devicetree live reconfigureable hardware (eoma68)

[Luke Kenneth Casson Leighton]

hi folks,

i raised the issue of dynamic reconfigureable hardware back in 2012,
and thank you (once again) to alan for kindly answering:
https://lkml.org/lkml/2012/3/10/71

since then, yaay!  beaglebone with "capes" has driven some
modifications to the linux kernel:
http://events.linuxfoundation.org/sites/events/files/slides/dynamic-dt-elce14.pdf

the hardware differences between beaglebone capes and EOMA68 hardware
is that EOMA68 CPU Cards may remain powered up whilst being removed
from a "chassis" (whether it be a laptop, a tablet, an engineering
board, a desktop - whatever) and plugged into a new one [i'm sure that
it's theoretically possible for beaglebone capes to be carefully
removed from a live-running beagleboard but i certainly wouldn't like
to try it].

the difference between the beaglebone cape on-board EEPROM and the
original idea that i came up with for EOMA68 is that the on-board
EEPROM in EOMA68 would contain actual device-tree configurations
describing the plugin-hardware, whereas from what i can gather
beaglebone cape EEPROMs merely contain an "identifier".

so, first thing that a CPU Card does is: read the EEPROM data, get the
various device-trees in it, and start totally reconfiguring the 68
pins and associated hardware.  not a problem, right? weelll... :)

here's the problem that i see, perhaps someone could tell me an
appropriate place to discuss this (if LKML isn't), and if the issue i
describe has already been catered for and is easily (and conveniently)
solvable.

first, EOMA68 is 68 pins, many of which share a GPIO function.  so,
for example, the UART TX and RX pins may also be multiplexed to plain
GPIO.  likewise all of the pins of SPI as well as SD/MMC, these may be
GPIO - just like with the beaglebone capes.

so, first complication: it is necessary to select "SD/MMC" interface
instead of the GPIO functions.... but then, 5 minutes later, whoops,
we have to select GPIO pins.  but, remember, there are at present
*three* totally different CPU Cards (based on the allwinner a20,
another on the ingenic jz4775, and a 64-bit ARM card is being
designed).

to clarify that: beaglebone is {1}-Board to {M}-Many-Capes, whereas
EOMA68 is {N}-CPUCards to {M}-Chassis.

so whereas with the beaglebone it's perfectly fine to just hard-code
"oh yeah, pin 50 is GPIO but it's also multiplexed with SD CLK line",
with EOMA68 it is simply flat-out impossible to do that.  you simply
cannot know, in the future, what GPIO will be connected on a CPU Card.
the *only* information that you have is, "EOMA68 pin 16 is GPIO0 or SD
Data line 3".  you have *NO IDEA* what *actual* GPIO that goes to on a
particular SoC, especially ones that will be available over the next
decade.

so that's complication (1).

however, there is a secondary complication in that a hardware designer
may choose *not* to use *all* of the pins associated with a particular
hardware function.  so for example, with SPI, it is possible to use
the 4-wire variant which has DDR capability (100mbytes/sec which is
pretty awesome, thanks to intel's work i believe).  or, you could go
down to a 2-wire data width bus.  or, you could go back to the
standard 4-wire configuration (NSS, CLK, MOSI, MISO), *or* you could
do the 3-wire SPI variant which is uniplexed single-wire data for both
transmit and send (NSS, CLK, MOSI shared).  and, not only that, but if
someone decides they don't want to use the standard NSS wire, that's
fine - they can use another GPIO pin for NSS!

another example: SD/MMC.  it's either SPI-compatibility mode (3 or 4
wire), or it's a 1-wire SD/MMC data bus, or a 2-wire, or a 4-wire bus.
oh, and there's a "Card Detect" GPIO... which is also optional, and
you use polling instead of EINT IRQ on that wire!

how in god's green earth are you supposed to describe this in
reconfigureable device-tree blocks??? :)

should we write:

* one device-tree file to describe the SPI 4-wire DDR mode
* one for the SPI 2-wire DDR mode
* one for the SPI 4-wire standard variant
* one for the SPI 3-wire standard variant
* one for... o fuk i forgot that you need to double those up because
of the NSS possibly being a different GPIO pin...

argh now we have *EIGHT* possible device-tree files to dynamically
choose from... and that's just the SPI interface!

now we have to do the same thing for SD/MMC... that's what... eight
possible device-tree files to choose from there, as well??

and we haven't started on the RGB/TTL interface yet!  there's even the
possibility, with the RGB/TTL interface, that it could be connected to
a converter IC, where a standard monitor (DVI, VGA) with EDID
capability could be connected to it.  oh and don't forget that it
could be 18-bit output or 15-bit output.


possible solution, here, which solves both complication (1) as well as
complication (2): a level of abstraction is needed.  the abstraction
is needed because, as mentioned in complication (1) the only
consistent naming is in the EOMA68 interface, *NOT* in the SoCs
*BEHIND* the EOMA68 interface.

the only possible way at the moment to do that is, i believe, to use
device tree "includes".  there would be files, on each SoC's kernel,
such as this:

* eoma68_pin_1_when_configured_as_gpio.dtsi
* eoma68_pin_35_when_configured_as_gpio.dtsi
* .... gpio ...
* etc.

and then:

* eoma68_sdmmc_in_4wire_data_mode.dtsi
* eoma68_sdmmc_in_2wire_data_mode.dtsi
* eoma68_sdmmc_in_1wire_data_mode.dtsi
* eoma68_sdmmc_in_spi_mode.dtsi

and so on and so forth.

then, having those available as well-known library files, which are
guaranteed to cover all and every possibility, a "Chassis" may then
have a single devicetree file which comprises:

/include/ eoma68_pin_1_when_configured_as_gpio.dtsi
....
....

and pretty much nothing else.

absolutely insane and guaranteed to make even the most sensible linux
kernel programmer spew and hurl their lunch, projectile-vomit-fashion,
over a deep and wide area.

so.

first question.

is there *anything* in devicetree that has a level of abstraction
where all these capabilities may be compressed into a single file?
naming, classes, anything?   the thought of splitting things up into
almost 100 separate dtsi files is genuinely making me cringe.

i have an even more horrible idea of overloading phandles, like
setting a variable "eoma68_pin_1_as_gpio = <0xe0680001>" or something
like that - using the phandle 32-bit values as a predefined convention
that, when it comes to loading the devicetree file off the chassis
EEPROM, those predefined 32-bit conventions are the necessary
abstraction.

... but honestly, it would be much much better to have strings, not
32-bit values.

any other ideas out there?

now, for anyone considering that this discussion might be irrelevant,
consider this: in the near future (if it hasn't happened already) some
bright spark is going to realise that the beaglebone "capes" is a
ready-made market for creating a far superior "compatible" form-factor
engineering board - perhaps something with a dual-core or quad-core
processor... just like has *already* happened with arduinos.

in other words, the current beaglebone black hardware expansion
interface becomes a de-facto industry standard, just like the arduino
shield interface already is [the only reason we haven't had this come
up in the linux kernel community with arduino shields and boards
already is because arduino boards aren't powerful enough to run the
linux kernel].

the implications there are that in a relatively short time, beaglebone
"capes" will likely turn from a 1-to-many scenario into the exact same
many-to-many scenario as faced by EOMA68 hardware right now.  if not
beaglebone, then something else.

so this is *not* a theoretical problem, isolated to a single hardware
design, it's a generic problem that's going to present itself and
smack people in the teeth fairly shortly.  yaay! :)

l.
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