To improve performance, the CPU uses internal store buffers to temporarily store data. This may affect the synchronization of device I/O operations. Therefore, the driver needs to take explicit steps to make sure that writes to registers are completed at the proper time.
For example, when access to device space (such as registers or a frame buffer) is synchronized by a lock, the driver needs to check that the store to the device space has actually completed before releasing the lock. Releasing the lock does not guarantee the flushing of I/O buffers.
To give another example, when acknowledging an interrupt, the driver usually sets or clears a bit in a device control register. The driver must ensure that the write to the control register has reached the device before the interrupt handler returns. Similarly, if the device requires a delay (the driver busy-waits) after writing a command to the control register, the driver must ensure that the write has reached the device before delaying.
If the device registers can be read without undesirable side effects, verification of a write can be as simple as reading the register immediately after writing to it. If that particular register cannot be read without undesirable side effects, another device register in the same register set can be used.