fdisk with option '-l' - how does it get info about device?

Question

I'm experimenting with a software defined device, trying to force it to work with block size = 512 bytes, but it looks like the Linux doesn't want that.

I've forced the device to report minimal blocks numbers for the SCSI INQUIRY command:

hekto@ubuntu3:~$ sudo sg_inq -p 0xb0 /dev/sde
VPD INQUIRY: Block limits page (SBC)
  Maximum compare and write length: 255 blocks
  Optimal transfer length granularity: 1 blocks
  Maximum transfer length: 1 blocks
  Optimal transfer length: 1 blocks
  Maximum prefetch transfer length: 0 blocks
  Maximum unmap LBA count: 0
  Maximum unmap block descriptor count: 0
  Optimal unmap granularity: 0
  Unmap granularity alignment valid: 0
  Unmap granularity alignment: 0
  Maximum write same length: 0x0 blocks
  Maximum atomic transfer length: 0
  Atomic alignment: 0
  Atomic transfer length granularity: 0

But the fdisk -l still reports physical block size = 4096:

hekto@ubuntu3:~$ sudo fdisk -l /dev/sde

Disk /dev/sde: 5 MB, 5120000 bytes
1 heads, 10 sectors/track, 1000 cylinders, total 10000 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes  <== see here
I/O size (minimum/optimal): 4096 bytes / 524288 bytes.  <== and here  

Where do these numbers (see above) come from?

OS: Ubuntu 14.04.5 LTS with kernel 3.19.0-78-generic

(working as virtual machine under VMware Fusion 8.5.8 on MacBook)

Answer 1

I suggested to use strace(1) to understand the syscalls(2) done by your fdisk command, i.e. to run as root:

  strace fdisk -l /dev/sde

you have discovered that it uses some ioctl like BLKIOMIN and BLKPBSZGET

I don't know how the kernel computes these parameters; perhaps read sd(4).