scmi: make socket_path required

Use clap's derive attributes to print device help instead of doing it
manually with the parsed arguments.

Previous output
===============

```text
Usage: vhost-device-scmi [OPTIONS]

Options:
  -s, --socket-path <SOCKET_PATH>  vhost-user socket to use (required)
  -d, --device <DEVICE>...         Devices to expose
      --help-devices               Print help on available devices
  -h, --help                       Print help
```

New output
==========

```text
vhost-user SCMI backend device

Usage: vhost-device-scmi [OPTIONS] --socket-path <SOCKET_PATH>

Options:
  -s, --socket-path <SOCKET_PATH>  vhost-user socket to use
  -d, --device <DEVICE>...         Devices to expose
  -h, --help                       Print help
  -V, --version                    Print version

Available devices:

- iio: industrial I/O sensor
  Parameters:
  - path: path to the device directory (e.g. /sys/bus/iio/devices/iio:device0)
  - channel: prefix of the device type (e.g. in_accel)
  - name: an optional name of the sensor, max. 15 characters

- fake: fake accelerometer
  A simple 3-axes sensor providing fake pre-defined values.
  Parameters:
  - name: an optional name of the sensor, max. 15 characters

Device specification example:
--device iio,path=/sys/bus/iio/devices/iio:device0,channel=in_accel
```

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>

vhost-device-scmi/src/devices/common.rs

@@ -189,7 +189,7 @@ pub fn available_devices() -> NameDeviceMapping {
     devices
 }
 
-fn devices_help() -> String {
+pub fn devices_help() -> String {
     let mut help = String::new();
     writeln!(help, "Available devices:").unwrap();
     for (name, specification) in available_devices().iter() {
@@ -216,11 +216,6 @@ fn devices_help() -> String {
     help
 }
 
-pub fn print_devices_help() {
-    let help = devices_help();
-    println!("{}", help);
-}
-
 // Common sensor infrastructure
 
 /// Basic information about the sensor.

vhost-device-scmi/src/main.rs

@@ -36,7 +36,7 @@ mod devices;
 mod scmi;
 mod vhu_scmi;
 
-use devices::common::{print_devices_help, DeviceDescription, DeviceProperties};
+use devices::common::{devices_help, DeviceDescription, DeviceProperties};
 
 use std::{
     path::PathBuf,
@@ -55,17 +55,16 @@ use vm_memory::{GuestMemoryAtomic, GuestMemoryMmap};
 type Result<T> = std::result::Result<T, String>;
 
 #[derive(Parser)]
+#[command(author, version, about, long_about = None, after_help = devices_help())]
 struct ScmiArgs {
     // Location of vhost-user Unix domain socket.
     // Required, unless one of the --help options is used.
-    #[clap(short, long, help = "vhost-user socket to use (required)")]
-    socket_path: Option<PathBuf>,
+    #[clap(short, long, help = "vhost-user socket to use")]
+    socket_path: PathBuf,
     // Specification of SCMI devices to create.
     #[clap(short, long, help = "Devices to expose")]
     #[arg(num_args(1..))]
     device: Vec<String>,
-    #[clap(long, help = "Print help on available devices")]
-    help_devices: bool,
 }
 
 pub struct VuScmiConfig {
@@ -77,9 +76,7 @@ impl TryFrom<ScmiArgs> for VuScmiConfig {
     type Error = String;
 
     fn try_from(cmd_args: ScmiArgs) -> Result<Self> {
-        let Some(socket_path) = cmd_args.socket_path else {
-            return Result::Err("Required argument socket-path was not provided".to_string());
-        };
+        let socket_path = cmd_args.socket_path;
         let mut devices: DeviceDescription = vec![];
         let device_iterator = cmd_args.device.iter();
         for d in device_iterator {
@@ -125,15 +122,6 @@ fn start_backend(config: VuScmiConfig) -> Result<()> {
     }
 }
 
-fn process_args(args: ScmiArgs) -> Option<ScmiArgs> {
-    if args.help_devices {
-        print_devices_help();
-        None
-    } else {
-        Some(args)
-    }
-}
-
 fn print_help(message: &String) {
     println!("{message}\n");
     let mut command = ScmiArgs::command();
@@ -142,17 +130,16 @@ fn print_help(message: &String) {
 
 fn main() {
     env_logger::init();
-    if let Some(args) = process_args(ScmiArgs::parse()) {
-        match VuScmiConfig::try_from(args) {
-            Ok(config) => {
-                if let Err(error) = start_backend(config) {
-                    error!("{error}");
-                    println!("{error}");
-                    exit(1);
-                }
+    let args = ScmiArgs::parse();
+    match VuScmiConfig::try_from(args) {
+        Ok(config) => {
+            if let Err(error) = start_backend(config) {
+                error!("{error}");
+                println!("{error}");
+                exit(1);
             }
-            Err(message) => print_help(&message),
         }
+        Err(message) => print_help(&message),
     }
 }
 
@@ -172,7 +159,7 @@ mod tests {
                      -d fake,name=bar"
         );
         let params: Vec<&str> = params_string.split_whitespace().collect();
-        let args: ScmiArgs = process_args(Parser::parse_from(params)).unwrap();
+        let args: ScmiArgs = Parser::parse_from(params);
         let config = VuScmiConfig::try_from(args).unwrap();
         assert_eq!(&config.socket_path, Path::new(&path));
         let devices = vec![
@@ -192,15 +179,6 @@ mod tests {
         assert_eq!(config.devices, devices);
     }
 
-    #[test]
-    fn test_device_help_processing() {
-        let params_string = "binary --help-devices".to_string();
-        let params: Vec<&str> = params_string.split_whitespace().collect();
-        let args: ScmiArgs = Parser::parse_from(params);
-        let processed = process_args(args);
-        assert!(processed.is_none());
-    }
-
     #[test]
     fn test_help() {
         // No way known to me to capture print_help() output from clap.