dw-i2c: deliver scheme capability fd to init in endpoint::serve (fix boot hang)

endpoint::serve used register_sync_scheme, which registers the scheme to
the daemon's own namespace but never returns the capability fd to init.
For type={scheme} services init blocks in call_ro(FD) waiting for that
fd, so the boot deadlocked right after 'Started Intel LPSS I2C
controller' (CPU halted, init asleep) — the ptyd-class bug.

Replace register_sync_scheme with an explicit capability-fd creation +
notify_init_scheme_fd() that writes the fd to the INIT_NOTIFY pipe
(same mechanism as SchemeDaemon::ready_with_fd / ready_sync_scheme,
which i2cd, ucsid, and ptyd already use). init then registers
/scheme/<name> and the boot proceeds.

Affects intel-lpss-i2cd and dw-acpi-i2cd, both type={scheme} services
using this endpoint.

drivers/i2c/designware/src/lib.rs

@@ -595,7 +595,7 @@ pub mod endpoint {
 
     use anyhow::{Context, Result};
     use i2c_interface::{I2cTransferRequest, I2cTransferResponse, I2cTransferStatus};
-    use redox_scheme::scheme::{register_sync_scheme, SchemeSync};
+    use redox_scheme::scheme::SchemeSync;
     use redox_scheme::{CallerCtx, OpenResult, Socket};
     use scheme_utils::{Blocking, HandleMap};
     use syscall::schemev2::NewFdFlags;
@@ -861,8 +861,17 @@ pub mod endpoint {
             handles: HandleMap::new(),
             controllers,
         };
-        register_sync_scheme(&socket, scheme_name, &mut endpoint)
-            .with_context(|| format!("failed to register {scheme_name} scheme"))?;
+
+        // type={scheme} handshake: deliver the capability fd to init over
+        // INIT_NOTIFY so init registers /scheme/<name> and returns from
+        // call_ro(FD). register_sync_scheme alone deadlocks init (ptyd-class).
+        let cap_id = endpoint
+            .scheme_root()
+            .context("failed to compute scheme root")?;
+        let cap_fd = socket
+            .create_this_scheme_fd(0, cap_id, 0, 0)
+            .context("failed to create scheme capability fd")?;
+        notify_init_scheme_fd(cap_fd).context("failed to deliver scheme fd to init")?;
 
         log::info!(
             "dw-i2c: serving /scheme/{scheme_name} with {} controller(s)",
@@ -882,6 +891,16 @@ pub mod endpoint {
         Ok(())
     }
 
+    fn notify_init_scheme_fd(cap_fd: usize) -> Result<()> {
+        let pipe_fd: usize = std::env::var("INIT_NOTIFY")
+            .context("INIT_NOTIFY is not set")?
+            .parse()
+            .context("INIT_NOTIFY is not a valid fd")?;
+        syscall::call_wo(pipe_fd, &cap_fd.to_ne_bytes(), syscall::CallFlags::FD, &[])
+            .context("failed to write scheme fd to the INIT_NOTIFY pipe")?;
+        Ok(())
+    }
+
     #[cfg(test)]
     mod tests {
         use super::*;