Documentation: Add x86/amd_hsmp driver

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

Signed-off-by: Naveen Krishna Chatradhi <nchatrad@amd.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20220222050501.18789-2-nchatrad@amd.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

Documentation/x86/amd_hsmp.rst

@@ -0,0 +1,86 @@
+.. 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

Documentation/x86/index.rst

@@ -24,6 +24,7 @@ x86-specific Documentation
    intel-iommu
    intel_txt
    amd-memory-encryption
+   amd_hsmp
    pti
    mds
    microcode