samples: rust_misc_device: Provide an example C program to exercise functionality

Here is the expected output (manually spliced together):

USERSPACE: Opening /dev/rust-misc-device for reading and writing
KERNEL: rust_misc_device: Opening Rust Misc Device Sample
USERSPACE: Calling Hello
KERNEL: rust_misc_device: IOCTLing Rust Misc Device Sample
KERNEL: rust_misc_device: -> Hello from the Rust Misc Device
USERSPACE: Fetching initial value
KERNEL: rust_misc_device: IOCTLing Rust Misc Device Sample
KERNEL: rust_misc_device: -> Copying data to userspace (value: 0)
USERSPACE: Submitting new value (1)
KERNEL: rust_misc_device: IOCTLing Rust Misc Device Sample
KERNEL: rust_misc_device: -> Copying data from userspace (value: 1)
USERSPACE: Fetching new value
KERNEL: rust_misc_device: IOCTLing Rust Misc Device Sample
KERNEL: rust_misc_device: -> Copying data to userspace (value: 1)
USERSPACE: Attempting to call in to an non-existent IOCTL
KERNEL: rust_misc_device: IOCTLing Rust Misc Device Sample
KERNEL: rust_misc_device: -> IOCTL not recognised: 20992
USERSPACE: ioctl: Succeeded to fail - this was expected: Inappropriate ioctl for device
USERSPACE: Closing /dev/rust-misc-device
KERNEL: rust_misc_device: Exiting the Rust Misc Device Sample
USERSPACE: Success

Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20241213134715.601415-6-lee@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

samples/rust/rust_misc_device.rs

@@ -4,6 +4,96 @@
 
 //! Rust misc device sample.
 
+/// Below is an example userspace C program that exercises this sample's functionality.
+///
+/// ```c
+/// #include <stdio.h>
+/// #include <stdlib.h>
+/// #include <errno.h>
+/// #include <fcntl.h>
+/// #include <unistd.h>
+/// #include <sys/ioctl.h>
+///
+/// #define RUST_MISC_DEV_FAIL _IO('|', 0)
+/// #define RUST_MISC_DEV_HELLO _IO('|', 0x80)
+/// #define RUST_MISC_DEV_GET_VALUE _IOR('|', 0x81, int)
+/// #define RUST_MISC_DEV_SET_VALUE _IOW('|', 0x82, int)
+///
+/// int main() {
+///   int value, new_value;
+///   int fd, ret;
+///
+///   // Open the device file
+///   printf("Opening /dev/rust-misc-device for reading and writing\n");
+///   fd = open("/dev/rust-misc-device", O_RDWR);
+///   if (fd < 0) {
+///     perror("open");
+///     return errno;
+///   }
+///
+///   // Make call into driver to say "hello"
+///   printf("Calling Hello\n");
+///   ret = ioctl(fd, RUST_MISC_DEV_HELLO, NULL);
+///   if (ret < 0) {
+///     perror("ioctl: Failed to call into Hello");
+///     close(fd);
+///     return errno;
+///   }
+///
+///   // Get initial value
+///   printf("Fetching initial value\n");
+///   ret = ioctl(fd, RUST_MISC_DEV_GET_VALUE, &value);
+///   if (ret < 0) {
+///     perror("ioctl: Failed to fetch the initial value");
+///     close(fd);
+///     return errno;
+///   }
+///
+///   value++;
+///
+///   // Set value to something different
+///   printf("Submitting new value (%d)\n", value);
+///   ret = ioctl(fd, RUST_MISC_DEV_SET_VALUE, &value);
+///   if (ret < 0) {
+///     perror("ioctl: Failed to submit new value");
+///     close(fd);
+///     return errno;
+///   }
+///
+///   // Ensure new value was applied
+///   printf("Fetching new value\n");
+///   ret = ioctl(fd, RUST_MISC_DEV_GET_VALUE, &new_value);
+///   if (ret < 0) {
+///     perror("ioctl: Failed to fetch the new value");
+///     close(fd);
+///     return errno;
+///   }
+///
+///   if (value != new_value) {
+///     printf("Failed: Committed and retrieved values are different (%d - %d)\n", value, new_value);
+///     close(fd);
+///     return -1;
+///   }
+///
+///   // Call the unsuccessful ioctl
+///   printf("Attempting to call in to an non-existent IOCTL\n");
+///   ret = ioctl(fd, RUST_MISC_DEV_FAIL, NULL);
+///   if (ret < 0) {
+///     perror("ioctl: Succeeded to fail - this was expected");
+///   } else {
+///     printf("ioctl: Failed to fail\n");
+///     close(fd);
+///     return -1;
+///   }
+///
+///   // Close the device file
+///   printf("Closing /dev/rust-misc-device\n");
+///   close(fd);
+///
+///   printf("Success\n");
+///   return 0;
+/// }
+/// ```
 use core::pin::Pin;
 
 use kernel::{