Add set_gpu_clocks for setting manual clock rates.

Also fix typos in dbus xml to match generated api signatures.

com.steampowered.SteamOSManager1.xml

@@ -136,28 +136,26 @@
     </method>
 
     <!--
-      SetGPUPerformanceLevel:
+      SetGpuPerformanceLevel:
       @level: The level to use 0 - auto, 1 - low, 2 - high, 3 - manual, 4 - profile_peak
       @success: True on success. False otherwise.
 
       Set the GPU performance level to the given value.
     -->
-    <method name="SetGPUPerformanceLevel">
+    <method name="SetGpuPerformanceLevel">
       <arg type="i" name="level" direction="in"/>
       <arg type="b" name="success" direction="out"/>
     </method>
 
     <!--
-      SetGPUClocks:
+      SetGpuClocks:
-      @clocks: The clocks to use.
+      @clocks: The clocks to use Range is 200-1600 but only takes effect when GpuPerformanceLevel is manual.
-      @clear: True if we should clear to go back to defaults. False to use clocks above.
       @success: True on success. False otherwise.
 
       Set the GPU clocks to the given value.
     -->
-    <method name="SetGPUClocks">
+    <method name="SetGpuClocks">
       <arg type="i" name="clocks" direction="in"/>
-      <arg type="b" name="clear" direction="in"/>
       <arg type="b" name="success" direction="out"/>
     </method>
 

src/manager.rs

@@ -163,6 +163,43 @@ impl SMManager {
         }
     }
 
+    async fn set_gpu_clocks(&self, clocks: i32) -> bool {
+        // Set gpu clocks to given value valid between 200 - 1600
+        // Only used when Gpu Performance Level is manual, but write whenever called.
+        // Writes value to /sys/class/drm/card0/device/pp_od_clk_voltage
+        if !(200..=1600).contains(&clocks) {
+            return false;
+        }
+
+        let result = File::create("/sys/class/drm/card0/device/pp_od_clk_voltage").await;
+        let mut myfile;
+        match result {
+            Ok(f) => myfile = f,
+            Err(message) => { println!("Error opening sysfs file for writing {message}"); return false; }
+        };
+
+        // write value
+        let data = format!("s 0 {clocks}\n");
+        let result = myfile.write(data.as_bytes()).await;
+        match result {
+            Ok(_worked) => {
+                let data = format!("s 1 {clocks}\n");
+                let result = myfile.write(data.as_bytes()).await;
+                match result {
+                    Ok(_worked) => {
+                        let result = myfile.write("c\n".as_bytes()).await;
+                        match result {
+                            Ok(_worked) => true,
+                            Err(message) => { println!("Error writing to sysfs file {message}"); false }
+                        }
+                    },
+                    Err(message) => { println!("Error writing to sysfs file {message}"); false }
+                }
+            },
+            Err(message) => { println!("Error writing to sysfs file {message}"); false }
+        }
+    }
+    
     /// A version property.
     #[dbus_interface(property)]
     async fn version(&self) -> u32 {