Linux Kernel Architecture

Chapter 6: Device Drivers

Table 6-2: Hardware Characteristics of a Request Queue.

Element Meaning

max_sectors Specifies the maximum number of sectors that the device can process in a

single request; the sector size of the specific device (hardsect_size)isused

as the size unit.

max_segment_size Maximum segment size (in bytes) for a single request.

max_phys_segments Specifies the maximum number of non-continuous segments for scatter-

gather requests used to transport non-contiguous data.

max_hw_segments Same asmax_phys_segmentsbut takes into account any remappings that

may be made by a (possible) I/O MMU. The constant specifies the maxi-

mum number of address/length pairs that the driver can pass to the device.

hardsect_size Specifies the physical sector size with which the device operates. This value

is almost always 512; only a few very new devices use different settings.

Adding Partitions

add_partitionis responsible for adding a new partitions into the generic hard disk data structures. I

shall discuss a slightly simplified version here. First of all, a new instance ofstruct hd_structis allo-

cated and filled with some basic information about the partition:

fs/partitions/check.c
void add_partition(struct gendisk disk, int part, sector_t start, sector_t len, int flags)
{
struct hd_struct p;

p = kzalloc(sizeof(*p), GFP_KERNEL);
...
p->start_sect = start;
p->nr_sects = len;
p->partno = part;
...

After assigning a name that shows up, for instance, in sysfs, the partition’s kernel object parent is set to be

the generic hard disk object. In contrast to complete disks, thektypeis notktype_block,butktype_part.

This allows for distinguishing uevents (see Section 7.4) that originate from disks from uevents that origi-

nate from partitions:

fs/partitions/check.c

kobject_set_name(&p->kobj, "%s%d",

kobject_name(&disk->kobj),part);

p->kobj.parent = &disk->kobj;

p->kobj.ktype = &ktype_part;

...