AST SOC gpio pass-through support

[Yong Li]

thanks for your quick response Andrew! i will read these info deeply. just a quick question, is it possible to enable/disable the pass through dynamically after kernel boots?  how?

> 在 2017年8月10日，16:28，Andrew Jeffery <andrew at aj.id.au> 写道：
> 
> Hi Yong,
> 
>> On Thu, 2017-08-10 at 15:50 +0800, Yong Li wrote:
>> Hi all,
> 
> Welcome to the list!
> 
>> 
>> AST SOC supports GPIOE pass-through function, I would like to know if
>> the kernel provides such API/interface to enable/disable pass-through?
> 
> Yes, the kernel's pinmux drivers can configure GPIO passthrough for
> you. In general, all the functions are documented in the pinctrl
> devicetree bindings:
> 
>    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/pinctrl/pinctrl-aspeed.txt?h=v4.12
> 
> Specifically, you are after functions/groups GPIE{0,2,4,6}. We already
> have these defined in the Aspeed DTSIs, so to configure them you can
> add something like the following snippet in your devicetree:
> 
>    &pinctrl {
>            pinctrl-names = "default";
>            pinctrl-0 = <&pinctrl_gpie0_default
>                    &pinctrl_gpie2_default
>                    &pinctrl_gpie4_default
>                    &pinctrl_gpie6_default>;
>    }
> 
> Specifying the mux configuration in the pinctrl node is probably as
> good a place as any, though if you have somewhere more relevant you
> should probably use that. Naturally you can remove any of these
> phandles to only select the specific pass-through configurations you
> need.
> 
> If you want to pass-through the whole bank you can also set the board
> strapping to make it the case right from power-on. The strapping
> register *is* writeable, however the kernel's pinctrl driver treats it
> as read-only, as really, the system designer ought configure it either
> via pull-ups or in the boot firmware.
> 
> The passthrough capability is a bit of an odd case - despite being GPIO
> I chose to represent it as a pinctrl function. Hope it makes sense to
> you.
> 
> Cheers,
> 
> Andrew