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Mon, 21 Jan 2019 23:09:00 +0000 Received: from linux by shell.armlinux.org.uk with local (Exim 4.89) (envelope-from ) id 1gligd-0001D2-W3; Mon, 21 Jan 2019 23:09:00 +0000 Date: Mon, 21 Jan 2019 23:08:59 +0000 From: Russell King - ARM Linux admin To: Lubomir Rintel Cc: Rob Herring , Mark Rutland , dri-devel , devicetree@vger.kernel.org, "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 4/6] dt-bindings: display: armada: Add display subsystem binding Message-ID: <20190121230859.b6bfmxjcrk4duknr@e5254000004ec.dyn.armlinux.org.uk> References: <20190120172534.24617-1-lkundrak@v3.sk> <20190120172534.24617-5-lkundrak@v3.sk> <3191cdde84978adf98d200a9db6f3c18e0a46390.camel@v3.sk> <20190121175301.lremzw6dul7dyff4@e5254000004ec.dyn.armlinux.org.uk> <4c21727d9753d4c0de667566972f2229dc9f1304.camel@v3.sk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4c21727d9753d4c0de667566972f2229dc9f1304.camel@v3.sk> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 21, 2019 at 09:45:22PM +0100, Lubomir Rintel wrote: > On Mon, 2019-01-21 at 17:53 +0000, Russell King - ARM Linux admin > wrote: > > On Mon, Jan 21, 2019 at 10:07:11AM -0600, Rob Herring wrote: > > > On Mon, Jan 21, 2019 at 9:46 AM Lubomir Rintel wrote: > > > > On Mon, 2019-01-21 at 09:35 -0600, Rob Herring wrote: > > > > > On Sun, Jan 20, 2019 at 11:26 AM Lubomir Rintel wrote: > > > > > > The Marvell Armada DRM master device is a virtual device needed to list all > > > > > > nodes that comprise the graphics subsystem. > > > > > > > > > > > > Signed-off-by: Lubomir Rintel > > > > > > --- > > > > > > .../display/armada/marvell-armada-drm.txt | 24 +++++++++++++++++++ > > > > > > 1 file changed, 24 insertions(+) > > > > > > > > > > > > diff --git a/Documentation/devicetree/bindings/display/armada/marvell-armada-drm.txt b/Documentation/devicetree/bindings/display/armada/marvell-armada-drm.txt > > > > > > index de4cca9432c8..3dbfa8047f0b 100644 > > > > > > --- a/Documentation/devicetree/bindings/display/armada/marvell-armada-drm.txt > > > > > > +++ b/Documentation/devicetree/bindings/display/armada/marvell-armada-drm.txt > > > > > > @@ -1,3 +1,27 @@ > > > > > > +Marvell Armada DRM master device > > > > > > +================================ > > > > > > + > > > > > > +The Marvell Armada DRM master device is a virtual device needed to list all > > > > > > +nodes that comprise the graphics subsystem. > > > > > > + > > > > > > +Required properties: > > > > > > + > > > > > > + - compatible: value should be "marvell,dove-display-subsystem", > > > > > > + "marvell,armada-display-subsystem" > > > > > > + - ports: a list of phandles pointing to display interface ports of CRTC > > > > > > + devices > > > > > > + - memory-region: phandle to a node describing memory to be used for the > > > > > > + framebuffer > > > > > > + > > > > > > +Example: > > > > > > + > > > > > > + display-subsystem { > > > > > > + compatible = "marvell,dove-display-subsystem", > > > > > > + "marvell,armada-display-subsystem"; > > > > > > + memory-region = <&display_reserved>; > > > > > > + ports = <&lcd0_port>; > > > > > > > > > > If there is only one device, you don't need this virtual node. > > > > > > > > By "one device" you mean one LCD controller (CRTC)? > > > > > > Yes. > > > > How does that work (as far as the Linux implementation) ? I can't see > > a way that could work, while allowing the flexibility that Armada DRM > > allows (two completely independent LCD controllers as two separate DRM > > devices vs one DRM device containing both LCD controllers.) > > > > > > I suppose in the (single CRTC) example case, the display-subsystem node > > > > used to associate it with the memory region reserved for allocating the > > > > frame buffers from. Could that be done differently? > > > > > > Move memory-region to the LCD controller node. > > > > That doesn't work - it would appear in the wrong part of the driver. > > > > > > Also, if the node is indeed made optional, then it's going to > > > > complicate things on the DRM side. Currently the driver that binds to > > > > the node creates the DRM device once it sees all the components > > > > connected to the ports appear. If we loose it, then the LCD controller > > > > driver would somehow need to find out that it's alone and create the > > > > DRM device itself. > > > > > > DT is not the only way to create devices. The DRM driver can bind to > > > the LCDC node and then create a child CRTC device (or even multiple > > > ones for h/w with multiple pipelines). > > > > That seems completely upside down and rediculous to me - are you > > really suggesting that we should have some kind of virtual device > > in DT, and omit the _real_ physical devices for that, having the > > driver create the device with all the appropriate SoC resources? > > Hmm, that's not how I read that. My understanding (putting aside > practicality of the solution) is that Rob was merely suggesting that > for the single LCDC case there would be just a single LCDC node in DT > and the driver that binds to it would create the DRM device & CRTC > device pair. How would we know that was the case when the driver binds to the CRTC node? There is no back-link from the CRTC to the display-subsystem when there's a display-subsystem node present, so there's no way for the CRTC driver to know whether it should create the DRM device or not. I just can't see how this works at a technical level. > > > You'll also notice that there are only 3 cases of this virtual node in > > > the tree: STi, i.MX IPU, and Rockchip. That's because we've deprecated > > > doing these virtual nodes for some time now. IOW, there are several > > > examples of how to do this without a virtual node. > > > > This driver has been in-tree with this setup for some time, although > > the documentation has been missing (we actually have a _lot_ of > > instances of that.) However, we have no in-tree DT using it. > > > > I don't really see how to satisfy your comments without totally > > restructuring the driver, which is going to be quite a big chunk > > of work. I'm not sure I have the motivation to do that right now. > > Note that the initial objection was against the display-subsystem node > being mandatory if "there is only one [LCDC] device". > > My understanding is that need to include the display-subsystem node for > the multiple LCDC setup (on Dove platform) anyways. Is that correct? > > Rob, I'm wondering if there would be a possibility of finding some > middle groud? Perhaps documenting, that the display-subsystem node > would ideally be optional for single LCDC setups, but indicating that > the Armada DRM driver actually requires is? > > Note that this is not a new driver -- it has been around since 2013, > though, without useful DT bindings. Maybe it would do just well in > company of the other three drivers you mentioned that use similar > bindings. > > (Also, there seem to have substantial discussion regarding the bindings > design back in '13, shedding some light into why the display-subsystem > node was deemed useful: [1]) > > [1] https://www.mail-archive.com/dri-devel@lists.freedesktop.org/msg40358.html > > Thanks, > Lubo > > -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up