Allow forcefully setting slave address

Closes #59

Author: iamkroot

CHANGELOG.md

@@ -10,6 +10,7 @@ Versioning](https://semver.org/spec/v2.0.0.html).
 
 - Updated nix to allow both version `0.22` or `0.23`.
 - Add PEC support for SMBus compatible adapters
+- Add `LinuxI2CDevice::force_new()` to open the device without checking if it is bound to a driver.
 
 ## [v0.5.0] - 2021-09-21
 

src/ffi.rs

@@ -153,9 +153,10 @@ pub struct i2c_rdwr_ioctl_data {
 mod ioctl {
     pub use super::i2c_rdwr_ioctl_data;
     pub use super::i2c_smbus_ioctl_data;
-    use super::{I2C_PEC, I2C_RDWR, I2C_SLAVE, I2C_SMBUS};
+    use super::{I2C_PEC, I2C_RDWR, I2C_SLAVE, I2C_SLAVE_FORCE, I2C_SMBUS};
 
     ioctl_write_int_bad!(set_i2c_slave_address, I2C_SLAVE);
+    ioctl_write_int_bad!(set_i2c_slave_address_force, I2C_SLAVE_FORCE);
     ioctl_write_int_bad!(set_smbus_pec, I2C_PEC);
     ioctl_write_ptr_bad!(i2c_smbus, I2C_SMBUS, i2c_smbus_ioctl_data);
     ioctl_write_ptr_bad!(i2c_rdwr, I2C_RDWR, i2c_rdwr_ioctl_data);
@@ -168,6 +169,13 @@ pub fn i2c_set_slave_address(fd: RawFd, slave_address: u16) -> Result<(), nix::E
     Ok(())
 }
 
+pub fn i2c_set_slave_address_force(fd: RawFd, slave_address: u16) -> Result<(), nix::Error> {
+    unsafe {
+        ioctl::set_i2c_slave_address_force(fd, i32::from(slave_address))?;
+    }
+    Ok(())
+}
+
 pub fn i2c_set_smbus_pec(fd: RawFd, enable: bool) -> Result<(), nix::Error> {
     unsafe {
         ioctl::set_smbus_pec(fd, i32::from(enable))?;

src/linux.rs

@@ -110,6 +110,27 @@ impl LinuxI2CDevice {
         Ok(device)
     }
 
+    /// Create a new I2CDevice for the specified path, without checking if the
+    /// device is bound to a driver
+    ///
+    /// # Safety
+    /// Using this can seriously confuse the original driver, and may cause all
+    /// future communication to perform the wrong operations and/or return wrong results.
+    pub unsafe fn force_new<P: AsRef<Path>>(
+        path: P,
+        slave_address: u16,
+    ) -> Result<LinuxI2CDevice, LinuxI2CError> {
+        let file = OpenOptions::new().read(true).write(true).open(path)?;
+        let mut device = LinuxI2CDevice {
+            devfile: file,
+            slave_address: 0, // will be set later
+            pec: false,
+        };
+        device.force_set_slave_address(slave_address)?;
+        device.set_smbus_pec(false)?;
+        Ok(device)
+    }
+
     /// Set the slave address for this device
     ///
     /// Typically the address is expected to be 7-bits but 10-bit addresses
@@ -127,6 +148,16 @@ impl LinuxI2CDevice {
         Ok(())
     }
 
+    /// Set the slave address for this device, even if it is already in use
+    /// by a driver
+    ///
+    /// This is private; use `force_new` instead.
+    unsafe fn force_set_slave_address(&mut self, slave_address: u16) -> Result<(), LinuxI2CError> {
+        ffi::i2c_set_slave_address_force(self.as_raw_fd(), slave_address)?;
+        self.slave_address = slave_address;
+        Ok(())
+    }
+
     /// Enable/Disable PEC support for this device
     ///
     /// Used only for SMBus transactions.  This request only has an effect if the