On Fri, Oct 20, 2017 at 07:02:27PM +1030, Andrew Jeffery wrote:

On Fri, 2017-10-20 at 09:43 +0200, Linus Walleij wrote:

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On Fri, Oct 20, 2017 at 9:17 AM, Linus Walleij [off-list ref] wrote:

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On Fri, Oct 20, 2017 at 5:37 AM, Andrew Jeffery [off-list ref] wrote:

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GPIO state reset tolerance is implemented in gpiolib through the
addition of a new pinconf parameter. With that, some renaming of helpers
is done to clarify the scope of the already existing
gpiochip_line_is_persistent(), as it's now ambiguous as to whether that
means on suspend, reset or both.

Isn't it most reasonable to say persistance covers both cases, reset
and/or sleep? This seems a bit like overdefined.

Seems reasonable to me to just expand the existing stuff to cover
reset as well, I don't think that should cause any issues for the
Arizona stuff.

quoted

I should also add: right now persistance is defined in negative terms,
you can supply the flag "may lose value", which means the subsystem
by default, and driver by default, will try to keep values persistent across
sleep.

Then it is possible to opt in for not doing so. (Usually to save power I
think.)

I think that especially for userspace use cases, saving power should
not really be the concern, but correct me if I'm wrong. I am thinking
of a box with a DC plug wired up to a factory line here.

What we have in the Arizona driver is an opt-in where the DT can
say "don't preserve the value  this line during system sleep" i.e. "lay lose
value" and we can extend that flag to mean "don't preserve this line
during reset either" but by default assume that we should.

Yeah, the preserve polarity was another thing I debated given the
current example with the Arizona driver. Not preserving is the default
for the Aspeed hardware, so that ended up influencing my choice. Not
that implementation details should necessarily influence interface
design, but it was at least more than a coin toss.

The way I think we ended up looking at this was when a user
requests a GPIO the least surprising thing for them is the value
maintains until they change it again. So it made sense to make
the allowing the value to be dropped the opt in side.

I don't have anything specific against preserving by default, just my
gut instinct and the hardware went the other way. As long as we expose
the option to opt out, which the additions for the Arizona already do.

Cheers,

Andrew

Thanks,
Charles