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Wed, 12 Jun 2024 08:11:09 -0700 (PDT) Received: from phenom.ffwll.local ([2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42286fe919bsm28783255e9.19.2024.06.12.08.11.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Jun 2024 08:11:09 -0700 (PDT) Date: Wed, 12 Jun 2024 17:11:07 +0200 From: Daniel Vetter To: Doug Anderson Cc: dri-devel@lists.freedesktop.org, Neil Armstrong , Maxime Ripard , Linus Walleij , Yuran Pereira , Chris Morgan , David Airlie , Jessica Zhang , Maarten Lankhorst , Thomas Zimmermann , linux-kernel@vger.kernel.org, Daniel Vetter Subject: Re: [PATCH] drm/panel: Avoid warnings w/ panel-simple/panel-edp at shutdown Message-ID: Mail-Followup-To: Doug Anderson , dri-devel@lists.freedesktop.org, Neil Armstrong , Maxime Ripard , Linus Walleij , Yuran Pereira , Chris Morgan , David Airlie , Jessica Zhang , Maarten Lankhorst , Thomas Zimmermann , linux-kernel@vger.kernel.org References: <20240611074846.1.Ieb287c2c3ee3f6d3b0d5f49b29f746b93621749c@changeid> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Operating-System: Linux phenom 6.8.9-amd64 On Wed, Jun 12, 2024 at 07:49:31AM -0700, Doug Anderson wrote: > Hi, > > On Wed, Jun 12, 2024 at 1:58 AM Daniel Vetter wrote: > > > > On Tue, Jun 11, 2024 at 07:48:51AM -0700, Douglas Anderson wrote: > > > At shutdown if you've got a _properly_ coded DRM modeset driver then > > > you'll get these two warnings at shutdown time: > > > > > > Skipping disable of already disabled panel > > > Skipping unprepare of already unprepared panel > > > > > > These warnings are ugly and sound concerning, but they're actually a > > > sign of a properly working system. That's not great. > > > > > > It's not easy to get rid of these warnings. Until we know that all DRM > > > modeset drivers used with panel-simple and panel-edp are properly > > > calling drm_atomic_helper_shutdown() or drm_helper_force_disable_all() > > > then the panel drivers _need_ to disable/unprepare themselves in order > > > to power off the panel cleanly. However, there are lots of DRM modeset > > > drivers used with panel-edp and panel-simple and it's hard to know > > > when we've got them all. Since the warning happens only on the drivers > > > that _are_ updated there's nothing to encourage broken DRM modeset > > > drivers to get fixed. > > > > > > In order to flip the warning to the proper place, we need to know > > > which modeset drivers are going to shutdown properly. Though ugly, do > > > this by creating a list of everyone that shuts down properly. This > > > allows us to generate a warning for the correct case and also lets us > > > get rid of the warning for drivers that are shutting down properly. > > > > > > Maintaining this list is ugly, but the idea is that it's only short > > > term. Once everyone is converted we can delete the list and call it > > > done. The list is ugly enough and adding to it is annoying enough that > > > people should push to make this happen. > > > > > > Implement this all in a shared "header" file included by the two panel > > > drivers that need it. This avoids us adding an new exports while still > > > allowing the panel drivers to be modules. The code waste should be > > > small and, as per above, the whole solution is temporary. > > > > > > Signed-off-by: Douglas Anderson > > > --- > > > I came up with this idea to help us move forward since otherwise I > > > couldn't see how we were ever going to fix panel-simple and panel-edp > > > since they're used by so many DRM Modeset drivers. It's a bit ugly but > > > I don't hate it. What do others think? > > > > I think it's terrible :-) > > Well, we're in agreement. It is terrible. However, in my opinion it's > still a reasonable way to move forward. > > > > Why does something like this now work? > > > > drm_panel_shutdown_fixup(panel) > > { > > /* if you get warnings here, fix your main drm driver to call > > * drm_atomic_helper_shutdown() > > */ > > if (WARN_ON(panel->enabled)) > > drm_panel_disable(panel); > > > > if (WARN_ON(panel->prepared)) > > drm_panel_unprepare(panel); > > } > > > > And then call that little helper in the relevant panel drivers? Also feel > > free to bikeshed the name and maybe put a more lengthly explainer into the > > kerneldoc for that ... > > > > Or am I completely missing the point here? > > The problem is that the ordering is wrong, I think. Even if the OS was > calling driver shutdown functions in the perfect order (which I'm not > convinced about since panels aren't always child "struct device"s of > the DRM device), the OS should be calling panel shutdown _before_ > shutting down the DRM device. That means that with your suggestion: > > 1. Shutdown starts and panel is on. > > 2. OS calls panel shutdown call, which prints warnings because panel > is still on. > > 3. OS calls DRM driver shutdown call, which prints warnings because > someone else turned the panel off. Uh, that's a _much_ more fundamental issue. The fix for that is telling the driver core about this dependency with device_link_add. Unfortuantely, despite years of me trying to push for this, drm_bridge and drm_panel still don't automatically add these, because the situation is a really complex mess. Probably need to read dri-devel archives for all the past attempts around device_link_add. But the solution is definitely not to have a manually tracked list, what's very architectural unsound way to tackle this problem. > Certainly if I goofed and the above is wrong then let me know--I did > my experiments on this many months ago and didn't try repeating them > again now. Oh the issue is very real and known since years. It also wreaks module unload and driver unbinding, since currently nothing makes sure your drm_panel lives longer than your drm_device. > In any case, the only way I could figure out around this was some sort > of list. As mentioned in the commit message, it's super ugly and > intended to be temporary. Once we solve all the current in-tree > drivers, I'd imagine that long term for new DRM drivers this kind of > thing would be discovered during bringup of new boards. Usually during > bringup of new boards EEs measure timing signals and complain if > they're not right. If some EE cared and said we weren't disabling the > panel correctly at shutdown time then we'd know there was a problem. You've stepped into an entire hornets nest with this device dependency issue unfortunately, I'm afraid :-/ Cheers, Sima -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch