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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Mon, 21 Aug 2017 15:27:25 +0200
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References: <20170808224750.23904-1-marcandre.lureau@redhat.com>
	<20170809050846-mutt-send-email-mst@kernel.org>
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Subject: Re: [Qemu-devel] [PATCH] virtio-input: send rel-wheel events for
 wheel buttons
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To: "Michael S. Tsirkin" <mst@redhat.com>, =?ISO-8859-1?Q?Marc-Andr=E9?= Lureau <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org

  Hi,

> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0.select=C2=A0=C2=
=A0=C2=A0=C2=A0=3D VIRTIO_INPUT_CFG_EV_BITS,
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0.subsel=C2=A0=C2=
=A0=C2=A0=C2=A0=3D EV_REL,
> > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0.size=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=3D 1,
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0.size=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=3D 2,
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0.u.bitmap=C2=A0=
=C2=A0=3D {
> > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0(1 << REL_X) | (1 << REL_Y),
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0(1 << REL_X) | (1 << REL_Y),=C2=A0=C2=A0(1 << (REL_WHEEL - 8))
>=20
> Works only when REL_WHEEL is between 8 and 15.
> Add BUILD_BUG_ON?

Not needed IMO, REL_WHEEL value is kernel/userspace abi and will never
change.

>=20
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0},
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0},
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0{ /* end of list */ },
>=20
> Is it problematic e.g. if you migrate from a host with REL_WHEEL
> to one without? Should we maintain a version without REL_WHEEL
> for old machine types?

Yes, we need a compat property.

Possibly it is easier to fix on the guest side, assuming it is actually
needed (see other reply).

cheers,
  Gerd