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 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...
> 
> That said, I don't find it significantly worse than the other options
> (none of which seems great), so I'd just like we reached some consesus
> and be done with it...
> 
> Cheers,
> Lukas
> 
> > cheers,
> >   Gerd
> > 

Personally I prefer the single function API, either with or without count.

About the not trivial for compatibility you'll have to support Qemu versions
that either:
- does not call new APIs or spice_qxl_set_max_monitors. A QXL card will have
  the maximum possible number of monitors in the guest;
- does not call new APIs but call spice_qxl_set_max_monitors. A QXL card will
  have a maximum of monitor specified by spice_qxl_set_max_monitors in the
  guest;
- call new APIs (the choice in this case to call spice_qxl_set_max_monitors
  or not is yours).

Frediano