February 1998
Intermediate to advanced
439 pages
13h 46m
English
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Removable Devices
When we discussed char drivers, we ignored the final two file
operations in the fops structure, because they exist only
for the sake of removable block devices. It’s now time to look at
them; sbull isn’t actually removable, but it pretends
to be, and therefore it implements the methods.
The operations I’m talking about 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 fops->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, ...
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