Re: [Qemu-devel] [RFC PATCH spice v2 1/2] QXL interface: add functions to identify monitors in the guest

["Lukáš Hrázký" <lhrazky@redhat.com>]

On Mon, 2018-11-05 at 14:08 +0100, Gerd Hoffmann wrote:
> On Mon, Nov 05, 2018 at 01:18:57PM +0100, Lukáš Hrázký wrote:
> > On Mon, 2018-11-05 at 07:52 +0100, Gerd Hoffmann wrote:
> > > > 2. Have a single function as follows:
> > > > 
> > > > void spice_qxl_set_device_info(QXLInstance *instance,
> > > >                                const char *device_address,
> > > >                                uint32_t device_display_id_start,
> > > >                                uint32_t device_display_id_count);
> > > 
> > > How about:
> > > 
> > > void spice_qxl_set_device_info(QXLInstance *instance,
> > >                                const char *device_address,
> > >                                uint32_t device_display_id);
> > > 
> > > I don't think we need start+count:
> > > 
> > >  * For single-head devices device_display_id will be zero.
> > >  * For one-channel-per-head multihead devices (i.e. virtio-gpu)
> > >    device_display_id will enumerate the heads (so everybody can figure
> > >    which channel is which head).
> > >  * For one-channel-per-device multihead devices (i.e. qxl/linux)
> > >    device_display_id will be zero too.  Number of heads is set via
> > >    spice_qxl_set_max_monitors().
> > 
> > That requires nontrivial and unexpected logic for the one-channel-per-
> > device multihead devices case. The API should be doing what it says and
> >  the dumber the better, this seems too smart to me...
> 
> Well, the device_display_id_count argument is redundant with
> spice_qxl_set_max_monitors().  That isn't a great API either.
> 
> I can see that it simplifies the logic in spice-server if we have a
> single function call instead of two.  So we could deprecate
> spice_qxl_set_max_monitors() in favour of your
> spice_qxl_set_device_info() variant.
> 
> spice_qxl_set_max_monitors() would then basically do this:
> 
>     spice_qxl_set_max_monitors(qxl, max)
>     {
>         spice_qxl_set_device_info(qxl, NULL, 0, max);
>     }

I can't actually do this, it does the wrong thing for the one-channel-
per-head (virtio-gpu) case. For that case it would send all
device_display_ids to 0 on all interfaces, but they need to be
different numbers.

For backwards compatibility when spice_qxl_set_device_info() is not
called I think we need to fallback to the old behaviour, as we are
missing the necessary information.

I'll send v3 with a single function using start + count and deprecate
the set_max_monitors one.

Cheers,
Lukas

> cheers,
>   Gerd
>