redox-driver-sys: add # Safety docs to all io.rs unsafe blocks

Document the safety contracts for:
- acquire_iopl: kernel fd validity, IOPL privilege, no concurrent calls
- inb/inw/inl: valid port, required privilege (IOPL or ring 0)
- outb/outw/outl: valid writable port, no destructive side effects

Closes the documentation gap for io.rs unsafe inline asm blocks.
Part of the systematic fix for ZERO # Safety docs across ~330 unsafe
blocks (NETWORKING-AND-DRIVERS-CODE-ASSESSMENT-2026-07-27.md §9.3).

local/recipes/drivers/redox-driver-sys/source/src/io.rs

@@ -2,6 +2,19 @@
 
 use syscall as redox_syscall;
 
+/// Acquire I/O Privilege Level for the current thread so that subsequent
+/// `inb`/`inw`/`inl`/`outb`/`outw`/`outl` calls do not raise #GP.
+///
+/// # Safety
+///
+/// The caller must ensure:
+/// - `acquire_iopl` is called from a thread that is permitted to hold IOPL
+///   (typically a driver thread under the `redox-driver-sys` umbrella).
+/// - No other code path is concurrently relying on the prior privilege level.
+/// - The kernel fd returned by `redox_cur_thrfd_v0()` is valid for the
+///   duration of this call (the function returns a duplicated handle which is
+///   closed before this function returns; the underlying fd must remain open
+///   at least until the close completes).
 #[cfg(all(target_arch = "x86_64", target_os = "redox"))]
 pub fn acquire_iopl() -> std::result::Result<(), crate::DriverError> {
     extern "C" {
@@ -26,6 +39,16 @@ pub fn acquire_iopl() -> std::result::Result<(), crate::DriverError> {
     )))
 }
 
+/// Read a byte from an x86 I/O port.
+///
+/// # Safety
+///
+/// The caller must ensure:
+/// - `port` is a valid x86 I/O port number (0..=0xFFFF) that is mapped for the
+///   current privilege level (IOPL must have been acquired via `acquire_iopl`,
+///   or the caller must be ring 0).
+/// - No other thread is concurrently modifying port-mapped device state in a
+///   way that makes single-byte reads racy.
 #[cfg(target_arch = "x86_64")]
 #[inline]
 pub fn inb(port: u16) -> u8 {
@@ -34,12 +57,28 @@ pub fn inb(port: u16) -> u8 {
     val
 }
 
+/// Write a byte to an x86 I/O port.
+///
+/// # Safety
+///
+/// The caller must ensure:
+/// - `port` is a valid x86 I/O port number (0..=0xFFFF) that is mapped and
+///   writable at the current privilege level (IOPL via `acquire_iopl`, or ring 0).
+/// - Writing `val` to `port` does not produce a side effect that puts the
+///   device in a state that violates the kernel's or another driver's
+///   invariants (e.g. disabling an interrupt source shared with the system).
 #[cfg(target_arch = "x86_64")]
 #[inline]
 pub fn outb(port: u16, val: u8) {
     unsafe { core::arch::asm!("out dx, al", in("dx") port, in("al") val, options(nomem, nostack, preserves_flags)) };
 }
 
+/// Read a 32-bit dword from an x86 I/O port.
+///
+/// # Safety
+///
+/// Same requirements as `inb`, but reads 4 contiguous bytes (little-endian)
+/// from the port. The device must support 32-bit port I/O at `port..port+3`.
 #[cfg(target_arch = "x86_64")]
 #[inline]
 pub fn inl(port: u16) -> u32 {
@@ -48,12 +87,24 @@ pub fn inl(port: u16) -> u32 {
     val
 }
 
+/// Write a 32-bit dword to an x86 I/O port.
+///
+/// # Safety
+///
+/// Same requirements as `outb`, but writes 4 contiguous bytes (little-endian)
+/// to `port..port+3`. The device must support 32-bit port I/O at this range.
 #[cfg(target_arch = "x86_64")]
 #[inline]
 pub fn outl(port: u16, val: u32) {
     unsafe { core::arch::asm!("out dx, eax", in("dx") port, in("eax") val, options(nomem, nostack, preserves_flags)) };
 }
 
+/// Read a 16-bit word from an x86 I/O port.
+///
+/// # Safety
+///
+/// Same requirements as `inb`, but reads 2 contiguous bytes (little-endian)
+/// from the port. The device must support 16-bit port I/O at `port..port+1`.
 #[cfg(target_arch = "x86_64")]
 #[inline]
 pub fn inw(port: u16) -> u16 {
@@ -62,8 +113,14 @@ pub fn inw(port: u16) -> u16 {
     val
 }
 
+/// Write a 16-bit word to an x86 I/O port.
+///
+/// # Safety
+///
+/// Same requirements as `outb`, but writes 2 contiguous bytes (little-endian)
+/// to `port..port+1`. The device must support 16-bit port I/O at this range.
 #[cfg(target_arch = "x86_64")]
 #[inline]
 pub fn outw(port: u16, val: u16) {
     unsafe { core::arch::asm!("out dx, ax", in("dx") port, in("ax") val, options(nomem, nostack, preserves_flags)) };
-}
+}