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+.. SPDX-License-Identifier: GPL-2.0
+
+============================================
+AMD HSMP interface
+============================================
+
+Newer Fam19h EPYC server line of processors from AMD support system
+management functionality via HSMP (Host System Management Port).
+
+The Host System Management Port (HSMP) is an interface to provide
+OS-level software with access to system management functions via a
+set of mailbox registers.
+
+More details on the interface can be found in chapter
+"7 Host System Management Port (HSMP)" of the family/model PPR
+Eg: https://www.amd.com/system/files/TechDocs/55898_B1_pub_0.50.zip
+
+HSMP interface is supported on EPYC server CPU models only.
+
+
+HSMP device
+============================================
+
+amd_hsmp driver under the drivers/platforms/x86/ creates miscdevice
+/dev/hsmp to let user space programs run hsmp mailbox commands.
+
+$ ls -al /dev/hsmp
+crw-r--r-- 1 root root 10, 123 Jan 21 21:41 /dev/hsmp
+
+Characteristics of the dev node:
+ * Write mode is used for running set/configure commands
+ * Read mode is used for running get/status monitor commands
+
+Access restrictions:
+ * Only root user is allowed to open the file in write mode.
+ * The file can be opened in read mode by all the users.
+
+In-kernel integration:
+ * Other subsystems in the kernel can use the exported transport
+ function hsmp_send_message().
+ * Locking across callers is taken care by the driver.
+
+
+An example
+==========
+
+To access hsmp device from a C program.
+First, you need to include the headers::
+
+ #include <linux/amd_hsmp.h>
+
+Which defines the supported messages/message IDs.
+
+Next thing, open the device file, as follows::
+
+ int file;
+
+ file = open("/dev/hsmp", O_RDWR);
+ if (file < 0) {
+ /* ERROR HANDLING; you can check errno to see what went wrong */
+ exit(1);
+ }
+
+The following IOCTL is defined:
+
+``ioctl(file, HSMP_IOCTL_CMD, struct hsmp_message *msg)``
+ The argument is a pointer to a::
+
+ struct hsmp_message {
+ __u32 msg_id; /* Message ID */
+ __u16 num_args; /* Number of input argument words in message */
+ __u16 response_sz; /* Number of expected output/response words */
+ __u32 args[HSMP_MAX_MSG_LEN]; /* argument/response buffer */
+ __u16 sock_ind; /* socket number */
+ };
+
+The ioctl would return a non-zero on failure; you can read errno to see
+what happened. The transaction returns 0 on success.
+
+More details on the interface and message definitions can be found in chapter
+"7 Host System Management Port (HSMP)" of the respective family/model PPR
+eg: https://www.amd.com/system/files/TechDocs/55898_B1_pub_0.50.zip
+
+User space C-APIs are made available by linking against the esmi library,
+which is provided by the E-SMS project https://developer.amd.com/e-sms/.
+See: https://github.com/amd/esmi_ib_library