GPIO - startup signals - where to put them?

Ben Henricksen

Hi I have a general linux question. We have our own board design, similar to beaglebone black. We have a couple of on-board devices which need GPIOs set to a value or a simple Lo / High 2 sec / Lo sequence. Where is the best place to do this?

I can put the code to do this in the \board\ti\am335x\board.c files and it works. However, subsequent initialisation (possibly the DTS stuff) resets the GPIOs.

I can set the GPIO type in a DTS file successfully (i.e. make it an output or input with pullup) but not set it's value, much less sequence it.

I have scripts which work which could be run from init.d but one of the signals starts a USB Hub and I would like this already booted when other USB udev rules run, hence the desire to get this set up earlier.

I get that devices have modules which can be loaded by mod_probe automatically - but to do this I'd need to create a kernel module to just set a gpio. Is this a world of pain I can avoid? Or is it quite simple really? Will setting up gpios in the board.c file always get over-written?

over 9 years ago

0 Titusrathinaraj Stalin over 9 years ago

TI__Guru** 116100 points

Dear Ben,
We can do this in n no of ways.

I can put the code to do this in the \board\ti\am335x\board.c files and it works. However, subsequent initialisation (possibly the DTS stuff) resets the GPIOs.

1) I hope, it won't reset if you remove that particular GPIO no (if used)

2) You can also access the GPIO via SYSFS interface, you can write simple application.

Command based:
processors.wiki.ti.com/.../Linux_PSP_GPIO_Driver_Guide

Bash script:
developer.ridgerun.com/.../How_to_use_GPIO_signals

'C' application:
http://elinux.org/GPIO
elinux.org/RPi_GPIO_Code_Samples

3) Also you can write the GPIO registers through devmem2 tool.

4) As you told, we can write kernel module driver (its also simple)

0 Yordan Kovachev over 9 years ago

TI__Guru**** 161600 points

Hi,

In addition to Titus comment.

Can yo try to put your code (which sequences the gpio to Lo / High 2 sec / Lo) in drivers/gpio/omap-gpio.c? I think the best place is the omap_gpio_probe() function, which is used to parse the dtb settings & set gpio registers. The omap-gpio.c driver is built in the kernel & should execute at the time kernel initializes the gpio.

Ben Henricksen said:
Will setting up gpios in the board.c file always get over-written?

Yes, kernel overwrites u-boot settings.

Best Regards,
Yordan

0 Ben Henricksen over 9 years ago in reply to Titusrathinaraj Stalin

Intellectual 855 points

Thanks for your suggestions.
I think the DTS setting up of the GPIO controller clears all the GPIOs which I have set in the board.c (I have removed my own individual GPIO settings).

My script which works uses the SYSFS interface. But I would like to get the hardware devices on the board out of reset right at the start of boot so that USB and attached devices are recognised at the end of the boot process.

I guess I'll have to look at a kernel module which is loaded by the mod_probe of the nodes in the DTS. Do you have any simple examples of this? Even the LED code looks tricky!

0 Ben Henricksen over 9 years ago in reply to Yordan Kovachev

Intellectual 855 points

Thanks for the suggestion Yordan. I'll see if I can add my code to the GPIO driver.

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GPIO - startup signals - where to put them?