AST SOC gpio pass-through support

[Yong Li]

Hi Andrew & Rick, what I want is I can change the gpio pass-trough
dynamically after kernel boots. For example:
The pass-through is disabled after I export gpioe2, I can control
these pins by writing value in /sys/class/gpio.

I still can enable the pass-through function later. As Rick mentioned,
the pass-through is disabled after export gpio, I would like to know
if the will be enable after I unexport it? Or I can use "runtime
pinmuxing" to change it? Let me study on them.

Thanks,
Yong

2017-08-11 7:45 GMT+08:00 Andrew Jeffery <andrew at aj.id.au>:
> On Thu, 2017-08-10 at 09:11 -0700, Rick Altherr wrote:
>>
>>
>> On Aug 10, 2017 3:12 AM, "Andrew Jeffery" <andrew at aj.id.au> wrote:
>> On Thu, 2017-08-10 at 17:01 +0800, Yong Li wrote:
>> > thanks for your quick response Andrew!
>>
>> No worries!
>>
>> > i will read these info deeply. just a quick question, is it
>> possible
>> > to enable/disable the pass through dynamically after kernel
>> boots?  how?
>>
>> So there is the concept of runtime pinmuxing[1]: It's possible for a
>> device to have several pinmux states that can be configured/selected
>> from process context.
>>
>>
>> On Quanta Q71L, the hardware strap is used to enable pass through so
>> the power button works even if the BMC fails to boot. The BMC
>> firmware is expected to disable the pass through once it is ready to
>> take over. I believe I sent the patch upstream to allow pinmux to
>> modify the strap register for this case. That is, pass through is
>> disabled when the user exports a GPIO to userspace.
>
> Ah yes, I forgot we added an exception there. It does require that the
> strapping is used for the configuration and not the alternative SCU
> bits that are set by the GPIE{0,2,4,6} functions, which were enabled by
> the devicetree snippet I sent previously. If the mux is requested with
> the GPIE{0,2,4,6} functions then the runtime pinmuxing is really the
> only choice.
>
> Yong: What is your use case? If it's similar to the Q71L as Rick has
> outlined, then please ensure the strapping is set correctly in hardware
> - setting it in U-Boot doesn't really gain you much unless you fail to
> load/boot the kernel.
>
> Cheers,
>
> Andrew