Re: [PATCH v2 1/4] drm/atomic: implement drm_atomic_helper_commit_tail for runtime_pm users

[Laurent Pinchart]

Hi Liviu,

On Wednesday 02 Aug 2017 14:32:06 Liviu Dudau wrote:
> On Wed, Aug 02, 2017 at 04:27:27PM +0300, Laurent Pinchart wrote:
> > On Wednesday 02 Aug 2017 13:46:48 Liviu Dudau wrote:
> >> On Wed, Aug 02, 2017 at 01:27:23PM +0200, Daniel Vetter wrote:
> >>> On Wed, Aug 2, 2017 at 1:20 PM, Liviu Dudau wrote:
> >>>> On Thu, Jul 20, 2017 at 03:01:16PM +0200, Maxime Ripard wrote:
> >>>>> +/**
> >>>>> + * drm_atomic_helper_commit_tail_rpm - commit atomic update to
> >>>>> hardware
> >>>>> + * @old_state: new modeset state to be committed
> >>>>> + *
> >>>>> + * This is an alternative implementation for the
> >>>>> + * &drm_mode_config_helper_funcs.atomic_commit_tail hook, for
> >>>>> drivers
> >>>>> + * that support runtime_pm or need the CRTC to be enabled to
> >>>>> perform a
> >>>>> + * commit. Otherwise, one should use the default implementation
> >>>>> + * drm_atomic_helper_commit_tail().
> >>>>> + */
> >>>>> +void drm_atomic_helper_commit_tail_rpm(struct drm_atomic_state
> >>>>> *old_state)
> >>>>> +{
> >>>>> +     struct drm_device *dev = old_state->dev;
> >>>>> +
> >>>>> +     drm_atomic_helper_commit_modeset_disables(dev, old_state);
> >>>>> +
> >>>>> +     drm_atomic_helper_commit_modeset_enables(dev, old_state);
> >>>>> +
> >>>>> +     drm_atomic_helper_commit_planes(dev, old_state,
> >>>>> +                                     DRM_PLANE_COMMIT_ACTIVE_ONLY);
> >>>>> +
> >>>>> +     drm_atomic_helper_commit_hw_done(old_state);
> >>>>> +
> >>>>> +     drm_atomic_helper_wait_for_vblanks(dev, old_state);
> >>>>> +
> >>>>> +     drm_atomic_helper_cleanup_planes(dev, old_state);
> >>>>> +}
> >>>>> +EXPORT_SYMBOL(drm_atomic_helper_commit_tail_rpm);
> >>>>> +
> >>>> 
> >>>> Given that this function is supposed to be used by runtime PM aware
> >>>> drivers and that the hook is called from the DRM core, should there
> >>>> not be some pm_runtime_{get,put} calls wrapping the body of this
> >>>> function?
> >> 
> >> Hi Daniel,
> >> 
> >>> No, because the drm atomic helpers have no idea which device is
> >>> backing which part of the drm device. Some drivers might on have one
> >>> device for the entire driver, some one device for just the display
> >>> part (which might or might not match drm_device->dev). And many arm
> >>> drivers have a device for each block separately (and you need to call
> >>> rpm_get/put on each). And some something in-between (e.g. core device
> >>> + external encoders).
> >> 
> >> Hmm, I understand your point about this function not having to care
> >> about pm_runtime_{get,put}, but I do not agree with your view that if it
> >> wanted to care about it, it wouldn't be able to do the right thing
> >> because it doesn't have the right device. After all, this function is
> >> about handling the updates that this atomic commit is concerned about,
> >> so having the old_state->dev drm_device pointer and its associated device
> >> should be enough. Am I missing something?
> > 
> > In embedded system it's quite common for display hardware to be made of
> > multiple IP cores. Depending on the SoC you could have to manage runtime
> > PM at the CRTC level for instance. The DRM core doesn't know about that,
> > and sees a single device only.
> > 
> > I've expressed my doubts previously about the need for a RPM version of
> > drm_atomic_helper_commit_tail(), as the resulting order of CRTC
> > enable/disable and plane update operations can lead to corrupt frames
> > when enabling the CRTC. I had a commit tail implementation in the rcar-du
> > driver that was very similar to drm_atomic_helper_commit_tail_rpm(), and
> > had to move back to the standard order to fix the corrupt frame issue.
> > The result isn't as clean as I would like, as power handling is split
> > between the CRTC enable/disable and the atomic begin/flush in a way that
> > is not straightforward.
> > 
> > It now occurred to me that a simpler implementation could be possible.
> > I'll have to experiment with it first, but the idea is as follows.
> > 
> > 1 Call pm_runtime_get_sync() at the beginning of .commit_tail() and
> > pm_runtime_put() at the end.
> > 
> > 2. Use the standard CRTC disable, plane update, CRTC enable order in
> > .commit_tail().
> > 
> > 3. Call pm_runtime_get() in the CRTC .enable() handler and
> > pm_runtime_put() in the CRTC .disable() handler;
> 
> Well, that is what mali-dp driver currently does, but according to Daniel
> (and I can see his POV) that is wrong.

Is it ? I might not have understood his arguments the same way (or possibly 
failed to even see them). Are you referring to this comments in this mail 
thread, or to something else ?

> I'm playing with removing all of that to see if there are any side effects
> in Mali DP like the ones you mentioned for RCAR.

Note that the first frame will usually not be noticed as it often takes a few 
frames for the display to turn on.

> > This should guarantee that the device won't be suspended between CRTC
> > disable and CRTC enable during a mode set, without requiring any special
> > collaboration between CRTC enable/disable and atomic begin/flush to
> > handle runtime PM. If drivers implement this, there should be no need for
> > drm_atomic_helper_commit_tail_rpm().
> > 
> > Maxime, Daniel, what do you think about this ?
> > 
> >>> I don't think there's anything the helpers can to to help support
> >>> this.
> >>> 
> >>> Also, just wrapping functions with rpm_get/put only papers over bugs
> >>> in your driver - either you enable something, then the rpm_get needs
> >>> to be done anyway (since the hw will be shut down otherwise), or you
> >>> disable something, same reasons. The only thing a rpm_get/put does is
> >>> paper over the bugs where you try to access the hw when it's off. As
> >>> soon as this function finishes, the hw is shut down again, drops all
> >> register values on the floor, so either that access wasn't needed, or
> >>> your driver has a bug (because it assumes the register values survive
> >>> when they don't).
> >>> 
> >>> So imo all around a bad idea, at least from my experience of doing rpm
> >>> enabling on i915 hw.
> >> 
> >> Understood. Thanks!

-- 
Regards,

Laurent Pinchart