Query regarding some blk_queue_XXX functions. (For kernel 2.6.32 SLES 11)

Pranay Kumar Srivastava Pranay.Shrivastava at hcl.com
Mon Sep 3 07:51:28 EDT 2012

Hi All,

I'm using SLES11 (kernel 2.6.32)

I recently did a dummy code for block device driver where the whole disk is made of pages in memory. I have some query regarding usage of some of the functions,

I've absolutely no idea what the first 2 do. What I was trying to do was that I wanted the block layer to give my driver requests which are aligned according to my device's minimum sector size. Since I wanted to play with the driver I chose 1024 bytes instead of 512, however I've tried with 2048 bytes device sector size as well and all seems well :D

Now the problem is kernel wants to give everything in 512 byte but I want to receive everything aligned properly according to my device's sector size. So I looked in kernel code and I thought I could use the functions below...

1. blk_limits_io_min: 
No clue about this one, in-fact the device didn't got added when I used this.

2. blk_queue_physical_block_size:
This one checks the logical block size before it sets physical block size itself. But not really sure about it's purpose

3. blk_queue_logical_block_size:
I think this is the one responsible that I get requests properly aligned to device's sector size? Not really sure...

4. set_capacity:
There's also a callback in block_device_operations by this name? However this function just sets the size of part0 of the gendisk. I've not seen the callback being called upon, I used some print statements and I never see them. So what's the purpose of the callback?

There's a field gd->start in gendisk which apparently sets the start of data sector? But why would block layer be bothered with that? Is this used by filesystems since application programs certainly can write to sector 0 as well right?

And a couple more :P ,

When I try to use fdisk on the disk, it seems to create partition table (I think), however when it tries to read the partition table again I get invalid argument to ioctl. So I think I'm missing an ioctl call in my driver correct? I've none yet though. 

However this issue aside, if I'm able to create a partition then my driver should be able to handle additional disks correct? And it would follow the same operations when I added my module and called add_disk the first time correct? 

Thanks a lot for reading!

Pranay Kr. Srivastava
pranay.shrivastava at hcl.com
Software Engineer
ERS,HCL Technologies
A-5, Sector 24, Noida 201301, U.P. (India)


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