[RFC] Expanding drm_mode_modeinfo flags

[Sean Paul]

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


> So we are trying to find ways to represent the same resolution in both
> video/cmd modes.
> 
> Thanks and Regards,
> Jeykumar S.
> 
> > 
> > > 
> > > --
> > > Ville Syrjälä
> > > Intel
> 
> -- 
> Jeykumar S

-- 
Sean Paul, Software Engineer, Google / Chromium OS