Documentation: Add document for UltraSoc SMB driver

Bring in documentation for UltraSoc SMB driver.
It simply describes the device, sysfs interface and the
firmware bindings.

Signed-off-by: Qi Liu <[e-mail]>
Signed-off-by: Junhao He <[e-mail]>
Reviewed-by: Jonathan Cameron <[e-mail]>
Reviewed-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Suzuki K Poulose <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/ABI/testing/sysfs-bus-coresight-devices-ultra_smb

···
+What:		/sys/bus/coresight/devices/ultra_smb<N>/enable_sink
+Date:		January 2023
+KernelVersion:	6.3
+Contact:	Junhao He <[e-mail]>
+Description:	(RW) Add/remove a SMB device from a trace path. There can be
+		multiple sources for a single SMB device.
+
+What:		/sys/bus/coresight/devices/ultra_smb<N>/mgmt/buf_size
+Date:		January 2023
+KernelVersion:	6.3
+Contact:	Junhao He <[e-mail]>
+Description:	(RO) Shows the buffer size of each UltraSoc SMB device.
+
+What:		/sys/bus/coresight/devices/ultra_smb<N>/mgmt/buf_status
+Date:		January 2023
+KernelVersion:	6.3
+Contact:	Junhao He <[e-mail]>
+Description:	(RO) Shows the value of UltraSoc SMB status register.
+		BIT(0) is zero means buffer is empty.
+
+What:		/sys/bus/coresight/devices/ultra_smb<N>/mgmt/read_pos
+Date:		January 2023
+KernelVersion:	6.3
+Contact:	Junhao He <[e-mail]>
+Description:	(RO) Shows the value of UltraSoc SMB Read Pointer register.
+
+What:		/sys/bus/coresight/devices/ultra_smb<N>/mgmt/write_pos
+Date:		January 2023
+KernelVersion:	6.3
+Contact:	Junhao He <[e-mail]>
+Description:	(RO) Shows the value of UltraSoc SMB Write Pointer register.

Documentation/trace/coresight/ultrasoc-smb.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+======================================
+UltraSoc - HW Assisted Tracing on SoC
+======================================
+   :Author:   Qi Liu <[e-mail]>
+   :Date:     January 2023
+
+Introduction
+------------
+
+UltraSoc SMB is a per SCCL (Super CPU Cluster) hardware. It provides a
+way to buffer and store CPU trace messages in a region of shared system
+memory. The device acts as a coresight sink device and the
+corresponding trace generators (ETM) are attached as source devices.
+
+Sysfs files and directories
+---------------------------
+
+The SMB devices appear on the existing coresight bus alongside other
+devices::
+
+	$# ls /sys/bus/coresight/devices/
+	ultra_smb0   ultra_smb1   ultra_smb2   ultra_smb3
+
+The ``ultra_smb<N>`` names SMB device associated with SCCL.::
+
+	$# ls /sys/bus/coresight/devices/ultra_smb0
+	enable_sink   mgmt
+	$# ls /sys/bus/coresight/devices/ultra_smb0/mgmt
+	buf_size  buf_status  read_pos  write_pos
+
+Key file items are:
+
+   * ``read_pos``: Shows the value on the read pointer register.
+   * ``write_pos``: Shows the value on the write pointer register.
+   * ``buf_status``: Shows the value on the status register.
+     BIT(0) is zero value which means the buffer is empty.
+   * ``buf_size``: Shows the buffer size of each device.
+
+Firmware Bindings
+-----------------
+
+The device is only supported with ACPI. Its binding describes device
+identifier, resource information and graph structure.
+
+The device is identified as ACPI HID "HISI03A1". Device resources are allocated
+using the _CRS method. Each device must present two base address; the first one
+is the configuration base address of the device, the second one is the 32-bit
+base address of shared system memory.
+
+Example::
+
+    Device(USMB) {                                               \
+      Name(_HID, "HISI03A1")                                     \
+      Name(_CRS, ResourceTemplate() {                            \
+          QWordMemory (ResourceConsumer, , MinFixed, MaxFixed, NonCacheable, \
+		       ReadWrite, 0x0, 0x95100000, 0x951FFFFF, 0x0, 0x100000) \
+          QWordMemory (ResourceConsumer, , MinFixed, MaxFixed, Cacheable, \
+		       ReadWrite, 0x0, 0x50000000, 0x53FFFFFF, 0x0, 0x4000000) \
+      })                                                         \
+      Name(_DSD, Package() {                                     \
+        ToUUID("ab02a46b-74c7-45a2-bd68-f7d344ef2153"),          \
+	/* Use CoreSight Graph ACPI bindings to describe connections topology */
+        Package() {                                              \
+          0,                                                     \
+          1,                                                     \
+          Package() {                                            \
+            1,                                                   \
+            ToUUID("3ecbc8b6-1d0e-4fb3-8107-e627f805c6cd"),      \
+            8,                                                   \
+            Package() {0x8, 0, \_SB.S00.SL11.CL28.F008, 0},       \
+            Package() {0x9, 0, \_SB.S00.SL11.CL29.F009, 0},       \
+            Package() {0xa, 0, \_SB.S00.SL11.CL2A.F010, 0},       \
+            Package() {0xb, 0, \_SB.S00.SL11.CL2B.F011, 0},       \
+            Package() {0xc, 0, \_SB.S00.SL11.CL2C.F012, 0},       \
+            Package() {0xd, 0, \_SB.S00.SL11.CL2D.F013, 0},       \
+            Package() {0xe, 0, \_SB.S00.SL11.CL2E.F014, 0},       \
+            Package() {0xf, 0, \_SB.S00.SL11.CL2F.F015, 0},       \
+          }                                                      \
+        }                                                        \
+      })                                                         \
+    }