Linux Device Drivers, Second Edition

Removable Devices

Thus far, we have ignored the final two file operations in the block_device_operations structure, which deal with devices that support removable media. It’s now time to look at them; sbull isn’t actually removable but it pretends to be, and therefore it implements these methods.

The operations in question are check_media_change and revalidate. The former is used to find out if the device has changed since the last access, and the latter re-initializes the driver’s status after a disk change.

As far as sbull is concerned, the data area associated with a device is released half a minute after its usage count drops to zero. Leaving the device unmounted (or closed) long enough simulates a disk change, and the next access to the device allocates a new memory area.

This kind of “timely expiration” is implemented using a kernel timer.

check_media_change

The checking function receives kdev_t as a single argument that identifies the device. The return value is 1 if the medium has been changed and 0 otherwise. A block driver that doesn’t support removable devices can avoid declaring the function by setting bdops->check_media_change to NULL.

It’s interesting to note that when the device is removable but there is no way to know if it changed, returning 1 is a safe choice. This is the behavior of the IDE driver when dealing with removable disks.

The implementation in sbull returns 1 if the device has already been removed from memory due to the timer expiration, ...