[PATCH v4 2/2] Documentation: Add x86/amd_hsmp driver

[Naveen Krishna Chatradhi <nchatrad@amd.com>]

This documentation for amd_hsmp driver explains how to use the
device interface.

Signed-off-by: Naveen Krishna Chatradhi <nchatrad@amd.com>
---
Changes since v3:
 remove change ids
Changes since v2:
 Add statement saying the HSMP interface is supported only on
 server cpu models from AMD.
Changes since v1:
 None
 Documentation/x86/amd_hsmp.rst | 86 ++++++++++++++++++++++++++++++++++
 1 file changed, 86 insertions(+)
 create mode 100644 Documentation/x86/amd_hsmp.rst

diff --git a/Documentation/x86/amd_hsmp.rst b/Documentation/x86/amd_hsmp.rst
new file mode 100644
index 000000000000..5dc2619ec3f0
--- /dev/null
+++ b/Documentation/x86/amd_hsmp.rst
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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 following PPR
+https://www.amd.com/system/files/TechDocs/55898_B1_pub_0.50.zip
+
+HSMP interface is supported on AMD 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 arguments in message */
+	__u16	response_sz;			/* Number of expected response words */
+	__u32	args[HSMP_MAX_MSG_LEN];		/* Argument(s) */
+	__u32	response[HSMP_MAX_MSG_LEN];	/* Response word(s) */
+	__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 can be found in chapter
+"7 Host System Management Port (HSMP)" of the following PPR
+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
-- 
2.17.1