[PATCH 5/8] rust: module_param: add from_setup_arg() to ModuleParam trait

[Matthew Wood <thepacketgeek@gmail.com>]

Extend the ModuleParam trait with from_setup_arg(), which constructs
a parameter value from a raw C string pointer received via a __setup
callback during early boot.

The default implementation converts the pointer to a CStr and
delegates to try_from_param_arg(), which handles the parse-from-string
path used by integer types.

StringParam overrides this method to store the raw pointer directly
without parsing, since the pointer originates from static_command_line
and remains valid for the kernel's lifetime.

Also add ModuleParamAccess::set_value() to allow the __setup callback
generated by the module! macro to populate the SetOnce container with
first-write-wins semantics.

Signed-off-by: Matthew Wood <thepacketgeek@gmail.com>
---
 rust/kernel/module_param.rs | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/rust/kernel/module_param.rs b/rust/kernel/module_param.rs
index 54379a2bba51..5e14dbe03865 100644
--- a/rust/kernel/module_param.rs
+++ b/rust/kernel/module_param.rs
@@ -45,6 +45,21 @@ unsafe impl Sync for ObsKernelParam {}
 pub trait ModuleParam: Sized + Copy {
     /// Parse a parameter argument into the parameter value.
     fn try_from_param_arg(arg: &BStr) -> Result<Self>;
+
+    /// Create a parameter value from a raw `__setup` callback argument.
+    ///
+    /// Default implementation: parse the null-terminated C string via
+    /// [`ModuleParam::try_from_param_arg`]. `StringParam` overrides this to store the pointer
+    /// directly.
+    ///
+    /// # Safety
+    ///
+    /// `val` must point to a valid null-terminated string.
+    unsafe fn from_setup_arg(val: *const c_char) -> Result<Self> {
+        // SAFETY: Caller guarantees `val` points to a valid null-terminated string.
+        let cstr = unsafe { CStr::from_char_ptr(val) };
+        Self::try_from_param_arg(cstr.as_ref())
+    }
 }
 
 /// Set the module parameter from a string.
@@ -226,6 +241,12 @@ fn try_from_param_arg(_arg: &BStr) -> Result<Self> {
         // when using PARAM_OPS_STRING.
         Err(EINVAL)
     }
+
+    unsafe fn from_setup_arg(val: *const c_char) -> Result<Self> {
+        // SAFETY: Caller guarantees `val` points to a valid null-terminated string.
+        // The pointer comes from `static_command_line` which is valid for the kernel's lifetime.
+        Ok(unsafe { StringParam::from_ptr(val) })
+    }
 }
 
 /// A wrapper for kernel parameters.
@@ -266,6 +287,14 @@ pub fn value(&self) -> &T {
     pub const fn as_void_ptr(&self) -> *mut c_void {
         core::ptr::from_ref(self).cast_mut().cast()
     }
+
+    /// Set the parameter value directly.
+    ///
+    /// Returns `true` if successfully set, `false` if already populated
+    /// (first-write-wins semantics via [`SetOnce`]).
+    pub fn set_value(&self, val: T) -> bool {
+        self.value.populate(val)
+    }
 }
 
 #[doc(hidden)]
-- 
2.52.0