Re: [PATCH v2 5/5] drm/tegra: Implement page-flipping support

[Thierry Reding <thierry.reding-RM9K5IK7kjKj5M59NBduVrNAH6kLmebB@public.gmane.org>]

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On Tue, Jan 22, 2013 at 10:31:11AM +0200, Terje Bergström wrote:
> On 14.01.2013 18:06, Thierry Reding wrote:
> > +static int tegra_dc_page_flip(struct drm_crtc *crtc, struct drm_framebuffer *fb,
> > +			      struct drm_pending_vblank_event *event)
> > +{
> > +	struct tegra_framebuffer *newfb = to_tegra_fb(fb);
> > +	struct tegra_dc *dc = to_tegra_dc(crtc);
> > +	struct drm_device *drm = crtc->dev;
> > +	unsigned long flags;
> > +
> > +	if (dc->event)
> > +		return -EBUSY;
> > +
> > +	tegra_dc_set_base(dc, 0, 0, newfb);
> > +
> > +	if (event) {
> > +		event->pipe = dc->pipe;
> > +
> > +		spin_lock_irqsave(&drm->event_lock, flags);
> > +		dc->event = event;
> > +		spin_unlock_irqrestore(&drm->event_lock, flags);
> > +
> > +		drm_vblank_get(drm, dc->pipe);
> > +	}
> > +
> > +	return 0;
> > +}
> 
> The patch seems fine to me. I have a question for a follow-up.
> 
> In downstream dc driver we initiate a page flip, and assign a fence
> (syncpt id, value) to it. We return the fence to user space. Then when
> the actual page flip is done, dc increments the sync point.
> 
> User space can take the fence and use it for synchronizing graphics
> operations. In downstream, we use that fence to be able to submit
> operations to graphics units and synchronize them to the flip by adding
> a host wait. It improves performance, because we can prepare and send
> the graphics operations to hardware while flip is still happening.
> 
> Is this something we could do in tegra-drm, too? DRM's page flip IOCTL
> doesn't seem to have a way to pass a fence back from fence, so we'd need
> to find a way to pass the fence back to user space.
> 
> Of course, this has the prerequisite of having support for syncpts,
> which I hoped we would get to 3.9. The review for host1x patches seem to
> have stalled, so that's iffy.

Yes, I haven't found as much time as I would have liked to look at the
latest patches. Perhaps there will be a time slot at the end of the
week. I thought there was also the remaining issue with the memory
allocator that Lucas (Cc'ed) was working on?

Thierry

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