From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rob Herring Subject: Re: [PATCH 4/6] dt-bindings: display: armada: Add display subsystem binding Date: Mon, 21 Jan 2019 17:58:50 -0600 Message-ID: References: <20190120172534.24617-1-lkundrak@v3.sk> <20190120172534.24617-5-lkundrak@v3.sk> <3191cdde84978adf98d200a9db6f3c18e0a46390.camel@v3.sk> <20190121175301.lremzw6dul7dyff4@e5254000004ec.dyn.armlinux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: <20190121175301.lremzw6dul7dyff4@e5254000004ec.dyn.armlinux.org.uk> Sender: linux-kernel-owner@vger.kernel.org To: Russell King - ARM Linux admin Cc: Lubomir Rintel , Mark Rutland , dri-devel , devicetree@vger.kernel.org, "linux-kernel@vger.kernel.org" List-Id: dri-devel@lists.freedesktop.org On Mon, Jan 21, 2019 at 11:53 AM 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. Why? You can fetch properties from other nodes. If you have 2 CRTCs, do you have 1 or 2 reserved memory regions? I'd think 2 with each one in the corresponding LCDC that uses them would be more flexible. Or just get the data out of the /reserved-memory node directly. Surely it has a compatible that you can find it with. > > > 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? We create child platform devices that inherit from the parent in DT all the time. MFD child drivers are a common case. Sometime the child devices have DT nodes and sometimes they don't. Otherwise, do it the other way around. Create a virtual DRM device conditioned on the SoC: if (of_machine_is_compatible("foo,bar")) platform_device_register_simple(...) > > > 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. The current Armada DRM driver has no binding to DT at all, so no, it is not just missing documentation or a dts file. > 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. It's not a big chunk of work. Look at commit 246774d17fc0 ("drm/etnaviv: remove the need for a gpu-subsystem DT node") for an example. Rob