Implement a read_write_in_place method for in-place transfers

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 [Full Changelog](https://github.com/rust-embedded/rust-spidev/compare/0.7.0...HEAD)
 
+- Implement a `read_write_in_place` method for in-place transfers.
+
 ## 0.7.0 / 2025-03-04
 
 [Full Changelog](https://github.com/rust-embedded/rust-spidev/compare/0.6.0...0.7.0)

src/spidevioctl.rs

@@ -108,6 +108,19 @@ impl<'a, 'b> spi_ioc_transfer<'a, 'b> {
         }
     }
 
+    /// Create a read/write transfer using the same buffer for reading
+    /// and writing (in-place transfer).
+    pub fn read_write_in_place(buf: &'a mut [u8]) -> Self {
+        // This is allowed according to a comment in the linux source tree:
+        // https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Documentation/spi/spidev.rst?id=211ddde0823f1442e4ad052a2f30f050145ccada#n191
+        spi_ioc_transfer {
+            rx_buf: buf.as_ptr() as *const () as usize as u64,
+            tx_buf: buf.as_ptr() as *const () as usize as u64,
+            len: buf.len() as u32,
+            ..Default::default()
+        }
+    }
+
     /// Create a delay transfer of a number of microseconds
     pub fn delay(microseconds: u16) -> Self {
         spi_ioc_transfer {