Block device - Initialize gendisk structure when not multiple of 512 bytes

Question

I'm currently learning kernel driver programmation using Linux Device Drivers - 3rd.

To define a block device, I have to declare a gendisk structure and to initialize it with relevant informations.

In chapter 16 its written:

sector_t capacity;

The capacity of this drive, in 512-byte sectors. The sector_t type can be 64 bits wide. Drivers should not set this field directly; instead, pass the number of sectors to set_capacity.

Usualy I set the capacity using a call like:

set_capacity(gendisk, sector_number*(hard_sector_size/KERNEL_SECTOR_SIZE));

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Imagine that I have a device with a memory zone that is not a multiple of 512 bytes.

Let's take 2000 bytes so there is three 512 bytes zones plus a 464 bytes zone

[-512b-]      [-512b-]      [-512b-]      [-464b-]
0     511    512    1023   1024   1535   1536   1999

• What should I pass to set_capacity ?

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Furthermore on some devices, memory is splitted into several areas. Imagine that I have a device containing multiple memory areas, each with different size and that I want to abstract this splitted memory into a single memory zone.

area1 => 32000 bytes
area2 => 512 bytes
area3 => 50 bytes
area4 => 45 bytes
area5 => 1024 bytes

In this case the hard sector size is not a constant within the device.

• How should I fill the gendisk structure ?

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UPDATE after @KamilCuk answer:

To continue with this example let's take the full device memory (33631 bytes) which is 65 sectors of 512 bytes + 1 sector of 351 bytes.

So the solutions are to present it either as:

• 33270 bytes memory zone ==> 65 sectors

or

• 33631 bytes memory zone ==> 66 sectors with 161 bytes discarded at the end of last sector

Are those the best solutions or there is another way to handle this specific case ?

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• Basically the question could be resumed to how to initialize a block device when it's not a multiple of KERNEL_SECTOR_SIZE?

Answer 1

• What should I pass to set_capacity ?
  From lwm.net article:

  The size value should be in 512-byte sectors, even if the hardware sector size used by your device is different.

Pass that you have 3 sectors, thus be compatible with kernel and ignore last sector.
Or pass that you have 4 sectors of 512, discard the bytes from 464 to 512 on the last sector on write and fill with zeros on read. Be sure, that all drivers that use your driver know, that the last sector is smaller and can act on it.

• How should I fill the gendisk structure ?
  To be true, with your block driver implementation. You are implementing block driver not filesystem driver, files are on the upper level. A filesystem can implement multiple files saved between blocks or in the same block of underlying block device. (like btrfs can compress small files to one block)