I2C concurrent access on Linux, mutex

Question

I'm writing a multithread C program in embedded Linux that accesses from userspace a number of I2C devices (slaves). Also, I access the same I2C device from multiple threads. I'm using SMBUS functions (i2c_smbus_write_byte_data, i2c_smbus_read_byte_data, i2c_smbus_read_i2c_block_data,...).

Is there any protection from concurrent access built in or do I need to add mutexes myself?

For instance: I have a Read function that read data from one sensor over I2C. But the same function can be called from another thread as well, resulting in possible concurrent access. Do I have to use some static mutex in that function or is it already in the I2C access functions?

Answer 1

I²C is a shared bus with multiple devices, which could be accessed from multiple processes as well as threads. So the Linux I²C driver code uses a mutex to manage access to each I²C bus.

For SMBus functions, see the Linux kernel function i2c_smbus_xfer() in i2c-core-smbus.c. It gets a lock for the I²C adapter before beginning a transfer (look at the source code, and see the call to __i2c_lock_bus_helper()). All SMBus transactions are based on that function.

For I²C functions, see the Linux kernel function i2c_transfer() in i2c-core-base.c. It gets a lock for the I²C adapter before beginning a transfer. All I²C transaction are based on that function.

So yes, there is protection from concurrent access built-in.

(links are for Linux kernel 5.13)