Hello all,
I saw this a few days ago, but was too busy to answer then -- sorry
about that. I've dug into this code a bit, but for the mt7620.
On 1/21/23 08:51, Sergio Paracuellos wrote:
Hi,
[+cc John Crispin]
On Sat, Jan 21, 2023 at 2:45 PM Arınç ÜNAL <arinc.u...@arinc9.com> wrote:
On 21.01.2023 10:56, Sergio Paracuellos wrote:
Hi,
On Sat, Jan 21, 2023 at 7:03 AM Arınç ÜNAL <arinc.u...@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";
};
};
First of all, to better understand what you're working with I highly
recommend you download the mt7621 Data Sheet and took at §2.4 Pin
Sharing Schemes. Here's a link to one I've found:
https://www.t-firefly.com/download/FireWRT/hardware/MT7621.pdf .
Microcontrollers come with a lot of nifty hardware -- more than they
have external pins for. So if you don't need a piece of hardware, you
can option to use that pin as a GPIO instead.
The kernel code for the other end of this device tree snippet that
Sergio gave you is in arch/mips/ralink/mt7621.c, which you'll probably
find in your OpenWRT build tree under
build_dir/target-mipsel_24kc_musl/linux-ramips_mt7620/linux-5.4.143/.
The various struct rt2880_pmx_func variables contain the valid values
for each of these sets of pins except for "gpio" -- which is implicit
for each one (not my favorite design choice, but oh well). Finally, each
of those are glued together with the struct rt2880_pmx_group
mt7621_pinmux_data[] array on line 96. You can use this to see what the
valid values are for each group, because until this all goes yaml,
there's nothing to tell you if you've used an invalid value.
So the point is that you can pretend to export a gpio to user space all
you like, change it's direction and value, and perhaps the
micocontroller really obeys the gpio commands you send it, but you'll
never see that outside of the silicone die if that pin group is set to a
mode that doesn't connect that pin to the internal GPIO device on the die.
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`.
/sys/class/gpio is marked as deprecated [0] since kernel version 4.8,
please, avoid using it. Use libgpiod instead.
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.
Yes, 'gpio_request_enable()' is thought to request gpio on the desired pin.
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
AFAICS, the Ralink driver sets gpio mode for a group of pins using
set_mux operation [1] so when the
gpio_request_enable() operation is called a check for that pin is set
as gpio is performed. Nothing else.
Maybe John Crispin who is the writer of this driver can explain a bit
more about this.
[0]: https://www.kernel.org/doc/Documentation/gpio/sysfs.txt
[1]:
https://github.com/torvalds/linux/blob/master/drivers/pinctrl/ralink/pinctrl-ralink.c#L117
Best regards,
Sergio Paracuellos
To the best of my (limited) knowledge we can only change the pin group
mode upon boot via device tree and I have not found a mechanism to query
the pin sharing scheme dynamically -- I don't think there is one. I
would LOVE to have one! But IIUC, to change them dynamically is
problematic because you have device drivers that have probed based upon
the pin sharing scheme set up in the device tree and changing them out
from underneath those respective drivers can resulted in undefined
behavior -- I think any such mechanism would have to remove devices that
depend upon it first -- and I'm willing to guess that not all of our
device drivers are properly coded to "go away."
Again, John Crispin will know more about this.
Daniel
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