[Arm-netbook] MIPI DSI instead of RGB/TTL - the only way out of the blind alley

[Luke Kenneth Casson Leighton]

On 12/14/16, Benson Mitchell <benson.mitchell+arm-netbook at gmail.com> wrote:
> On Tue, Dec 13, 2016 at 9:51 AM, dumblob <dumblob at gmail.com> wrote:
>> Can we provide both interfaces (RGB/TTL + MIPI DSI) on the same pins while
>> having a HW way to choose from these?
>>
>> Yes we can! EOMA already counts on several types of PC Cards (originally
>> called PCMCIA). At minimum two - thinner (Type I - 3.3 mm) and thicker
>> (Type
>> II - 5 mm). Let's declare the thicker cards to be high-end and offer only
>> MIPI DSI while thinner cards low-end with just RGB/TTL. Problem solved!
> To be specific, EOMA68 includes Type I, Type II and Type III already. There
> are differences in permitted power, and RGB resolution. These differences
> run in opposite directions, in order to make sure that any combination that
> fits will work.
>
> WRT resolution, Type I is the high-end, with card-minimum/housing-maximum
> 1920x1080, because a Type I housing will accept _only_ Type I cards. Thus
> anything with a 1920x1080 screen has a Type I slot, and any card that
> physically fits will drive that display.

 nooo, 5.0mm is the 1366x768 because the 5.0mm needs to be prevented
and prohibited from being physically inserted into incompatible 3.3mm
(1920x1080) slots.

> (AIUI, a Type II housing can have a 1920x1080 display, as long as that
> display will accept and upscale a 1366x768 signal.

 NO.  absolutely not.  that is a completely unacceptable technical
burden on the manufacturers of the housings, forcing them to have
additional circuitry which may or may not be used.... and may or may
not be actually available on the open market.... and may actually end
up being far more costly than the processor utilised in the Card.

 any kind of resolution scaling at these framerates and buffer sizes
it actually needs a full processor - with several hundred megabytes of
DDR2 / DDR3 RAM - to perform the conversion.

 so no - absolutely not.  you connect the LCD to the EOMA68 bus on the
Housing, the LCD's resolution is fixed as decided by the manufacturer
of the Housing, and that's the end of it.



> Then either a Type I card
> or a Type II card exceeding the minimum specs can output full 1920x1080, but
> it will work with even the minimum Type II card's 1366x768.)
>
> The point is, any card _must_ work in any housing it physically fits in. So
> if you want Type I to support MIPI, that's great -- but that Type I still
> fits in a Type II, so it must also be able to output RGB/TTL on the same
> pins,

 ... correct....  but worse than that it must be on the *exact*
dual-function pins as arbitrarily specified in the propsed [completely
not-thought-through] standard.

> and there must be a mechanism to autonegotiate this depending on the
> housing.

 correct.

> I don't think adding autonegotiation here is particularly hard (basically
> just defining a flag in the I2C EEPROM),

 correct.

> but the need to support both
> interfaces negates some of the benefits of MIPI,

 not really relevant

> and adds complexity to all
> cards supporting MIPI,

 "insane, hard to implement and with ICs that probably don't actually
exist" complexity

> and I'm not sure that complexity (multiplexer, and in
> some cases, MIPI->RGB conversion IC) can actually fit on a crowded EOMA68
> card.

 correct.

>> The "high-end" specification shall then also be extended allowing higher
>> thermal dissipation (5W is too low - maybe 15W would be OK as it's still
>> easy to cool passively) etc. to accommodate "high-end" (actually
>> mainstream,
>> but in this context it's high-end) requirements.
> Type I and Type II are currently limited to 5W, while Type III is limited to
> 10W. Again, these have to be in this order, because any card must work in
> any housing it physically fits. So a 5W card can be powered by a 10W power
> supply, but not the other way around.
>
> I think 10W is a hardware limit of the connector (4 pins at 0.5A per pin);

 correct.  ah, i'd forgotten about that.  yeah you do not want to be
overheating the pins.  so, 10W limit it is.

>> Speaking about thermal dissipation, I'm not sure that in case of the
>> high-end card type, this limit should be a fixed one.
> Obviously, technically skilled people will overclock and overwatt specific
> EOMA68 cards in housings that they know can supply more power.

 outside of the standard.... probably.

> But the
> minute you change this behavior from "hackers breaking the rules" to
> "there are no rules", you've made it so that the whole promise of EOMA
> ("Just plug it in; it will work") can no longer be kept.

 correct. and that's why these things have to be thought through very,
very carefully, and simplly not permitted - outright banned - if there
is even the slightest possibility of confusion or harm.

 remember the goal is 100 million units and above per year.  even the
SLIGHTEST chance of confusion could result in millions of units
returned, resulting in a catastrophic loss of confidence in the
standard.

 there is NO WAY the standard can be "sacrificed" just for the benefit
of some arbitrary
"nice-to-have" decision or short-term profit.


> I personally would be fine with that -- in fact, I would be fine with a lot
> of things that are defined in the EOMA68 standard being just a matter of
> labeling, and leave it on the user to choose compatible parts.

 as long as the compatibility is "everything, always works [even if
it's a bit slower]" i don't care.

 the MOMENT that becomes "it MIGHT work, but it might not" then the
standard's fucked and six years (and counting) have been utterly
wasted, irrevocably destroyed in an instant.

> But Luke's not designing this standard for me; he's designing it for people
> who would get confused and buy a 25W card and a 10W-max tablet housing, and
> not understand why they don't work.

 absolutely correct.

> If you're targeting those people (which
> you have to, to get volume), you have to make it work for them.

 absolutely correct.

 this isn't a "techie standard wanna plug in yer favurit memry upgrad
just undo da scruz n read duh instrucshunns on da in'ur'ne' "

 just as that pc journalist said a couple months back (about gaming
pcs being too hard), it's for people with big fat fingers who are
afraid to drop the screwdriver and damage things.

 one button.

 press it.

 card comes out.

 put new one in.

 it will work.

 that simple.

 and it stays that simple.

 this is not negotiable.

l.