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

[Pekka Paalanen <ppaalanen@gmail.com>]

On Sun, 19 Jul 2015 17:20:32 -0700
Stéphane Marchesin <stephane.marchesin@gmail.com> wrote:

> On Thu, Jul 16, 2015 at 11:08 PM, Pekka Paalanen <ppaalanen@gmail.com> wrote:
> >
> > On Thu, 16 Jul 2015 20:20:39 +0800
> > John Hunter <zhjwpku@gmail.com> wrote:
> >
> > > From: Zhao Junwang <zhjwpku@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. 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, if the driver cannot implement proper semantics (which IMO
includes the throttling) for vblank-sync'd operations and it does not
want to fake them with a clock, it should just refuse vblank-synced
operations. That would push the problem to userspace, and it would be
obvious what's going wrong. Naturally, it would break some userspace
programs that expect vblank-synced operations to work, but is that
so much different to the current unfixed situation?


Thanks,
pq
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