[Arm-netbook] microdesktop v1.7 arrived, works fine... but...

[Luke Kenneth Casson Leighton]

ok so the story goes like this:

back in august 2016 i got the A20 card up and running with mainline
4.7rc1 (or thereabouts), including patching in NAND support for a
proper mtd device.  it worked... up to a point... except when u-boot
did a complete scan it CORRUPTED the NAND flash... just from reads.

i figured this was "just software" as it's pretty experimental, and
the 3.4.104 sunxi-nand works perfectly.

fast-foward to a few months ago, when i was testing the external
micro-sd slot, weirdly it resulted in a kernel segfault *from the
sunxi-nand driver*.  i thought, "that's weird, might be to do with
current-fighting from the micro-desktop PCB not having level-shifting"

sooo... that's now fixed: the micro-sd slot works perfectly through
the new revision 1.7 microdesktop's level-shifting IC.  however i got
exactly the same kernel segfault in the sunxi-nand driver, so i
temporarily took it out of the kernel config, tested again, and yes,
the micro-sd worked fine.

... except that when i booted it up again, the nand flash had been
corrupted.  now, bear in mind there's *NO DRIVER INSTALLED*.

this is just too weird for me to deal with.  not to mention, because
of the age of the A20's Boot ROM there is now a limited set of
"legacy" NAND ICs available i've had it with them.

now, i _could_ convert to eMMC but it's too much of a major redesign:
it involves disrupting the RGB/TTL tracks, and may require at least
two more rounds of pre-production prototyping...

... it's too much: it's too risky... and i'm getting fed up.  so.
what i'm going to do instead is: cut the NAND IC entirely, then wire
SDC2 (which is the same pins as the NAND) to the *ON-BOARD* Micro-SD
card slot, instead.

what that will give is *two* bootable Micro-SD card options, priority
being first the external one (MMC0) and second the on-board one
(MMC2).

the reason why this will be possible when it was not possible before
was described in another post to the list a couple weeks back: there's
simply no way to route the tracks on such a small 6-layer PCB with the
NAND tracks going from right-to-left, MMC0 has no room to go
left-to-right and at the same time MMC3 going left-to-right.  that's
over 30 tracks trying to cross through space that will only take about
15.

BUT...

... if the NAND tracks are *removed*.... *NOW* i can route MMC2
(right-hand-side of the A20 pins) to the on-board MicroSD slot (right
of the Card).

i can always do a variant which has eMMC later.... but right now i
just want to get something out the door as soon as possible with as
little risk as possible.  this approach does have the advantage that
the BoM is about $5 less.... but it does mean i will need to get
something like...  1,000 MicroSD cards, instead!

l.