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

[Frediano Ziglio <fziglio@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.
> 

This function is only called for QXL cards devices so no virtio-gpu,
no cirrus, no svga or anything else.

> 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
> > 
> 

Frediano