Best practice device tree design for display subsystems/DRM

Tue Jul 2 23:55:59 PDT 2013

On Wed, Jul 03, 2013 at 08:02:05AM +1000, Dave Airlie wrote:
> On Wed, Jul 3, 2013 at 7:50 AM, Sascha Hauer <s.hauer at pengutronix.de> wrote:
> > On Tue, Jul 02, 2013 at 09:25:48PM +0100, Russell King wrote:
> >> On Tue, Jul 02, 2013 at 09:57:32PM +0200, Sebastian Hesselbarth wrote:
> >> > I am against a super node which contains lcd and dcon/ire nodes. You can
> >> > enable those devices on a per board basis. We add them to dove.dtsi but
> >> > disable them by default (status = "disabled").
> >> >
> >> > The DRM driver itself should get a video-card node outside of
> >> > soc/internal-regs where you can put e.g. video memory hole (or video
> >> > mem size if it will be taken from RAM later)
> >> >
> >> > About the unusual case, I guess we should try to get both lcd
> >> > controllers into one DRM driver. Then support mirror or screen
> >> > extension X already provides. For those applications where you want
> >> > X on one lcd and some other totally different video stream - wait
> >> > for someone to come up with a request or proposal.
> >>
> >> Well, all I can say then is that the onus is on those who want to treat
> >> the components as separate devices to come up with some foolproof way
> >> to solve this problem which doesn't involve making assumptions about
> >> the way that devices are probed and doesn't end up creating artificial
> >> restrictions on how the devices can be used - and doesn't end up burdening
> >> the common case with lots of useless complexity that they don't need.
> >>
> >> It's _that_ case which needs to come up with a proposal about how to
> >> handle it because you _can't_ handle it at the moment in any sane
> >> manner which meets the criteria I've set out above, and at the moment
> >> the best proposal by far to resolve that is the "super node" approach.
> >>
> >> There is _no_ way in the device model to combine several individual
> >> devices together into one logical device safely when the subsystem
> >> requires that there be a definite point where everything is known.
> >> That applies even more so with -EPROBE_DEFER.  With the presence of
> >> such a thing, there is now no logical point where any code can say
> >> definitively that the system has technically finished booting and all
> >> resources are known.
> >>
> >> That's the problem - if you don't od the super-node approach, you end
> >> up with lots of individual devices which you have to figure out some
> >> way of combining, and coping with missing ones which might not be
> >> available in the order you want them to be, etc.
> >>
> >> That's the advantage of the "super node" approach - it's a container
> >> to tell you what's required in order to complete the creation of the
> >> logical device, and you can parse the sub-nodes to locate the
> >> information you need.
> >
> > I think such an approach would lead to drm drivers which all parse their
> > "super nodes" themselves and driver authors would become very creative
> > how such a node should look like.
> >
> > Also this gets messy with i2c devices which are normally registered
> > under their i2c bus masters. With the super node approach these would
> > have to live under the super node, maybe with a phandle to the i2c bus
> > master. This again probably leads to very SoC specific solutions. It
> > also doesn't solve the problem that the i2c bus master needs to be
> > registered by the time the DRM driver probes.
> >
> > On i.MX the IPU unit not only handles the display path but also the
> > capture path. v4l2 begins to evolve an OF model in which each (sub)device
> > has its natural position in the devicetree; the devices are then
> > connected with phandles. I'm not sure how good this will work together
> > with a super node approach.
> >
> >>
> >> An alternative as I see it is that DRM - and not only DRM but also
> >> the DRM API and Xorg - would need to evolve hotplug support for the
> >> various parts of the display subsystem.  Do we have enough people
> >> with sufficient knowledge and willingness to be able to make all
> >> that happen?  I don't think we do, and I don't see that there's any
> >> funding out there to make such a project happen, which would make it
> >> a volunteer/spare time effort.
> >
> > +1 for this solution, even if this means more work to get from the
> > ground.
> >
> > Do we really need full hotplug support in the DRM API and Xorg? I mean
> > it would really be nice if Xorg detected a newly registered device, but
> > as a start it should be sufficient when Xorg detects what's there when
> > it starts, no?
> 
> Since fbdev and fbcon sit on top of drm to provide the console
> currently I'd also expect some fun with them. How do I get a console
> if I have no outputs at boot, but I have crtcs? do I just wait around
> until an output appears.

I thought the console/fb stuff should go away.

> 
> There are a number of issues with hotplugging encoders and connectors
> at runtime, when really the SoC/board designer knows what it provides
> and should be able to tell the driver in some fashion.
> 
> The main problems when I played with hot adding eDP on Intel last
> time, are we have grouping of crtc/encoder/connectors for multi-seat
> future use, these groups need to be updated, and I think the other
> issue was updating the possible_crtcs/possible_clones stuff. In theory
> sending X a uevent will make it reload the list, and it mostly deals
> with device hotplug since 1.14 when I added the USB hotplug support.
> 
> I'm not saying this is a bad idea, but really it seems pointless where
> the hardware is pretty much hardcoded, that DT can't represent that
> and let the driver control the bring up ordering.

SoC hardware normally does not change during runtime, that's right.
That's why I don't want to have full hotplug support up to xorg, but
only a way of adding/removing crtcs, encoders and connectors on an
already registered DRM device. We already do this in the i.MX DRM driver
(see drivers/staging/imx-drm/imx-drm-core.c). I'm sure this is not
without problems, but I think it would be doable.

> 
> Have you also considered how suspend/resume works in such a place,
> where every driver is independent? The ChromeOS guys have bitched
> before about the exynos driver which is has lots of sub-drivers, how
> do you control the s/r ordering in a crazy system like that? I'd
> prefer a central driver, otherwise there is too many moving parts.

Composing a DRM device out of subdevices doesn't necessarily mean the
components should be suspended/resumed in arbitrary order. The DRM
device should always be suspended first (thus deactivating sub devices
as necessary and as done already) and resumed last.

Note that a super node approach does not solve this magically. We would
still have to make sure that the i2c bus masters on our SoC are suspended
after the DRM device.

Sascha

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