[PATCH 1/2] drm/hyperv: Add DRM driver for hyperv synthetic video device

[Deepak Rawat]

On Tue, 2021-01-05 at 12:30 +0100, Thomas Zimmermann wrote:
> Hi
> 
> Am 05.01.21 um 12:04 schrieb Gerd Hoffmann:
> >    Hi,
> > 
> > > > It's not possible to do page flip with this virtual device. The
> > > > call to
> > > > SYNTHVID_VRAM_LOCATION is only honoured once. So unfortunately
> > > > need to
> > > > use SHMEM helpers.
> > > 
> > > I was thinking about using struct
> > > video_output_situation.vram_offset; in
> > > case you want to tinker with that. There's a comment in the patch
> > > that
> > > vram_offset should always be 0. But this comment seems incorrect
> > > for devices
> > > with more than one output.
> > 
> > Where does the comment come from?  fbdev drivers support a single
> > framebuffer only so for a fbdev driver it makes sense to set the
> > offset
> > to 0 unconditionally.  With drm you probably can handle things
> > differently ...
> 
> I cannot find it in hyperv_fb.c; it must have gotten introduced here.
> 
> Only one display is supported by this DRM driver, so the comment is 
> correct. In the future, having support for multiple displays might be
> an 
> option.
> 
> 

Beside that offset should be 0, another problem is that
SYNTHVID_SITUATION_UPDATE will cause the whole screen to flicker (I
suppose this call is more for changing resolution), so can't really use
this for page-flip.