[Arm-netbook] Early UART output vs. housing board discovery

Luke Kenneth Casson Leighton lkcl at lkcl.net
Wed Jan 10 13:59:19 GMT 2018

On Wed, Jan 10, 2018 at 1:44 PM, Jonathan Neuschäfer
<j.neuschaefer at gmx.net> wrote:

> On Wed, Jan 10, 2018 at 11:12:24AM +0000, Luke Kenneth Casson Leighton wrote:

>>  it is only PRODUCTION where the EOMA68 Specification is REQUIRED -
>> unconditionally - to be followed.
> I can certainly see cases where a Technical Enduser, as the spec calls
> them, wants to modify or replace low-level software on an already
> produced card, and perform "testing":
> - A card only ships with the SoC vendor's fork of Linux, but the user
>   wants to run/port mainline Linux.
> - The user wants to change something about the bootloader.
> - The card only runs Linux, but the user wants to port BSD.
> Thus it would be rather useful to have the UART available as an early
> debugging facility after production, and in a standardized way.

 yes, absolutely.... and there is absolutely nothing stopping the
"User" from, in effect, upgrading themselves mentally to the status of
"Technical EndUser" - at which point there are completely different
rules / state-diagram.

 so they would be perfectly and absolutely within their rights to take
the Card out, plug it into a.... testing station of some kind (even if
that's just a breakout board), plug in a MicroSD card with "UART debug
enabled" and off they go.

> A Technical Enduser could of course open a card, find the right test
> points for RX/TX, solder wires to them, run them out of the case, and
> close the case again, but this procedure is highly card-specific, and
> probably not always possible, e.g. when the RX/TX lines are routed in
> a way that makes soldering hard.

 no it's even more basic / simpler than that.  they don't have to do
that, they can just put the Card into a break-out PCMCIA holder
socket.  or anything else.

> In short: Thank you for the clarification. Now I disagree with this
> decision in the spec. :-/

 ok, so bear in mind that the UART wires double up as GPIO, and that
it is the HOUSING DESIGNER's right, under the EOMA68 specification, to
make the decision to allocate eiher one (or both) of the UART wires to
GPIO - as either Input or Output.

 .... so what do you think would happen, in this case, if someone
plugged in a Card where it was FORCIBLY REQUIRED that UART *ABSOLUTELY
MUST* transmit "early boot messages" on those two wires?

> (BTW, just to be very clear about the word "early": I mean early in the
> card's "run"-cycle (from power-up to power-down), not early in the the
> card's life-cycle (from production to destruction).)

 yep understood.

>>  could i leave it with you to alter the rest of what you wrote to take
>> that into account before we proceed further?  also, before proceeding,
>> perhaps we should discuss how to make the above absolutely clear.  it
>> is very important that there be zero misunderstandings in the EOMA68
>> Standard.
> I listed some hard(er) to understand phrases in my initial mail:

 i know... let's not forget about them but deal with things one at a
time, if that's ok.  i know from experience, from past discussions,
that ANY clarification requires a HUGE amount of effort and
discussion, with an exchange that can carry on for several days.

 i trust that you can appreciate that it would overwhelm both me and
you and everyone on this list to have three ongoing *simultaneous*
separate and distinct highly-technical logical reasoning discussions,


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