Thinp: how to enable discards?

[Spelic <spelic@xxxxxxxxxxxxx>]

Hello all
I have booted a kernel 3.4.2 and compiled latest LVM2 tools.
I have created a thin pool and a thin LV.

in dmesg it says:
[72244.278644] device-mapper: thin: Discard unsupported by data device 
(dm-1): Disabling discard passdown.

and in fact I cannot discard data on that volume. Not only the passdown 
doesn't work, but the unmapping doesn't work either.
How can I enable this?

Thank you
S.

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