[RFC v2 0/5] Common Display Framework

[Laurent Pinchart]

Hi Jani,

On Wednesday 19 December 2012 16:57:56 Jani Nikula wrote:
> On Tue, 18 Dec 2012, Laurent Pinchart wrote:
> > On Monday 17 December 2012 18:53:37 Jani Nikula wrote:
> >> I can see the need for a framework for DSI panels and such (in fact Tomi
> >> and I have talked about it like 2-3 years ago already!) but what is the
> >> story for HDMI and DP? In particular, what's the relationship between
> >> DRM and CDF here? Is there a world domination plan to switch the DRM
> >> drivers to use this framework too? ;) Do you have some rough plans how
> >> DRM and CDF should work together in general?
> > 
> > There's always a world domination plan, isn't there ? :-)
> > 
> > I certainly want CDF to be used by DRM (or more accurately KMS). That's
> > what the C stands for, common refers to sharing panel and other display
> > entity drivers between FBDEV, KMS and V4L2.
> > 
> > I currently have no plan to expose CDF internals to userspace through the
> > KMS API. We might have to do so later if the hardware complexity grows in
> > such a way that finer control than what KMS provides needs to be exposed
> > to userspace, but I don't think we're there yet. The CDF API will thus
> > only be used internally in the kernel by display controller drivers. The
> > KMS core might get functions to handle common display entity operations,
> > but the bulk of the work will be in the display controller drivers to
> > start with. We will then see what can be abstracted in KMS helper
> > functions.
> > 
> > Regarding HDMI and DP, I imagine HDMI and DP drivers that would use the
> > CDF API. That's just a thought for now, I haven't tried to implement them,
> > but it would be nice to handle HDMI screens and DPI/DBI/DSI panels in a
> > generic way.
> > 
> > Do you have thoughts to share on this topic ?
> 
> It just seems to me that, at least from a DRM/KMS perspective, adding
> another layer (=CDF) for HDMI or DP (or legacy outputs) would be
> overengineering it. They are pretty well standardized, and I don't see there
> would be a need to write multiple display drivers for them. Each display
> controller has one, and can easily handle any chip specific requirements
> right there. It's my gut feeling that an additional framework would just get
> in the way. Perhaps there could be more common HDMI/DP helper style code in
> DRM to reduce overlap across KMS drivers, but that's another thing.
>
> So is the HDMI/DP drivers using CDF a more interesting idea from a non-DRM
> perspective? Or, put another way, is it more of an alternative to using DRM?
> Please enlighten me if there's some real benefit here that I fail to see!

As Rob pointed out, you can have external HDMI/DP encoders, and even internal 
HDMI/DP encoder IPs can be shared between SoCs and SoC vendors. CDF aims at 
sharing a single driver between SoCs and boards for a given HDMI/DP encoder.

CDF isn't an alternative to DRM/KMS. It should be seen as a framework that 
helps DRM/KMS drivers (as well as V4L2 drivers, and possibly FBDEV drivers, 
although those should be ported to DRM/KMS) sharing encoder and connector 
code.

> For DSI panels (or DSI-to-whatever bridges) it's of course another story.
> You typically need a panel specific driver. And here I see the main point of
> the whole CDF: decoupling display controllers and the panel drivers, and
> sharing panel (and converter chip) specific drivers across display
> controllers. Making it easy to write new drivers, as there would be a model
> to follow. I'm definitely in favour of coming up with some framework that
> would tackle that.

That's the main (and original) goal of CDF (originally called Generic Panel 
Framwork, and renamed to CDF to support encoder drivers as explained above). 
I'm glad to know that you're in favour of it :-)

-- 
Regards,

Laurent Pinchart