[RFC PATCH v4 3/8] staging: imx-drm: Document updated imx-drm device tree bindings

[Philipp Zabel]

Hi Tomi,

Am Donnerstag, den 27.02.2014, 15:55 +0200 schrieb Tomi Valkeinen:
> On 27/02/14 15:00, Russell King - ARM Linux wrote:
> > On Thu, Feb 27, 2014 at 02:06:25PM +0100, Philipp Zabel wrote:
> >> For the i.MX6 display subsystem there is no clear single master device,
> >> and the physical configuration changes across the SoC family. The
> >> i.MX6Q/i.MX6D SoCs have two separate display controller devices IPU1 and
> >> IPU2, with two output ports each.
> > 
> > Not also forgetting that there's another scenario too: you may wish
> > to drive IPU1 and IPU2 as two completely separate display subsystems
> > in some hardware, but as a combined display subsystem in others.
> > 
> > Here's another scenario.  You may have these two IPUs on the SoC, but
> > there's only one display output.  You want to leave the second IPU
> > disabled, as you wouldn't want it to be probed or even exposed to
> > userland.
> 
> I first want to say I don't see anything wrong with such a super node.
> As you say, it does describe hardware. But I also want to say that I
> still don't see a need for it. Or, maybe more exactly, I don't see a
> need for it in general. Maybe there are certain cases where two devices
> has to be controlled by a master device. Maybe this one is one of those.
> 
> In the imx case, why wouldn't this work, without any master node, with
> the IPU nodes separate in the DT data:
> 
> - One IPU enabled, one disabled: nothing special here, just set the
> other IPU to status="disabled" in the DT data. The driver for the
> enabled IPU would register the required DRM entities.

that should work. Let the enabled IPU create the imx-drm platform device
on probe, parse the device tree and ignore everything only hanging off
of the disabled IPU.

[Reordering a bit...]
>- Two IPUs in combined mode:
> 
> Pick one IPU as the master, and one as slave. Link the IPU nodes in DT
> data with phandles, say: master=<&ipu1> on the slave IPU and
> slave=<&ipu0> on the master.
> 
> The master one will register the DRM entities, and the slave one will
> just do what the master says.

That might work, too. Just let the each IPU scan the graph and try to
find the imx-drm master before creating the imx-drm platform device.
The first IPU fill find no preexisting master and create the imx-drm
platform device as above, adding the other IPU as well as the other
components with component_master_add_child. It just has to make sure
that the other IPU is added to the list before the encoders are.

The second IPU will scan the graph, find a preexisting master for the
other IPU node, register its component and just wait to be bound by the
master.

> - Two IPUs as separate units: almost the same as above, but both would
> independently register the DRM entities.

Here the second IPU would not be connected to the first IPU via the
graph - it would not find a preexisting imx-drm device when scanning its
graph and create its own imx-drm device just like the first IPU did.
As a result there are two completely separate DRM devices.

That being said, this change could be made at any time in the future,
in a backwards compatible fashion, by just declaring the imx-drm node
optional and ignoring it if it exists.

> As for the probe time "are we ready yet?" problem, the IPU driver can
> just delay registering the DRM entities until all the nodes in its graph
> have been probed. The component helpers can probably be used here.

This is what is happening right now, except that the two IPUs are not
obtained from the graph but are given as starting points via the ports
property in the imx-drm node.

> > On the face of it, the top-level super-device node doesn't look very
> > hardware-y, but it actually is - it's about how a board uses the
> > hardware provided.  This is entirely in keeping with the spirit of DT,
> > which is to describe what hardware is present and how it's connected
> > together, whether it be at the chip or board level.
> 
> No disagreement there. I'm mostly put off by the naming. The binding doc
> says it's a "DRM master device", compatible with "fsl,imx-drm". Now,
> naming may not be the most important thing in the world, but I'd rather
> use generic terms, not linux driver stack names.

Did anybody propose such a generic term? How about:

-imx-drm {
-	compatible = "fsl,imx-drm";
-	ports = <&ipu1_di0>, <&ipu1_di1>;
-};
+display-subsystem {
+	compatible = "fsl,imx-display-subsystem";
+	ports = <&ipu1_di0>, <&ipu1_di1>;
+};

> > If this wasn't the case, we wouldn't even attempt to describe what devices
> > we have on which I2C buses - we'd just list the hardware on the board
> > without giving any information about how it's wired together.
> > 
> > This is no different - however, it doesn't have (and shouldn't) be
> > subsystem specific... but - and this is the challenge we then face - how
> > do you decide that on one board with a single zImage kernel, with both
> > DRM and fbdev built-in, whether to use the DRM interfaces or the fbdev
> > interfaces?  We could have both matching the same compatible string, but
> > we'd also need some way to tell each other that they're not allowed to
> > bind.
> 
> Yes, that's an annoying problem, we have that on OMAP. It's a clear sign
> that our video support is rather messed up.
> 
> My opinion is that the fbdev and drm drivers for a single hardware
> should be exclusive at compile time. We don't allow multiple drivers for
> single device for other subsystems either, do we? Eventually we should
> have only one driver for one hardware device.
> 
> If that's not possible, then the drivers in question could have an
> option to enable or disable themselves, passed via the kernel command
> line, so that the user can select which subsystem to use.

That is the exact same problem as having multiple drivers that can bind
to the same device.

> > Before anyone argues against "it isn't hardware-y", stop and think.
> > What if I design a board with two Epson LCD controllers on board and
> > put a muxing arrangement on their output.  Is that one or two devices?
> > What if I want them to operate as one combined system?  What if I have
> > two different LCD controllers on a board.  How is this any different
> > from the two independent IPU hardware blocks integrated inside an iMX6
> > SoC with a muxing arrangement on their output?
> 
> Well, generally speaking, I think one option is to treat the two
> controllers separately and let the userspace handle it. That may or may
> not be viable, depending on the hardware, but to me it resembles very
> much a PC with two video cards.

And two graphics cards connected to the same output with a multiplexer
are a mess. This only works well if it is tightly integrated.

[...]
> So no, I don't have a problem with master device nodes in DT. I have a
> problem having pure SW stack nomenclature in the DT data (or even worse,
> SW stack entities in the DT data), and I have a problem requiring
> everyone to have a master device node if it's only needed for special cases.
>
> And yes, this series is about IMX bindings, not generic ones. And I'm
> also fine with requiring everyone to have a master device node, if it
> can be shown that it's the only sensible approach.

For i.MX, for now, let's keep the mandatory imx-drm node. Maybe rename
it so nobody can say we are leaking linux subsystem names into the
device tree.

regards
Philipp