feat(boot): clear screen before boot handoff, if --retain-boot-console is not specified on the command-line (#51)

crates/boot/src/actions/chainload.rs

@@ -1,4 +1,5 @@
 use crate::context::SproutContext;
+use crate::phases::before_handoff;
 use crate::utils;
 use alloc::boxed::Box;
 use alloc::rc::Rc;
@@ -88,6 +89,10 @@ pub fn chainload(context: Rc<SproutContext>, configuration: &ChainloadConfigurat
     BootloaderInterface::mark_exec(context.root().timer())
         .context("unable to mark execution of boot entry in bootloader interface")?;
 
+    // Since we are about to hand off control to another image, we need to execute the handoff hook.
+    // This will perform operations like clearing the screen.
+    before_handoff(&context).context("unable to execute before handoff hook")?;
+
     // Start the loaded image.
     // This call might return, or it may pass full control to another image that will never return.
     // Capture the result to ensure we can return an error if the image fails to start, but only

crates/boot/src/options.rs

@@ -24,6 +24,8 @@ pub struct SproutOptions {
     pub force_menu: bool,
     /// The timeout for the boot menu in seconds.
     pub menu_timeout: Option<u64>,
+    /// Retains the boot console before boot.
+    pub retain_boot_console: bool,
 }
 
 /// The default Sprout options.
@@ -35,6 +37,7 @@ impl Default for SproutOptions {
             boot: None,
             force_menu: false,
             menu_timeout: None,
+            retain_boot_console: false,
         }
     }
 }
@@ -42,7 +45,7 @@ impl Default for SproutOptions {
 /// The options parser mechanism for Sprout.
 impl SproutOptions {
     /// Produces [SproutOptions] from the arguments provided by the UEFI core.
-    /// Internally we utilize the `jaarg` argument parser which has excellent no_std support.
+    /// Internally, we use the `jaarg` argument parser which has excellent no_std support.
     pub fn parse() -> Result<Self> {
         enum ArgID {
             Help,
@@ -51,6 +54,7 @@ impl SproutOptions {
             Boot,
             ForceMenu,
             MenuTimeout,
+            RetainBootConsole,
         }
 
         // All the options for the Sprout executable.
@@ -65,6 +69,8 @@ impl SproutOptions {
             Opt::flag(ArgID::ForceMenu, &["--force-menu"]).help_text("Force showing the boot menu"),
             Opt::value(ArgID::MenuTimeout, &["--menu-timeout"], "TIMEOUT")
                 .help_text("Boot menu timeout, in seconds"),
+            Opt::flag(ArgID::RetainBootConsole, &["--retain-boot-console"])
+                .help_text("Retain boot console before boot"),
         ]);
 
         // Acquire the arguments as determined by the UEFI core.
@@ -99,6 +105,10 @@ impl SproutOptions {
                         // The timeout for the boot menu in seconds.
                         result.menu_timeout = Some(value.parse::<u64>()?);
                     }
+                    ArgID::RetainBootConsole => {
+                        // Retain the boot console before booting.
+                        result.retain_boot_console = true;
+                    }
                     ArgID::Help => {
                         let ctx = HelpWriterContext {
                             options: &OPTIONS,

crates/boot/src/phases.rs

@@ -24,3 +24,13 @@ pub fn phase(context: Rc<SproutContext>, phase: &[PhaseConfiguration]) -> Result
     }
     Ok(())
 }
+
+/// Manual hook called by code in the bootloader that hands off to another image.
+/// This is used to perform actions like clearing the screen.
+pub fn before_handoff(context: &SproutContext) -> Result<()> {
+    // If we have not been asked to retain the boot console, then we should clear the screen.
+    if !context.root().options().retain_boot_console {
+        uefi::system::with_stdout(|stdout| stdout.reset(true)).context("unable to clear screen")?;
+    }
+    Ok(())
+}