Re: [RFC v2 0/5] Common Display Framework

On 2012-12-19 17:26, Rob Clark wrote:

On Wed, Dec 19, 2012 at 8:57 AM, Jani Nikula
[off-list ref] wrote:

quoted

Hi Laurent -

On Tue, 18 Dec 2012, Laurent Pinchart [off-list ref] wrote:

quoted

Hi Jani,

On Monday 17 December 2012 18:53:37 Jani Nikula wrote:

quoted

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!

fwiw, I think there are at least a couple cases where multiple SoC's
have the same HDMI IP block.

And, there are also external HDMI encoders (for example connected over
i2c) that can also be shared between boards.  So I think there will be
a number of cases where CDF is appropriate for HDMI drivers.  Although
trying to keep this all independent of DRM (as opposed to just
something similar to what drivers/gpu/i2c is today) seems a bit
overkill for me.  Being able to use the helpers in drm and avoiding an
extra layer of translation seems like the better option to me.  So my
vote would be drivers/gpu/cdf.

Well, we need to think about that. I would like to keep CDF independent
of DRM. I don't like tying different components/frameworks together if
there's no real need for that.

Also, something that Laurent mentioned in our face-to-face discussions:
Some IPs/chips can be used for other purposes than with DRM.

He had an example of a board, that (if I understood right) gets video
signal from somewhere outside the board, processes the signal with some
IPs/chips, and then outputs the signal. So there's no framebuffer, and
the image is not stored anywhere. I think the framework used in these
cases is always v4l2.

The IPs/chips in the above model may be the exact same IPs/chips that
are used with "normal" display. If the CDF was tied to DRM, using the
same drivers for normal and these streaming cases would probably not be
possible.

 Tomi