Re: outcome of DRM/KMS DT bindings session

[Thierry Reding <thierry.reding@gmail.com>]

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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

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