Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752242AbdHBNtF (ORCPT ); Wed, 2 Aug 2017 09:49:05 -0400 Received: from galahad.ideasonboard.com ([185.26.127.97]:52068 "EHLO galahad.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751918AbdHBNtE (ORCPT ); Wed, 2 Aug 2017 09:49:04 -0400 From: Laurent Pinchart To: Liviu Dudau Cc: Daniel Vetter , Maxime Ripard , linux-samsung-soc , dri-devel , Seung-Woo Kim , Chen-Yu Tsai , Linux Kernel Mailing List , "open list:ARM/Rockchip SoC..." , Kyungmin Park , Kukjin Kim , Krzysztof Kozlowski , Daniel Vetter , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH v2 1/4] drm/atomic: implement drm_atomic_helper_commit_tail for runtime_pm users Date: Wed, 02 Aug 2017 16:49:16 +0300 Message-ID: <1828962.5J4WLQuEBS@avalon> User-Agent: KMail/4.14.10 (Linux/4.9.34-gentoo; KDE/4.14.32; x86_64; ; ) In-Reply-To: <20170802133206.GQ970@e110455-lin.cambridge.arm.com> References: <3036323.BRqfrJhm0Z@avalon> <20170802133206.GQ970@e110455-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6169 Lines: 135 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