riscv: add fallible functions to satp

Adds fallible access functions for `Satp` ASID and PPN fields.

Author: rmsyn

riscv/src/register/satp.rs

@@ -1,5 +1,7 @@
 //! satp register
 
+use crate::result::{Error, Result};
+
 /// satp register
 #[derive(Clone, Copy, Debug)]
 pub struct Satp {
@@ -105,6 +107,28 @@ pub unsafe fn set(mode: Mode, asid: usize, ppn: usize) {
     _write(bits);
 }
 
+/// Attempts to set the register to corresponding page table mode, physical page number and address space id.
+#[inline]
+#[cfg(target_pointer_width = "32")]
+pub unsafe fn try_set(mode: Mode, asid: usize, ppn: usize) -> Result<()> {
+    if asid != asid & 0x1FF {
+        Err(Error::InvalidValue {
+            field: "asid",
+            value: asid,
+            bitmask: 0x1FF,
+        })
+    } else if ppn != ppn & 0x3F_FFFF {
+        Err(Error::InvalidValue {
+            field: "ppn",
+            value: ppn,
+            bitmask: 0x3F_FFFF,
+        })
+    } else {
+        let bits = (mode as usize) << 31 | (asid << 22) | ppn;
+        _try_write(bits)
+    }
+}
+
 /// Sets the register to corresponding page table mode, physical page number and address space id.
 #[inline]
 #[cfg(target_pointer_width = "64")]
@@ -114,3 +138,25 @@ pub unsafe fn set(mode: Mode, asid: usize, ppn: usize) {
     let bits = (mode as usize) << 60 | (asid << 44) | ppn;
     _write(bits);
 }
+
+/// Sets the register to corresponding page table mode, physical page number and address space id.
+#[inline]
+#[cfg(target_pointer_width = "64")]
+pub unsafe fn try_set(mode: Mode, asid: usize, ppn: usize) -> Result<()> {
+    if asid != asid & 0xFFFF {
+        Err(Error::InvalidValue {
+            field: "asid",
+            value: asid,
+            bitmask: 0xFFFF,
+        })
+    } else if ppn != ppn & 0xFFF_FFFF_FFFF {
+        Err(Error::InvalidValue {
+            field: "ppn",
+            value: ppn,
+            bitmask: 0xFFF_FFFF_FFFF,
+        })
+    } else {
+        let bits = (mode as usize) << 60 | (asid << 44) | ppn;
+        _try_write(bits)
+    }
+}

riscv/src/result.rs

@@ -15,6 +15,12 @@ pub enum Error {
     },
     /// Unimplemented function or type.
     Unimplemented,
+    /// Invalid field value.
+    InvalidValue {
+        field: &'static str,
+        value: usize,
+        bitmask: usize,
+    },
 }
 
 impl fmt::Display for Error {
@@ -25,6 +31,14 @@ impl fmt::Display for Error {
                 "out-of-bounds access, index: {index}, min: {min}, max: {max}"
             ),
             Self::Unimplemented => write!(f, "unimplemented"),
+            Self::InvalidValue {
+                field,
+                value,
+                bitmask,
+            } => write!(
+                f,
+                "invalid {field} field value: {value:#x}, valid bitmask: {bitmask:#x}",
+            ),
         }
     }
 }