Problems mounting a ubi image using QEMU

Question

I'm trying to emulate a nand flash with qemu and use that to mount an existent ubifs image on the virtual machine.

I added a nand device and a drive of the type mtd, resulting on the following command:

$ qemu-system-arm -nographic -M virt -m 64 -device nand,chip_id=0x59  -drive if=mtd,format=raw,file=data.ubi -kernel openwrt-armvirt-32-zImage-initramfs 
Warning: Orphaned drive without device: id=mtd0,file=data.ubi,if=mtd,bus=0,unit=0
[    0.000000] Booting Linux on physical CPU 0x0
[    0.000000] Linux version 4.19.56 (buildbot@builds) (gcc version 7.4.0 (OpenWrt GCC 7.4.0 r10348-577174cf60)) #0 SMP Tue Jun 25 14:46:01 2019
[    0.000000] CPU: ARMv7 Processor [412fc0f1] revision 1 (ARMv7), cr=30c5387d
[    0.000000] CPU: div instructions available: patching division code
[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, PIPT instruction cache
[    0.000000] OF: fdt: Machine model: linux,dummy-virt
[    0.000000] Memory policy: Data cache writealloc
[    0.000000] psci: probing for conduit method from DT.
[    0.000000] psci: PSCIv0.2 detected in firmware.
...

I can't access to the data.ubi probably because of the following warning:

"Warning: Orphaned drive without device"

How can I add the ubi image to the nand device correctly?

Answer 1

You have to link the -drive and -device via an id parameter:

qemu-system-arm \
  `: [...]` \
  -device nand,chip_id=0x59,id=myubiflash \
  -drive if=mtd,format=raw,file=data.ubi,id=myubiflash \
  `: [...]`

If you really read the message carefully, you'll notice that the drive id defaults to mtd0:

Warning: Orphaned drive without device: id=mtd0,file=data.ubi,if=mtd,bus=0,unit=0

And of course qemu can't magically guess that you meant to define the -device nand for the drive.

The error message is exactly on the spot here.

---

Edit

I'll admit, no perfect understanding of the layers of syntax in QEMU commandline on my side either. Also, the QEMU doc isn't the easiest to read; however it has this passage:

A block driver node created with -blockdev can be used for a guest device by specifying its node name for the drive property in a -device argument that defines a block device.

-blockdev is synonym to -drive in this context.

If I can interpret it right, the meaning of this is that instead of

• -device […],id=foo, -drive […],id=foo

you're supposed to use

• -device […],drive=foo, -drive […],id=foo

Can't test this ATM, but either of those should make it work for you.