Unifying cape overlays into boot .dtb for BeagleBoard.org boards

Pantelis Antoniou panto at antoniou-consulting.com
Tue Jun 17 09:59:21 PDT 2014

Hi Russell,

On Jun 17, 2014, at 7:01 PM, Russell King - ARM Linux wrote:

> On Tue, Jun 17, 2014 at 04:32:11PM +0300, Pantelis Antoniou wrote:
>> The complexity is absolutely required, and it has nothing to do with
>> beaglebone capes.
>> The fact of the matter is that reconfigurable hardware is here, on
>> shipping system, and we, as the linux kernel community have to make
>> sure it works, and that it works in a sane way.
> Right, so looking back in the git history, this project has been going
> on for... at least four years?  Probably longer than the length of time
> that we've been converting ARM to DT.  At no point during that has anyone
> brought up the issue of DT being dynamic, so none of the drivers which
> we have been converting caters for this.

The timeline does not go that far back. The first (non-DT overlay based)
capebus patchset was posted on October 2012 and after feedback (i.e. it
was deemed to suck) it was pointed that a DT based generic method should
be used; hence DT overlays.

The dynamic DT part has always been part of PPC DT support for pSeries.

> Isn't this a bit of a missed opportunity, if this is a direction that
> OF wishes to head towards?

I am going to let Grant answer that, but personally I believe a lot of
the troubles we've been having with DT and dynamic device graph changes
can be handled elegantly using something DT overlays (and the follow up
work which is transaction DT support).
> Wouldn't it have been relevant to the discussion at kernel summit too,
> concerning DRM/v4l2/componentised systems?  What if someone comes along
> tomorrow with part of their multimedia based system inside a FPGA
> which they program up at runtime?

That case might already work on FPGA people's trees. I know Altera for sure
uses overlays, and some xilinx guys popped up on past discussions.
Their vendor trees probably use an older revision of the patches.

BTW, there is nothing special about DRM/v4l2 that can't be handled by a generic
DT mechanism. What makes this such a big problem?


-- Pantelis

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