On Mon, Jul 22, 2019 at 04:50:43PM -0700, Jeykumar Sankaran wrote:
On 2019-07-19 07:29, Sean Paul wrote:
> On Fri, Jul 19, 2019 at 05:15:28PM +0300, Ville Syrjälä wrote:
> > On Fri, Jul 19, 2019 at 09:55:58AM -0400, Sean Paul wrote:
> > > On Fri, Jul 19, 2019 at 11:05:53AM +0200, Daniel Vetter wrote:
> > > > On Thu, Jul 18, 2019 at 11:18:42AM -0700, Jeykumar Sankaran
wrote:
> > > > > On 2019-07-16 02:07, Daniel Vetter wrote:
> > > > > > On Thu, Jul 11, 2019 at 11:46:44AM -0700, Jeykumar Sankaran
wrote:
>
> /snip
>
> > > > > > > drm: add mode flags in uapi for seamless mode switch
> > > > > >
> > > > > > I think the uapi is the trivial part here, the real deal is
how
> > > > > > userspace
> > > > > > uses this. Can you pls post the patches for your compositor?
> > > > > >
> > > > > > Also note that we already allow userspace to tell the kernel
whether
> > > > > > flickering is ok or not for a modeset. msm driver could use
that to at
> > > > > > least tell userspace whether a modeset change is possible.
So you can
> > > > > > already implement glitch-free modeset changes for at least
video mode.
> > > > > > -Daniel
> > > > >
> > > > > I believe you are referring to the below tv property of the
connector.
> > > > >
> > > > > /**
> > > > > * @tv_flicker_reduction_property: Optional TV property to
control the
> > > > > * flicker reduction mode.
> > > > > */
> > > > > struct drm_property *tv_flicker_reduction_property;
> > > >
> > > > Not even close :-)
> > > >
> > > > I mean the DRM_MODE_ATOMIC_ALLOW_MODESET flag for the atomic
ioctl. This
> > > > is not a property of a mode, this is a property of a
_transition_ between
> > > > configurations. Some transitions can be done flicker free,
others can't.
> > >
> > > Agree that an atomic flag on a commit is the way to accomplish
this. It's pretty
> > > similar to the psr transitions, where we want to reuse most of the
atomic
> > > circuitry, but in a specialized way. We'd also have to be careful
to only
> > > involve the drm objects which are seamless modeset aware (you
could imagine
> > > a bridge chain where the bridges downstream of the first bridge
don't care).
> > >
> > > >
> > > > There's then still the question of how to pick video vs command
mode, but
> > > > imo better to start with implementing the transition behaviour
correctly
> > > > first.
> > >
> > > Connector property? Possibly a terrible idea, but I wonder if we
could [re]use
> > > the vrr properties for command mode. The docs state that the
driver has the
> > > option of putting upper and lower bounds on the refresh rate.
> >
> > Not really sure why this needs new props and whatnot. This is kinda
> > what
> > the i915 "fastset" stuff already does:
> > 1. userspace asks for something to be changed via atomic
> > 2. driver calculates whether a modeset is actually required
> > 3. atomic validates need_modeset() vs. DRM_MODE_ATOMIC_ALLOW_MODESET
> > 4. if (need_modeset) heavyweight_commit() else lightweight_commit()
> >
> > Ie. why should userspace really care about anything except the
> > "flickers are OK" vs. "flickers not wanted" thing?
>
> Agree, I don't think the seamless modeset (ie: changing resolution
> without
> flicker) needs a property. Just need to test the commit without
> ALLOW_MODESET
> and commit it if the test passes.
>
Agreed that a TEST_ONLY commit without ALLOW_MODESET flag can be used
to
check
whether the modeset can be done seamless. But since there are no error
code
returns,
the client cannot distinguish the test_only commit failures from other
invalid config failures.
Also, note that when the client sees two 1080p modes (vid/cmd) and it
is
interested only
to do *only* seamless switches, without any additional flag it cannot
distinguish between
these two 1080p modes. The client has to invoke two test_only commits
with
these
modes to identify the seamless one. Is that a preferred approach?
Hi Jey!
Yeah, pretty much. Stepping back a bit though, why is the kernel
exposing
2
1080p modes in the first place? If you just expose one mode and then
use a
property to enter "low-latency operation" (or overloading vrr for cmd
mode), you
shouldn't need to do the mode switch, just flip the property and let
the
kernel
figure out how to transition to video/cmd mode.
Intel's "fastset" calculates the need for modeset internally based on
the
configuration and chooses the best commit path. But the requirement
here
is to expose the information up-front since the use case cannot afford
to fall back to the normal modeset when it has requested for a
seamless
one.
> >
> > Also what's the benefit of using video mode if your panel supportes
> > command mode? Can you turn off the memory in the panel and actually
> > save power that way? And if there is a benefit can't the driver just
> > automagically switch between the two based on how often things are
> > getting updated? That would match how eDP PSR already works.
>
> I'm guessing video mode might have some latency benefits over command
> mode?
>
> Sean
Yes. Pretty much those are reasons we need to switch to video mode.
But
instead
of making the decision internal to the driver based on the frequency
of
frame updates,
we have proprietary use cases where the client has to trigger the
switch
explicitly.
Unsolicited advice: if you find yourself typing "proprietary" while
justifying
an upstream proposal, reword your argument immediately :-)
Now that I've filled my awful joke quota for the mail, I can move on.
Generally
speaking, it's better to you expand on why userspace needs this.
There's a
very
good chance that if it makes sense for Qualcomm, it makes sense for
others. If
we have an open discussion about it, others might chime in with "me
too"
and
that will help your case (and you might even hook someone into doing
the
work
for you!). Alternatively, we might be able to solve the problem in a
different
way that makes everyone happy (or most everyone happy).
There's 100% a case for allowing userspace to trigger a low-latency
mode
at the
expense of power (we should have it disable the psr entry timer as
well).
Let's
steer this conversation in the direction of "can we use something that
already
exists, or should we add something new?"
Sean
