Declaring a Variable Volatile

volatile is a keyword that must be applied when declaring any variable that will reference a device register. Without the use of volatile, the compile-time optimizer can inadvertently delete important accesses. Neglecting to use volatile might result in bugs that are difficult to track down.

The correct use of volatile is necessary to prevent elusive bugs. The volatile keyword instructs the compiler to use exact semantics for the declared objects, in particular, not to remove or reorder accesses to the object. Two instances where device drivers must use the volatile qualifier are:

• When data refers to an external hardware device register, that is, memory that has side effects other than just storage. Note, however, that if the DDI data access functions are used to access device registers, you do not have to use volatile.

• When data refers to global memory that is accessible by more than one thread, that is not protected by locks, and that relies on the sequencing of memory accesses. Using volatileconsumes fewer resources than using lock.

The following example uses volatile. A busy flag is used to prevent a thread from continuing while the device is busy and the flag is not protected by a lock:

while (busy) {
    /* do something else */
}

The testing thread will continue when another thread turns off the busy flag:

busy = 0;

Because busy is accessed frequently in the testing thread, the compiler can potentially optimize the test by placing the value of busy in a register and test the contents of the register without reading the value of busy in memory before every test. The testing thread would never see busy change and the other thread would only change the value of busy in memory, resulting in deadlock. Declaring the busy flag as volatile forces its value to be read before each test.

An alternative to the busy flag is to use a condition variable. See Condition Variables in Thread Synchronization.

When using the volatile qualifier, avoid the risk of accidental omission. For example, the following code

struct device_reg {
    volatile uint8_t csr;
    volatile uint8_t data;
};
struct device_reg *regp;

is preferable to the next example:

struct device_reg {
    uint8_t csr;
    uint8_t data;
};
volatile struct device_reg *regp;

Although the two examples are functionally equivalent, the second one requires the writer to ensure that volatile is used in every declaration of type struct device_reg. The first example results in the data being treated as volatile in all declarations and is therefore preferred. As mentioned above, using the DDI data access functions to access device registers makes qualifying variables as volatile unnecessary.