Documentation/devicetree/bindings/arm/arm,scmi.txt
+++ b/Documentation/devicetree/bindings/arm/arm,scmi.txt
@@ -13,6 +13,9 @@ the device tree.
 Required properties:
 
 The scmi node with the following properties shall be under the /firmware/ node.
+Some properties are specific to a transport type.
+
+shmem-based transports (mailbox, smc/hvc):
 
 - compatible : shall be "arm,scmi" or "arm,scmi-smc" for smc/hvc transports
 - mboxes: List of phandle and mailbox channel specifiers. It should contain
@@ -21,6 +24,17 @@ The scmi node with the following properties shall be under the /firmware/ node.
 	  supported.
 - shmem : List of phandle pointing to the shared memory(SHM) area as per
 	  generic mailbox client binding.
+
+Virtio transport:
+
+- compatible : shall be "arm,scmi-virtio".
+- virtio_transport : phandle of the virtio device. This device must support one
+                     virtqueue for transmitting commands ("tx", cmdq), and,
+		     optionally, one more virtqueue for receiving notifications
+		     and delayed responses ("rx", eventq).
+
+Additional required properties:
+
 - #address-cells : should be '1' if the device has sub-nodes, maps to
 	  protocol identifier for a given sub-node.
 - #size-cells : should be '0' as 'reg' property doesn't have any size
@@ -42,7 +56,8 @@ Each protocol supported shall have a sub-node with corresponding compatible
 as described in the following sections. If the platform supports dedicated
 communication channel for a particular protocol, the 3 properties namely:
 mboxes, mbox-names and shmem shall be present in the sub-node corresponding
-to that protocol.
+to that protocol. The virtio transport does not support dedicated communication
+channels.
 
 Clock/Performance bindings for the clocks/OPPs based on SCMI Message Protocol
 ------------------------------------------------------------
@@ -106,7 +121,8 @@ Required sub-node properties:
 [4] Documentation/devicetree/bindings/sram/sram.yaml
 [5] Documentation/devicetree/bindings/reset/reset.txt
 
-Example:
+Example (mailbox transport):
+----------------------------
 
 sram@50000000 {
 	compatible = "mmio-sram";
@@ -195,3 +211,21 @@ thermal-zones {
 		...
 	};
 };
+
+Example (virtio transport):
+---------------------------
+
+virtio_mmio_scmi: virtio_mmio@4b001000 {
+	compatible = "virtio,mmio";
+	...
+};
+
+firmware {
+	...
+	scmi {
+		compatible = "arm,scmi-virtio";
+		virtio_transport = <&virtio_mmio_scmi>;
+		...
+
+The rest is similar to the mailbox transport example, when omitting the
+mailbox/shmem-specific properties.
-- 
2.25.1

Re: [RFC PATCH 6/7] dt-bindings: arm: Add virtio transport for SCMI

[Rob Herring]

On Fri, Sep 18, 2020 at 06:55:58PM +0200, Peter Hilber wrote:
> From: Igor Skalkin <igor.skalkin@opensynergy.com>
> 
> Document the properties for arm,scmi-virtio compatible nodes. The
> backing virtio SCMI device is described in patch [1].
> 
> [1] https://lists.oasis-open.org/archives/virtio-comment/202005/msg00096.html
> 
> Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
> Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
> Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
> ---
>  .../devicetree/bindings/arm/arm,scmi.txt      | 38 ++++++++++++++++++-
>  1 file changed, 36 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/arm/arm,scmi.txt b/Documentation/devicetree/bindings/arm/arm,scmi.txt
> index 55deb68230eb..844ff3c40a49 100644
> --- a/Documentation/devicetree/bindings/arm/arm,scmi.txt
> +++ b/Documentation/devicetree/bindings/arm/arm,scmi.txt
> @@ -13,6 +13,9 @@ the device tree.
>  Required properties:
>  
>  The scmi node with the following properties shall be under the /firmware/ node.
> +Some properties are specific to a transport type.
> +
> +shmem-based transports (mailbox, smc/hvc):
>  
>  - compatible : shall be "arm,scmi" or "arm,scmi-smc" for smc/hvc transports
>  - mboxes: List of phandle and mailbox channel specifiers. It should contain
> @@ -21,6 +24,17 @@ The scmi node with the following properties shall be under the /firmware/ node.
>  	  supported.
>  - shmem : List of phandle pointing to the shared memory(SHM) area as per
>  	  generic mailbox client binding.
> +
> +Virtio transport:
> +
> +- compatible : shall be "arm,scmi-virtio".
> +- virtio_transport : phandle of the virtio device. This device must support one
> +                     virtqueue for transmitting commands ("tx", cmdq), and,
> +		     optionally, one more virtqueue for receiving notifications
> +		     and delayed responses ("rx", eventq).

Isn't what the virtio device provides discoverable? We don't have virtio 
protocols in DT for anything else. Why is SCMI special?

Rob

Re: [RFC PATCH 6/7] dt-bindings: arm: Add virtio transport for SCMI

[Peter Hilber]

On 23.09.20 22:54, Rob Herring wrote:
> On Fri, Sep 18, 2020 at 06:55:58PM +0200, Peter Hilber wrote:
>> From: Igor Skalkin <igor.skalkin@opensynergy.com>
>>
>> Document the properties for arm,scmi-virtio compatible nodes. The
>> backing virtio SCMI device is described in patch [1].
>>
>> [1] https://lists.oasis-open.org/archives/virtio-comment/202005/msg00096.html
>>
>> Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
>> Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
>> Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
>> ---
>>  .../devicetree/bindings/arm/arm,scmi.txt      | 38 ++++++++++++++++++-
>>  1 file changed, 36 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/devicetree/bindings/arm/arm,scmi.txt b/Documentation/devicetree/bindings/arm/arm,scmi.txt
>> index 55deb68230eb..844ff3c40a49 100644
>> --- a/Documentation/devicetree/bindings/arm/arm,scmi.txt
>> +++ b/Documentation/devicetree/bindings/arm/arm,scmi.txt
>> @@ -13,6 +13,9 @@ the device tree.
>>  Required properties:
>>  
>>  The scmi node with the following properties shall be under the /firmware/ node.
>> +Some properties are specific to a transport type.
>> +
>> +shmem-based transports (mailbox, smc/hvc):
>>  
>>  - compatible : shall be "arm,scmi" or "arm,scmi-smc" for smc/hvc transports
>>  - mboxes: List of phandle and mailbox channel specifiers. It should contain
>> @@ -21,6 +24,17 @@ The scmi node with the following properties shall be under the /firmware/ node.
>>  	  supported.
>>  - shmem : List of phandle pointing to the shared memory(SHM) area as per
>>  	  generic mailbox client binding.
>> +
>> +Virtio transport:
>> +
>> +- compatible : shall be "arm,scmi-virtio".
>> +- virtio_transport : phandle of the virtio device. This device must support one
>> +                     virtqueue for transmitting commands ("tx", cmdq), and,
>> +		     optionally, one more virtqueue for receiving notifications
>> +		     and delayed responses ("rx", eventq).
> 
> Isn't what the virtio device provides discoverable? We don't have virtio 
> protocols in DT for anything else. Why is SCMI special?
> 
> Rob
> 

Does your comment refer to the presence of the `virtio_transport'
phandle, or to the entire "arm,scmi-virtio" node? The protocol child
nodes of "arm,scmi-virtio" can be clock providers, power domain
providers, etc.

The author and me are currently looking into how to replace
the `virtio_transport' phandle.

Best regards,

Peter

[RFC PATCH 7/7] firmware: arm_scmi: Add virtio transport

[Peter Hilber]

From: Igor Skalkin <igor.skalkin@opensynergy.com>

This transport enables accessing an SCMI platform as a virtio device.

Implement an SCMI virtio driver according to the virtio SCMI device spec
patch v5 [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.

After the preparatory patches, implement the virtio transport as
paraphrased:

Call scmi-virtio init from arm-scmi to probe for the virtio device before
arm-scmi will call any transport ops.

Use the scmi_xfer tx/rx buffers for data exchange with the virtio device
in order to avoid redundant maintenance of additional buffers. Allocate
the buffers in the SCMI transport, and prepend room for a small header
used by the virtio transport to the tx/rx buffers.

For simplicity, restrict the number of messages which can be pending
simultaneously according to the virtqueue capacity. (The virtqueue sizes
are negotiated with the virtio device.)

As soon as rx channel message buffers are allocated or have been read
out by the arm-scmi driver, feed them 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.

Limitations:

Do not adjust the other SCMI timeouts for delayed response and polling
for now, since these timeouts are only relevant in special cases which
are not yet deemed relevant for this transport.

To do (as discussed in the cover letter):

- Avoid re-use of buffers still being used by the virtio device on
  timeouts.

- Avoid race conditions on receiving messages during/after channel free
  on driver probe failure or remove.

[1] https://lists.oasis-open.org/archives/virtio-comment/202005/msg00096.html
[2] https://www.oasis-open.org/committees/ballot.php?id=3496

Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
---
 MAINTAINERS                        |   1 +
 drivers/firmware/Kconfig           |  12 +-
 drivers/firmware/arm_scmi/Makefile |   1 +
 drivers/firmware/arm_scmi/common.h |  14 +
 drivers/firmware/arm_scmi/driver.c |  11 +
 drivers/firmware/arm_scmi/virtio.c | 470 +++++++++++++++++++++++++++++
 include/uapi/linux/virtio_ids.h    |   1 +
 include/uapi/linux/virtio_scmi.h   |  41 +++
 8 files changed, 550 insertions(+), 1 deletion(-)
 create mode 100644 drivers/firmware/arm_scmi/virtio.c
 create mode 100644 include/uapi/linux/virtio_scmi.h

diff --git a/MAINTAINERS b/MAINTAINERS
index deaafb617361..8df73d6ddfc1 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -16772,6 +16772,7 @@ F:	drivers/firmware/arm_scpi.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>
diff --git a/drivers/firmware/Kconfig b/drivers/firmware/Kconfig
index bdde51adb267..c4bdd84f7405 100644
--- a/drivers/firmware/Kconfig
+++ b/drivers/firmware/Kconfig
@@ -9,7 +9,7 @@ menu "Firmware Drivers"
 config ARM_SCMI_PROTOCOL
 	tristate "ARM System Control and Management Interface (SCMI) Message Protocol"
 	depends on ARM || ARM64 || COMPILE_TEST
-	depends on ARM_SCMI_HAVE_SHMEM
+	depends on ARM_SCMI_HAVE_SHMEM || VIRTIO_SCMI
 	help
 	  ARM System Control and Management Interface (SCMI) protocol is a
 	  set of operating system-independent software interfaces that are
@@ -34,6 +34,16 @@ config ARM_SCMI_HAVE_SHMEM
 	  This declares whether a shared memory based transport for SCMI is
 	  available.
 
+config VIRTIO_SCMI
+	bool "Virtio transport for SCMI"
+	default n
+	depends on VIRTIO
+	help
+	  This enables the virtio based transport for SCMI.
+
+	  If you want to use the ARM SCMI protocol between the virtio guest and
+	  a host providing a virtio SCMI device, answer Y.
+
 config ARM_SCMI_POWER_DOMAIN
 	tristate "SCMI power domain driver"
 	depends on ARM_SCMI_PROTOCOL || (COMPILE_TEST && OF)
diff --git a/drivers/firmware/arm_scmi/Makefile b/drivers/firmware/arm_scmi/Makefile
index 3cc7fa40a464..25caea5e1969 100644
--- a/drivers/firmware/arm_scmi/Makefile
+++ b/drivers/firmware/arm_scmi/Makefile
@@ -4,6 +4,7 @@ scmi-driver-y = driver.o notify.o
 scmi-transport-$(CONFIG_ARM_SCMI_HAVE_SHMEM) = shmem.o
 scmi-transport-$(CONFIG_MAILBOX) += mailbox.o
 scmi-transport-$(CONFIG_HAVE_ARM_SMCCC_DISCOVERY) += smc.o
+scmi-transport-$(CONFIG_VIRTIO_SCMI) += virtio.o
 scmi-protocols-y = base.o clock.o perf.o power.o reset.o sensors.o system.o
 scmi-module-objs := $(scmi-bus-y) $(scmi-driver-y) $(scmi-protocols-y) \
 		    $(scmi-transport-y)
diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
index c6071ffe4346..865364dea9ea 100644
--- a/drivers/firmware/arm_scmi/common.h
+++ b/drivers/firmware/arm_scmi/common.h
@@ -161,6 +161,17 @@ int scmi_base_protocol_init(struct scmi_handle *h);
 int __init scmi_bus_init(void);
 void __exit scmi_bus_exit(void);
 
+#ifdef CONFIG_VIRTIO_SCMI
+int __init virtio_scmi_init(void);
+void __exit virtio_scmi_exit(void);
+#else
+static inline int __init virtio_scmi_init(void)
+{
+	return 0;
+}
+#define virtio_scmi_exit() do { } while (0)
+#endif
+
 #define DECLARE_SCMI_REGISTER_UNREGISTER(func)		\
 	int __init scmi_##func##_register(void);	\
 	void __exit scmi_##func##_unregister(void)
@@ -257,6 +268,9 @@ extern const struct scmi_desc scmi_mailbox_desc;
 #ifdef CONFIG_HAVE_ARM_SMCCC
 extern const struct scmi_desc scmi_smc_desc;
 #endif
+#ifdef CONFIG_VIRTIO_SCMI
+extern const struct scmi_desc scmi_virtio_desc;
+#endif
 
 void scmi_rx_callback(struct scmi_chan_info *cinfo, u32 msg_hdr);
 void scmi_free_channel(struct scmi_chan_info *cinfo, struct idr *idr, int id);
diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
index 2e25f6f068f5..326f69f2a45a 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -947,6 +947,9 @@ static const struct of_device_id scmi_of_match[] = {
 #endif
 #ifdef CONFIG_HAVE_ARM_SMCCC_DISCOVERY
 	{ .compatible = "arm,scmi-smc", .data = &scmi_smc_desc},
+#endif
+#ifdef CONFIG_VIRTIO_SCMI
+	{ .compatible = "arm,scmi-virtio", .data = &scmi_virtio_desc},
 #endif
 	{ /* Sentinel */ },
 };
@@ -965,8 +968,14 @@ static struct platform_driver scmi_driver = {
 
 static int __init scmi_driver_init(void)
 {
+	int ret;
+
 	scmi_bus_init();
 
+	ret = virtio_scmi_init();
+	if (ret)
+		return ret;
+
 	scmi_clock_register();
 	scmi_perf_register();
 	scmi_power_register();
@@ -989,6 +998,8 @@ static void __exit scmi_driver_exit(void)
 	scmi_sensors_unregister();
 	scmi_system_unregister();
 
+	virtio_scmi_exit();
+
 	platform_driver_unregister(&scmi_driver);
 }
 module_exit(scmi_driver_exit);
diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
new file mode 100644
index 000000000000..126c964d9f49
--- /dev/null
+++ b/drivers/firmware/arm_scmi/virtio.c
@@ -0,0 +1,470 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Virtio Transport driver for Arm System Control and Management Interface
+ * (SCMI).
+ *
+ * Copyright (C) 2020 OpenSynergy.
+ */
+
+/**
+ * DOC: Theory of Operation
+ *
+ * The scmi-virtio transport implements a driver for the virtio SCMI device
+ * proposed in virtio spec patch v5[1].
+ *
+ * There is one tx channel (virtio cmdq, A2P channel) and at most one rx
+ * channel (virtio eventq, P2A channel). Each channel is implemented through a
+ * virtqueue. Access to each virtqueue is protected by a spinlock.
+ *
+ * This SCMI transport uses the scmi_xfer tx/rx buffers for data exchange with
+ * the virtio device to avoid maintenance of additional buffers.
+ *
+ * [1] https://lists.oasis-open.org/archives/virtio-comment/202005/msg00096.html
+ */
+
+#include <linux/of.h>
+#include <linux/of_platform.h>
+#include <linux/platform_device.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/virtio.h>
+#include <linux/virtio_config.h>
+#include <uapi/linux/virtio_ids.h>
+#include <uapi/linux/virtio_scmi.h>
+
+#include "common.h"
+
+#define VIRTIO_SCMI_MAX_MSG_SIZE 128 /* Value may be increased. */
+#define DESCR_PER_TX_MSG 2
+
+struct scmi_vio_channel {
+	spinlock_t lock;
+	struct virtqueue *vqueue;
+	struct scmi_chan_info *cinfo;
+	u8 is_rx;
+};
+
+union virtio_scmi_input {
+	__virtio32 hdr;
+	struct virtio_scmi_response response;
+	struct virtio_scmi_notification notification;
+};
+
+struct scmi_vio_msg {
+	struct virtio_scmi_request *request;
+	union virtio_scmi_input *input;
+	u8 completed;
+};
+
+static int scmi_vio_populate_vq_rx(struct scmi_vio_channel *vioch,
+				   struct scmi_xfer *xfer)
+{
+	struct scatterlist sg_in;
+	struct scmi_vio_msg *msg = xfer->extra_data;
+	int rc;
+
+	msg->completed = false;
+
+	sg_init_one(&sg_in, msg->input,
+		    sizeof(*msg->input) + VIRTIO_SCMI_MAX_MSG_SIZE);
+
+	rc = virtqueue_add_inbuf(vioch->vqueue, &sg_in, 1, xfer, GFP_ATOMIC);
+	if (rc)
+		dev_err(vioch->cinfo->dev, "%s() rc=%d\n", __func__, rc);
+	else
+		virtqueue_kick(vioch->vqueue);
+
+	return rc;
+}
+
+static void scmi_vio_complete_cb(struct virtqueue *vqueue)
+{
+	struct scmi_vio_channel *vioch = vqueue->priv;
+	unsigned long iflags;
+	unsigned int length;
+
+	spin_lock_irqsave(&vioch->lock, iflags);
+
+	do {
+		struct scmi_xfer *xfer;
+
+		virtqueue_disable_cb(vqueue);
+
+		while ((xfer = virtqueue_get_buf(vqueue, &length))) {
+			struct scmi_vio_msg *msg = xfer->extra_data;
+			u32 msg_hdr =
+				virtio32_to_cpu(vqueue->vdev, msg->input->hdr);
+			u8 msg_type = MSG_XTRACT_TYPE(msg_hdr);
+
+			if (!vioch->is_rx) { /* tx queue response */
+				msg->completed = true;
+				xfer->rx.len =
+					length - sizeof(msg->input->response);
+				if (!xfer->hdr.poll_completion)
+					scmi_rx_callback(vioch->cinfo, msg_hdr);
+				continue;
+			}
+
+			/* rx queue - notification or delayed response */
+			switch (msg_type) {
+			case MSG_TYPE_NOTIFICATION:
+				xfer->rx.len = length -
+					       sizeof(msg->input->notification);
+				xfer->rx.buf = msg->input->notification.data;
+				break;
+			case MSG_TYPE_DELAYED_RESP:
+				xfer->rx.len =
+					length - sizeof(msg->input->response);
+				xfer->rx.buf = msg->input->response.data;
+				break;
+			default:
+				dev_warn_once(vioch->cinfo->dev,
+					      "rx: unknown message_type %d\n",
+					      msg_type);
+				scmi_vio_populate_vq_rx(vioch, xfer);
+				continue;
+			}
+
+			scmi_rx_callback(vioch->cinfo, msg_hdr);
+			scmi_vio_populate_vq_rx(vioch, xfer);
+		}
+
+		if (unlikely(virtqueue_is_broken(vqueue)))
+			break;
+	} while (!virtqueue_enable_cb(vqueue));
+
+	spin_unlock_irqrestore(&vioch->lock, iflags);
+}
+
+static const char *const scmi_vio_vqueue_names[] = { "tx", "rx" };
+
+static vq_callback_t *scmi_vio_complete_callbacks[] = {
+	scmi_vio_complete_cb,
+	scmi_vio_complete_cb
+};
+
+static bool virtio_chan_available(struct device *dev, int idx)
+{
+	struct platform_device *pdev;
+	struct virtio_device *vdev;
+	struct device_node *vioch_node;
+	struct scmi_vio_channel **vioch;
+
+	vioch_node = of_parse_phandle(dev->of_node, "virtio_transport", 0);
+	if (!vioch_node)
+		return false;
+
+	pdev = of_find_device_by_node(vioch_node);
+	of_node_put(vioch_node);
+	if (!pdev)
+		return false;
+
+	vdev = (struct virtio_device *)pdev->dev.driver_data;
+	if (!vdev)
+		return false;
+
+	vioch = vdev->priv;
+	if (!vioch)
+		return false;
+
+	return vioch[idx] && vioch[idx]->vqueue;
+}
+
+static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
+			     bool tx)
+{
+	struct platform_device *pdev;
+	struct virtio_device *vdev;
+	struct device_node *vioch_node;
+	struct scmi_vio_channel **vioch;
+	int vioch_index = tx ? VIRTIO_SCMI_VQ_TX : VIRTIO_SCMI_VQ_RX;
+
+	vioch_node =
+		of_parse_phandle(cinfo->dev->of_node, "virtio_transport", 0);
+
+	pdev = of_find_device_by_node(vioch_node);
+	of_node_put(vioch_node);
+	if (!pdev) {
+		dev_err(dev, "virtio_transport device not found\n");
+		return -1;
+	}
+
+	vdev = (struct virtio_device *)pdev->dev.driver_data;
+	if (!vdev)
+		return -1;
+
+	vioch = (struct scmi_vio_channel **)vdev->priv;
+	if (!vioch) {
+		dev_err(dev, "Data from scmi-virtio probe not found\n");
+		return -1;
+	}
+	cinfo->transport_info = vioch[vioch_index];
+	vioch[vioch_index]->cinfo = cinfo;
+
+	return 0;
+}
+
+static int virtio_chan_free(int id, void *p, void *data)
+{
+	struct scmi_chan_info *cinfo = p;
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+
+	if (vioch) {
+		cinfo->transport_info = NULL;
+		kfree(vioch);
+	}
+
+	scmi_free_channel(cinfo, data, id);
+	return 0;
+}
+
+static int virtio_get_max_msg(bool tx, struct scmi_chan_info *base_cinfo,
+			      int *max_msg)
+{
+	struct scmi_vio_channel *vioch = base_cinfo->transport_info;
+
+	*max_msg = virtqueue_get_vring_size(vioch->vqueue);
+
+	/* Tx messages need multiple descriptors. */
+	if (tx)
+		*max_msg /= DESCR_PER_TX_MSG;
+
+	if (*max_msg > MSG_TOKEN_MAX) {
+		dev_notice(
+			base_cinfo->dev,
+			"Only %ld messages can be pending simultaneously, while the virtqueue could hold %d\n",
+			MSG_TOKEN_MAX, *max_msg);
+		*max_msg = MSG_TOKEN_MAX;
+	}
+
+	return 0;
+}
+
+static int virtio_xfer_init_buffers(struct scmi_chan_info *cinfo,
+				    struct scmi_xfer *xfer, int max_msg_size)
+{
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+	struct scmi_vio_msg *msg;
+
+	msg = devm_kzalloc(cinfo->dev, sizeof(*msg), GFP_KERNEL);
+	if (!msg)
+		return -ENOMEM;
+
+	xfer->extra_data = msg;
+
+	if (vioch->is_rx) {
+		int rc;
+		unsigned long iflags;
+
+		msg->input = devm_kzalloc(cinfo->dev,
+					  sizeof(*msg->input) + max_msg_size,
+					  GFP_KERNEL);
+		if (!msg->input)
+			return -ENOMEM;
+
+		/*
+		 * xfer->rx.buf will be set to notification or delayed response
+		 * specific values in the receive callback, according to the
+		 * type of the received message.
+		 */
+
+		spin_lock_irqsave(&vioch->lock, iflags);
+		rc = scmi_vio_populate_vq_rx(vioch, xfer);
+		spin_unlock_irqrestore(&vioch->lock, iflags);
+		if (rc)
+			return rc;
+	} else {
+		msg->request =
+			devm_kzalloc(cinfo->dev,
+				     sizeof(*msg->request) + max_msg_size,
+				     GFP_KERNEL);
+		if (!msg->request)
+			return -ENOMEM;
+
+		xfer->tx.buf = msg->request->data;
+
+		msg->input = devm_kzalloc(
+			cinfo->dev, sizeof(msg->input->response) + max_msg_size,
+			GFP_KERNEL);
+		if (!msg->input)
+			return -ENOMEM;
+
+		xfer->rx.buf = msg->input->response.data;
+	}
+
+	return 0;
+}
+
+static int scmi_vio_send(struct scmi_vio_channel *vioch, struct scmi_xfer *xfer)
+{
+	struct scatterlist sg_out;
+	struct scatterlist sg_in;
+	struct scatterlist *sgs[DESCR_PER_TX_MSG] = { &sg_out, &sg_in };
+	struct scmi_vio_msg *msg = xfer->extra_data;
+	unsigned long iflags;
+	int rc;
+
+	msg->completed = false;
+
+	sg_init_one(&sg_out, msg->request,
+		    sizeof(*msg->request) + xfer->tx.len);
+	sg_init_one(&sg_in, &msg->input->response,
+		    sizeof(msg->input->response) + xfer->rx.len);
+
+	spin_lock_irqsave(&vioch->lock, iflags);
+	rc = virtqueue_add_sgs(vioch->vqueue, sgs, 1, 1, xfer, GFP_ATOMIC);
+	if (rc)
+		dev_err(vioch->cinfo->dev, "%s() rc=%d\n", __func__, rc);
+	else
+		virtqueue_kick(vioch->vqueue);
+	spin_unlock_irqrestore(&vioch->lock, iflags);
+
+	return rc;
+}
+
+static int virtio_send_message(struct scmi_chan_info *cinfo,
+			       struct scmi_xfer *xfer)
+{
+	uint32_t hdr;
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+	struct virtio_device *vdev = vioch->vqueue->vdev;
+	struct scmi_vio_msg *msg = xfer->extra_data;
+
+	hdr = pack_scmi_header(&xfer->hdr);
+
+	msg->request->hdr = cpu_to_virtio32(vdev, hdr);
+
+	return scmi_vio_send(vioch, xfer);
+}
+
+static void virtio_fetch_response(struct scmi_chan_info *cinfo,
+				  struct scmi_xfer *xfer)
+{
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+	struct scmi_vio_msg *msg = xfer->extra_data;
+
+	xfer->hdr.status = virtio32_to_cpu(vioch->vqueue->vdev,
+					   msg->input->response.status);
+}
+
+static void dummy_fetch_notification(struct scmi_chan_info *cinfo,
+				     size_t max_len, struct scmi_xfer *xfer)
+{
+	(void)cinfo;
+	(void)max_len;
+	(void)xfer;
+}
+
+static void dummy_clear_channel(struct scmi_chan_info *cinfo)
+{
+	(void)cinfo;
+}
+
+static bool virtio_poll_done(struct scmi_chan_info *cinfo,
+			     struct scmi_xfer *xfer)
+{
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+	struct scmi_vio_msg *msg = xfer->extra_data;
+	unsigned long iflags;
+	bool completed;
+
+	spin_lock_irqsave(&vioch->lock, iflags);
+	completed = msg->completed;
+	spin_unlock_irqrestore(&vioch->lock, iflags);
+
+	return completed;
+}
+
+static const struct scmi_transport_ops scmi_virtio_ops = {
+	.chan_available = virtio_chan_available,
+	.chan_setup = virtio_chan_setup,
+	.chan_free = virtio_chan_free,
+	.get_max_msg = virtio_get_max_msg,
+	.send_message = virtio_send_message,
+	.fetch_response = virtio_fetch_response,
+	.fetch_notification = dummy_fetch_notification,
+	.clear_channel = dummy_clear_channel,
+	.poll_done = virtio_poll_done,
+	.xfer_init_buffers = virtio_xfer_init_buffers,
+};
+
+const struct scmi_desc scmi_virtio_desc = {
+	.ops = &scmi_virtio_ops,
+	.max_rx_timeout_ms = 60000, /* for non-realtime virtio devices */
+	.max_msg = 0, /* overridden by virtio_get_max_msg() */
+	.max_msg_size = VIRTIO_SCMI_MAX_MSG_SIZE,
+};
+
+static int scmi_vio_probe(struct virtio_device *vdev)
+{
+	struct device *dev = &vdev->dev;
+	struct scmi_vio_channel **vioch;
+	bool have_vq_rx;
+	int vq_cnt;
+	int i;
+	struct virtqueue *vqs[VIRTIO_SCMI_VQ_MAX_CNT];
+
+	vioch = devm_kcalloc(dev, VIRTIO_SCMI_VQ_MAX_CNT, sizeof(*vioch),
+			     GFP_KERNEL);
+	if (!vioch)
+		return -ENOMEM;
+
+	have_vq_rx = virtio_has_feature(vdev, VIRTIO_SCMI_F_P2A_CHANNELS);
+	vq_cnt = have_vq_rx ? VIRTIO_SCMI_VQ_MAX_CNT : 1;
+
+	for (i = 0; i < vq_cnt; i++) {
+		vioch[i] = devm_kzalloc(dev, sizeof(**vioch), GFP_KERNEL);
+		if (!vioch[i])
+			return -ENOMEM;
+	}
+
+	if (have_vq_rx)
+		vioch[VIRTIO_SCMI_VQ_RX]->is_rx = true;
+
+	if (virtio_find_vqs(vdev, vq_cnt, vqs, scmi_vio_complete_callbacks,
+			    scmi_vio_vqueue_names, NULL)) {
+		dev_err(dev, "Failed to get %d virtqueue(s)\n", vq_cnt);
+		return -1;
+	}
+	dev_info(dev, "Found %d virtqueue(s)\n", vq_cnt);
+
+	for (i = 0; i < vq_cnt; i++) {
+		spin_lock_init(&vioch[i]->lock);
+		vioch[i]->vqueue = vqs[i];
+		vioch[i]->vqueue->priv = vioch[i];
+	}
+
+	vdev->priv = vioch;
+
+	virtio_device_ready(vdev);
+
+	return 0;
+}
+
+static unsigned int features[] = {
+	VIRTIO_SCMI_F_P2A_CHANNELS,
+};
+
+static const struct virtio_device_id id_table[] = {
+	{ VIRTIO_ID_SCMI, VIRTIO_DEV_ANY_ID },
+	{ 0 }
+};
+
+static struct virtio_driver virtio_scmi_driver = {
+	.driver.name = "scmi-virtio",
+	.driver.owner = THIS_MODULE,
+	.feature_table = features,
+	.feature_table_size = ARRAY_SIZE(features),
+	.id_table = id_table,
+	.probe = scmi_vio_probe,
+};
+
+int __init virtio_scmi_init(void)
+{
+	return register_virtio_driver(&virtio_scmi_driver);
+}
+
+void __exit virtio_scmi_exit(void)
+{
+	unregister_virtio_driver(&virtio_scmi_driver);
+}
diff --git a/include/uapi/linux/virtio_ids.h b/include/uapi/linux/virtio_ids.h
index b052355ac7a3..57d233c02720 100644
--- a/include/uapi/linux/virtio_ids.h
+++ b/include/uapi/linux/virtio_ids.h
@@ -48,5 +48,6 @@
 #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 */
 
 #endif /* _LINUX_VIRTIO_IDS_H */
diff --git a/include/uapi/linux/virtio_scmi.h b/include/uapi/linux/virtio_scmi.h
new file mode 100644
index 000000000000..9f21b3dbbfe2
--- /dev/null
+++ b/include/uapi/linux/virtio_scmi.h
@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause) */
+/*
+ * Copyright (C) 2020 OpenSynergy GmbH
+ */
+
+#ifndef _UAPI_LINUX_VIRTIO_SCMI_H
+#define _UAPI_LINUX_VIRTIO_SCMI_H
+
+#include <linux/virtio_types.h>
+
+/* Feature bits */
+
+/* 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
+
+struct virtio_scmi_request {
+	__virtio32 hdr;
+	__u8 data[];
+};
+
+struct virtio_scmi_response {
+	__virtio32 hdr;
+	__virtio32 status;
+	__u8 data[];
+};
+
+struct virtio_scmi_notification {
+	__virtio32 hdr;
+	__u8 data[];
+};
+
+#endif /* _UAPI_LINUX_VIRTIO_SCMI_H */
-- 
2.25.1