Linux Kernel Architecture

Chapter 6: Device Drivers

❑ disk->fops->openinvokes the appropriateopenfunction for the file to perform hardware-

specific initialization tasks.

❑ If the partition information has become invalid as indicated byblock_device->bd_invalidated,

rescan_partitionsis called to re-read the partition information. Partition information can

become invalid if a removable medium is exchanged.

If the device has not been opened before, some more work is required. First suppose that not a

partition, but the main block device is opened — which may nevertheless contain partitions. In this

case, the required actions are modulo some bookkeeping details identical to the case shown above:

disk->fops->openhandles the low-level work of opening the device, andrescan_partitionsreads the

partition table if the existing information is invalid.

This is, however, usually the first time that partition information is read in. When a new disk is registered

usingadd_disk,thekernelsetsgendisk->bd_invalidatedto 1, which signals an invalid partition table

on the block device (in fact, since there is no partition table at all, it cannot really be calledvalid!). Then a

fake file is constructed as a parameter passed todo_open, and this, in turn, triggers reading the partition

table.

If the opened block device represents a partition that has not been opened before, the kernel needs to

associate theblock_deviceinstance for the partition with theblock_devicethat contains the partition.

Essentially this works as follows:

fs/block_dev.c

struct hd_struct *p;

struct block_device *whole;

whole = bdget_disk(disk, 0);

...

bdev->bd_contains = whole;

p = disk->part[part - 1];

...

bdev->bd_part = p;

After finding theblock_deviceinstance that represents the whole disk that includes the partition, a link

between partition and container is set up usingblock_device->bd_contains. Note that the kernel can

find the whole block device starting from the partition’s block device, but not vice versa! Additionally, the

partition information inhd_structis now shared between the generic hard disk and theblock_device

instance for the partition, as indicated in Figure 6-10.

6.5.5 Request Structure

The kernel provides its own data structure to describe a request to a block device.

<blkdev.h>

struct request {

struct list_head queuelist;

struct list_head donelist;

struct request_queue *q;

unsigned int cmd_flags;