KMS timings (Re: [PATCH 6/8] drm/bochs: phase 3: provide a custom ->atomic_commit implementation)

[Pekka Paalanen]

On Tue, 21 Jul 2015 11:02:58 +0200
Daniel Vetter <daniel at ffwll.ch> wrote:

> On Tue, Jul 21, 2015 at 10:06:09AM +0300, Pekka Paalanen wrote:
> > On Mon, 20 Jul 2015 10:32:31 -0700
> > Stéphane Marchesin <stephane.marchesin at gmail.com> wrote:
> > 
> > > On Mon, Jul 20, 2015 at 7:21 AM, Daniel Vetter <daniel at ffwll.ch> wrote:
> > > > On Mon, Jul 20, 2015 at 12:35:48PM +0300, Pekka Paalanen wrote:
> > > >> On Mon, 20 Jul 2015 01:58:33 -0700
> > > >> Stéphane Marchesin <stephane.marchesin at gmail.com> wrote:
> > > >>
> > > >> > On Mon, Jul 20, 2015 at 12:46 AM, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> > > >> > > On Sun, 19 Jul 2015 17:20:32 -0700
> > > >> > > Stéphane Marchesin <stephane.marchesin at gmail.com> wrote:
> > > >> > >
> > > >> > >> On Thu, Jul 16, 2015 at 11:08 PM, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> > > >> > >> >
> > > >> > >> > On Thu, 16 Jul 2015 20:20:39 +0800
> > > >> > >> > John Hunter <zhjwpku at gmail.com> wrote:
> > > >> > >> >
> > > >> > >> > > From: Zhao Junwang <zhjwpku at gmail.com>
> > > >> > >> > >
> > > >> > >> > > This supports the asynchronous commits, required for page-flipping
> > > >> > >> > > Since it's virtual hw it's ok to commit async stuff right away, we
> > > >> > >> > > never have to wait for vblank.
> > > >> > >> >
> > > >> > >> > Hi,
> > > >> > >> >
> > > >> > >> > in theory, yes. This is what a patch to bochs implemented not too long
> > > >> > >> > ago, so AFAIK you are only replicating the existing behaviour.
> > > >> > >> >
> > > >> > >> > However, if userspace doing an async commit (or sync, I suppose) does
> > > >> > >> > not incur any waits in the kernel in e.g. sending the page flip event,
> > > >> > >> > then flip driven programs (e.g. a Wayland compositor, say, Weston)
> > > >> > >> > will be running its rendering loop as a busy-loop, because the kernel
> > > >> > >> > does not throttle it to the (virtual) display refresh rate.
> > > >> > >> >
> > > >> > >> > This will cause maximal CPU usage and poor user experience as
> > > >> > >> > everything else needs to fight for CPU time and event dispatch to get
> > > >> > >> > through, like input.
> > > >> > >> >
> > > >> > >> > I would hope someone could do a follow-up to implement a refresh cycle
> > > >> > >> > emulation based on a clock. Userspace expects page flips to happen at
> > > >> > >> > most at refresh rate when asking for vblank-synced flips. It's only
> > > >> > >> > natural for userspace to drive its rendering loop based on the vblank
> > > >> > >> > cycle.
> > > >> > >>
> > > >> > >>
> > > >> > >> I've been asking myself the same question (for the UDL driver) and I'm
> > > >> > >> not sure if this policy should go in the kernel. After all, there
> > > >> > >> could be legitimate reasons for user space to render lots of frames
> > > >> > >> per second. It seems to me that if user space doesn't want too many
> > > >> > >> fps, it should just throttle itself.
> > > >> > >
> > > >> > > If userspace wants to render lots of frames per second, IMO it should
> > > >> > > not be using vblank-synced operations in a way that may throttle it.
> > > >> > > The lots of frames use case is already non-working for the majority of
> > > >> > > the drivers without DRM_MODE_PAGE_FLIP_ASYNC, right?
> > > >> > >
> > > >> > > The problem here I see is that one DRM driver decides to work different
> > > >> > > to other DRM drivers. All real-hardware DRM drivers, when asked to do
> > > >> > > vblank-synced update, actually do throttle to the vblank AFAIK.
> > > >> >
> > > >> > udl is an exception here. It is (arguably) real hardware but doesn't throttle.
> > > >> >
> > > >> > > Is it
> > > >> > > too much to assume, that the video mode set in a driver (refresh rate)
> > > >> > > corresponds to the vblank rate which implicitly delays the completion
> > > >> > > of vblank-sync'd operations to at least the next vblank boundary?
> > > >> >
> > > >> > I think it's wrong to make user space think that a vsynced display
> > > >> > always matches the refresh rate in a world where:
> > > >> >
> > > >> > - some displays have variable refresh rates (not just the fancy new
> > > >> > stuff like g-sync, look for lvds_downclock in the intel driver for
> > > >> > example, also consider DSI displays)
> > > >> >
> > > >> > - some displays have no refresh rate (the ones we are talking about
> > > >> > here: udl, bochs...)
> > 
> > > > Imo aiming for vrefresh to be accurate is good. For gsync and friends I
> > > > think we should have an explicit range or flag to make userspace aware of
> > > > what's going on.
> > > 
> > > I think the concept of vrefresh is flawed and not really future-proof
> > > (I gave a few examples in my previous email). I agree we should keep
> > > it as legacy, but we should add something else for the more advanced
> > > cases.
> > 
> > Right, so let's add something new for new hardware features and keep
> > the existing behavior existing.
> > 
> > I suppose the problem is that the existing behavior is not really
> > documented so we have to resort to screaming users?
> > 
> > If one does not ask for ASYNC with a page flip, does it mean flipping
> > on the next vblank, or flipping such that it cannot tear but allowing
> > techiques like scanline waits?
> 
> Since legacy page_flip is always for the full primary plane you can't do
> scanline waits - it covers everything anyway.

Ah, nice. Obvious in hindsight, at least if the display server does
not mark dirty regions.

But there seems to be drmModeDirtyFB(), can that not be used with page
flip? If userspace provided dirty regions, would scanline-wait be
possible in theory? If yes, would it be acceptable for drivers to do?

> > Stéphane also raised the concern that scanout downclocking etc. may
> > cause flips to be quite late from predicted. This is less of a problem,
> > rendering can also take arbitrary times and software is usually written
> > to deal with missing deadlines. It's a problem that happens all the
> > time anyway. As it is a problem with prediction accuracy, we could just
> > wave it off by setting the imaginary "no constant refresh rate" flag.
> > IMHO this is a problem that should be solved at another occasion, if
> > necessary.
> 
> Downclocking should only be able to delay the very first frame in a
> sequence (video playback, animation). Imo we can shrug that off as "clock
> skew" ;-)

Indeed, and at least Weston has special repaint loop start logic to
deal with that.

> Imo once someone updates frames regularly vblank timestamps should be
> evenly spaced and pageflips not instant (at least by default).

Glad to hear. :-)

> > > Yeah I don't think I care about the old interface, it is what it is.
> > > But we should design something which works well for the future use
> > > cases.
> > 
> > Are you referring to the atomic API as the old thing?
> > 
> > In the end, the question is how paranoid should display servers be
> > about the timings? Do we need to fix Weston, Mutter and likely others
> > to deal with page flips being signalled immediately, or is it more
> > appropriate to fix the DRM drivers to keep up the illusion of providing
> > what the API appears to suggest?
> 
> I guess they can't assume too much about vblanks (too many drivers don't
> even bother with precise/irq-delay-correct timestamps), but I think
> assuming that doing a page_flip completion event based renderer shouldn't
> result in spinning is reasonable.

Right. Reliability of timestamps is a whole another can.

> I guess for bochs/udl and others we could create a small drm driver which
> keeps track of the last vblank ts (we have those already) and suitable
> delays the even/timestamp to keep up the illusion. Or we just rip out
> pageflip support for those drivers. But if weston&co can't cope with that
> that would be worse.

Now that you ask, I'm not even really sure what the non-page-flip KMS
display update way is, so no, I would say Weston cannot deal with it as
is.

Who's up for the job? :-)


Thanks,
pq