Re: outcome of DRM/KMS DT bindings session

On Tue, Oct 29, 2013 at 01:52:57PM +1000, Dave Airlie wrote:

So we had a sessions at kernel summit to discuss the driver model and
DT interactions for a display pipeline,

we had good attendance from a few sides and I hope to summarise the
recommendations below,

a) Device Tree bindings

We should create a top-level virtual device binding that a top level
driver can bind to, like alsa asoc does.

We should separate the CDF device tree model from CDF as a starting
point and refine it outside of CDF, and produce a set of bindings that
cover the current drivers we have, exynos, imx, tegra, msm, armada
etc. This set of bindings should not be tied on CDF being merged or
anything else.

Display pipelines should be modelered in the device tree, but the
level of detail required for links between objects may be left up to
the SoC developer, esp wrt tightly coupled SoCs.

Externally linked devices like bridges and panels should be explicitly linked.

According to the above, the device tree bindings for simple panels that
I proposed earlier should be fine. However there was so much controversy
involved that I've decided not to make that part of my pull request this
cycle. Also they haven't been reviewed by DT bindings maintainers yes,
so according to our new rules they cannot be merged.

I think Laurent was more or less fine with them too, although he had
some objections to how DSI panels were represented and wanted those to
be sub-nodes of the DSI controller. I'll see if I can come up with
something to address that.

We should probably aim for a common binding for things like DSI.

b) Driver Model

The big thing here is that the device tree description we use should
not dictate the driver model we use. This is the biggest thing I
learned, so what does it mean?

We aren't required to write a device driver per device tree object.

We shouldn't be writing device drivers per device tree object.

I may remember this wrongly, but that's the opposite recommendation that
I got back when I started to work on Tegra DRM.

For tightly-coupled SoCs where the blocks come from one vendor and are
reused a lot, a top level driver should use the DT as configuration
information source for the list of blocks it needs to initialise on
the card, not as a list of separate drivers. There may be some
external drivers required and the code should deal with this, like how
alsa asoc does.

To share code between layers we should refactor it into a helper
library not a separate driver, the kms/v4l/fbdev can use the library.

This should allow us to move forward a bit clearer esp with new
drivers and following these recommendations, and I think porting
current drivers to a sane model, especially exynos and imx.

Now I saw we here but I'm only going to be donating my use of a big
stick and review abilities to making this happen, but I'm quite
willing to enforce some of these rules going forward as I think it
will make life easier.

After looking at some of the ordering issues we've had with x86 GPUs
(which are really just a tightly coupled SoC) I don't want separate
drivers all having their own init, suspend/resume paths in them as I
know we'll have to start making special vtable entry points etc to
solve some random ordering issues that crop up.

Where does that leave the Tegra driver? I've spent a significant amount
of time to get it to some sane state where having multiple subdrivers
are handled fairly nicely (in my opinion). Rewriting all of it isn't
something that I look forward to at all.

Thierry