Re: Add Allwinner Q8 tablets hardware manager

Hi Pierre,

On Oct 27, 2016, at 19:59 , Pierre-Hugues Husson [off-list ref] wrote:

2016-10-27 17:52 GMT+02:00 Pantelis Antoniou [off-list ref]:

quoted

Yes there is no EEPROM but you might be able to map probing results to
a fake ‘model’ number.

Let me expand a bit:

Assume that you have a number of probing steps, for example A, B, C each
returning true or false, and C being executed only when B is ‘true’ you
could do this to generate a bit field that identifies it.

For example let’s assume that model FOO’s probing steps are

A false, B true, C false -> 010

Model’s BAR

A true, B false, C don’t care -> 10x

Mapping these to models could be

Model FOO, (010 & 111) == 010 (all three probing steps must match)

Model BAR, (10x & 110) = 100 (the first two probing steps must match)

This method looks too complex on multiple grounds.
Assuming your method, I'm not too sure how this would actually be
described in a DTS.
Such probing steps should include reading/matching IDs in an EEPROM/on
an ADC, but it should also include the result of a driver's probe.
Also, drivers should have a way to report an ID/OTP instead of just a
boolean.

Err, I don’t think you got the point.

The probing steps are done by a board specific probe driver.
This driver performs the probing steps (which is exactly what Hans’s
method now does) but instead of applying changes to the device tree
programmatically generates a model string.

This model string can be used by a general purpose overlay manager to apply
the overlay(s) for the specific board. The plural part is important - read
below.

As you mentioned, it is a way to distinguish models, not just a set of
parameters.
Does this mean that this DT would lead to loading various DT based on
the matching model, which would look like a FIT?
Also there is a modularity problem there. If I have phones with either
screen A or screen B, and with either accelerometer A or accelerometer
B, I would have to implement all four combinations.

The model lookup need not result in a simple overlay to apply.

So for your case it would be:

model corp,0 -> overlay screen A + overlay accel A
model corp,1 -> overlay screen A + overlay accel B
model corp,2 -> overlay screen B + overlay accel A
model corp,3 -> overlay screen B + overlay accel B

You don’t need the combinatorial number of overlays.

I'm starting to agree with Hans, and to be able to implement
everything he needs, would require a turing complete device-tree,
which can include and apply device-tree overlays.
This doesn't mean it can't be done, nor that it shouldn't be done, but
that's a lot of work.

Hans' i2c-probe-stop-at-first-match does make sense for most usecases,
but I have two problems with it:
1. It is I2C specific (as I've mentioned earlier, I have the same
needs with DSI panels)
2. This looks like a temporary solution if a turing-complete solution
is to be implemented.

A custom I2C method would not be optimal IMO.

Regards

— Pantelis

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