mt7621 GPIO mapping mystery

[Arınç ÜNAL]

On 21.01.2023 10:56, Sergio Paracuellos wrote:
> Hi,
> 
> On Sat, Jan 21, 2023 at 7:03 AM Arınç ÜNAL <arinc.unal at arinc9.com> wrote:
>>
>> Pins from 22 to 33 are on the rgmii2 pin group. They don't function as
>> GPIO by default. Requesting a gpio by either from devicetree or `echo
>> 203 >  /sys/class/gpio/export` won't change anything. You have to claim
>> the pin group as gpio on the devicetree.
> 
> Yes, you have to claim the pin group as gpio on the device tree to
> make this work. Ralink has the concept of "GPIO mode" but actually is
> just an electrical configuration for a certain device. So if the mode
> (function) is not requested as a real GPIO nothing is going to work.
> So in your board's dts file you have to add something like the
> following with the groups you want to claim as real gpio function:
> 
> #include "mt7621.dtsi"
> ...
> 
> &state_default {
>      gpio {
>          groups = "jtag", "uart3", "wdt";
>          function = "gpio";
>      };
> };
> 
>>
>> Quoting my response from [0]:
>>
>>> state_default is there to explicitly set the function of a pin group to gpio, this is done because the bootloader may have set the function of a pin group to something else before booting OpenWrt which would render the pins of that group uncontrollable for general purpose aka GPIO.
>>>
>>>      Actually I think @arinc9 did some work around that.
>>>
>>> Not yet, I plan to modify the gpio_request_enable pinmux operation to set the pin group as gpio when there's a gpio request for a pin in that pin group. gpio_request_enable pinmux operation can only set the function of an individual pin currently. Since ralink pinctrl driver can only set the function of a group of pins, the operation currently cannot be used.
>>>
>>> If we make it work, any GPIO defined on devicetree or exported from userspace will automatically have the function of the pin group it's in set to gpio, completely getting rid of the need for explicitly defining functions of certain pin groups on the devicetree.
>>
>> Of course when I said "I plan to modify this code" I actually meant I
>> was going to talk this through with Sergio but I never had the
>> opportunity to do so. I guess this thread is a good place to start
>> talking about this.
>>
>> I had this case on a user:
>>
>> They got an LED wired to wdt pin. GPIO is already exported on the DT.
>> However their LED just won't work.
>>
>> It turns out the bootloader sets the wdt pin's function to something
>> other than gpio. And when OpenWrt boots, the pinctrl driver makes no
>> changes to the pin's function.
> 
> Bootloader always sets its own configuration for the pinctrl. The
> linux pinctrl driver sets every single group default mode [0] as it is
> in the Mediatek's Mt7621 datasheet.
> 
>>
>> So we had to specifically claim that pin as gpio to make the LED work.
>> Now there is already a solution for this which is the
>> gpio_request_enable pinmux operation but it's not supposed to be used on
>> pinctrl drivers that cannot control pins individually.
>>
>> Sergio, you think we can somehow make this pinmux operation mux a pin
>> group as gpio instead of a single pin?
> 
> I am not an expert in pinmux drivers but I think there are strong
> reasons why only a single pin is allowed to be requested.
> 
> See kernel doc about this here: [1]
> 
>>
>> Or introduce a new pinmux operation that can do this?
> 
> I think you should send an email to kernel gpio / pinctrl kernel mail
> list to get feedback from Bart and Linus as gpio and pin control
> maintainers to properly understand the way to go but I don't really
> understand what is the problem requesting the group as gpio in the
> device tree like any other single platform is doing and seems to be
> the correct way to go. Maybe I am missing something :)

 From what I understand, a gpio is requested by exporting a gpio by 
either from devicetree or `echo 203 >  /sys/class/gpio/export`.

Now, the pinctrl driver must somehow know that the pin which translates 
to the GPIO number needs to function as gpio.

Doing this manually on DT bindings is an option but it's not very 
viable. I believe this is why the gpio_request_enable operation was made 
for pinctrl drivers to implement. Now a pin can be made to function as 
GPIO from userspace dynamically instead of hardcoding it on the devicetree.

Boards with pinouts, like Raspberrypi, Bananapi, etc. benefit this the 
most. Because it'd be extremely hard to hardcode every pin with pinouts 
on the devicetree for each different device.

For example, my Unielec U7621 board uses the rgmii2 pins for ethernet 
but at the same time it's got pinouts for them. If the pinmux operation 
worked, I could just export the gpio number and the pin would function 
as gpio. When I'm done, I could just unexport and the pin group would go 
back to function as an rgmii bus.

I believe this is already the case with pinctrl drivers that can control 
pins individually. There's no state-default node on DT where some pins 
are hardcoded to function as gpio.

MediaTek Moore Pinctrl driver which can control pins individually 
implements gpio_request_enable.

https://github.com/torvalds/linux/blob/master/drivers/pinctrl/mediatek/pinctrl-moore.c#L77

gpio_request_enable is also there on the Ralink Pinctrl driver but I 
don't think it does anything.

https://github.com/torvalds/linux/blob/master/drivers/pinctrl/ralink/pinctrl-ralink.c#L161

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