[Arm-netbook] 2.7.4 preproduction sample, 1.7 MD, Questions

[Paul Boddie]

On Sunday, 19 July 2020 22:43:59 CEST Luke Kenneth Casson Leighton wrote:
> On Sun, Jul 19, 2020 at 9:35 PM Paul Boddie <paul at boddie.org.uk> wrote:
> >
> > It looks like the device tree (sun7i-a20-eoma68-a20.dts) is very similar
> > to that from the Cubietruck 2.
> 
> that sounds about right.  possibly the original cubieboard.

OK. From what I can see, there are strong similarities between the above file 
and both sun4i-a10-cubieboard.dts and sun7i-a20-cubieboard2.dts. (I got mixed 
up and wrote Cubietruck 2 above.)

The principal differences appear to be as follows:

The EOMA68-A20 DT doesn't have HDMI nodes whereas the Cubieboard DTs have 
nodes for HDMI connectors and the usual peripheral nodes (&hdmi, &hdmi_out). 
Given the effort that went into putting micro-HDMI on the computer card, I 
guess there might be some definitions to add here.

The EOMA68-A20 DT has a special pinctrl node for mmc3_cd_pin (MMC3 card 
detect, I guess). There are also LED pin differences, but I think these are 
explained by pin definitions migrating to the sun7i-a20.dtsi file.

Some I2C definitions seem to be different in the Cubieboard DTs, but that 
could also be due to pin definitions migrating to the sun7i-a20.dtsi file.

The Cubieboard DTs enable the display engine (&de) and power 
(&ac_power_supply).

There are some changes to the Ethernet node (&gmac). (I didn't think Ethernet 
was exposed by the computer cards when the specification was updated after the 
first round of cards being made.)

The Cubieboard DTs enable USB OTG (&usb_otg) whereas the EOMA68-A20 DT has a 
node &reg_usb0_vbus that the other DTs do not have.

So, it seems possible that an updated DT file would be a minor variation on 
the Cubieboard 2 file.

> > Were the differences documented and are there any
> > schematics to provide any necessary insight when updating the device tree?
> 
> there's PDFs somewhere... http://hands.com/~lkcl/eoma/
> such as this: http://hands.com/~lkcl/eoma/DS113-V2.7-2017-02-17.pdf

And, as usual, when I go to download it, I find that I already have it 
alongside a schematic for the Cubietruck!

> the thing is that it is critically important that the dts file *do
> not* include the peripherals associated with the "Boards"
> (Micro-Desktop, 15in laptop).  the peripheral sets *MUST* go into a
> "dynamic include" file that is detected at boot time and "inserted"
> into the live device-tree.

Indeed. I guess the computer card device tree sits at a level between the 
chipset device tree and what would normally be the machine device tree.

> this functionality - dynamic runtime "insertion" of device-tree
> fragments - was an idea *in development* back in.... 2013  we have had
> to wait several years for this functionality to make its way into both
> u-boot and the linux kernel.

I guess I should ask whether such support made it into either of them.

Paul