topic Re: Re: Vybrid GPIO on /sys/class/gpio does not switch properly in Vybrid Processors

Vybrid GPIO on /sys/class/gpio does not switch properly

johannestraxler — Thu, 26 Feb 2015 16:24:11 GMT

I have a GPIO (PTE8 - Pin Nr. 113) exported via the /sys/class/gpio device tree and configured as OUTPUT direction.

From my C-Code I do the following

int fd = open ("/sys/class/gpio/gpio113/value", O_WRONLY);

...

while (1)
{
     ...
     write (fd, "1", 1);
     ...
     ...
     write (fd, "0", 1);
     ...
}

The problem is, that write() does not work every time. It seems as if the value is not written into the Port-Register of the processor. I did measurements with a scope directly on the processor pin and the value showed 1 after the second write statement in line 11, but it should be 0. In the same step I checked in shell with "cat /sys/class/gpio/gpio113/value" and here the value is also shown as 0. When I then typed "echo 0 > /sys/class/gpio/gpio113/value" in the shell the pin went to 0. I also tried to modify the driver strength in IOMUX to 25Ohm, 50Ohm and 150Ohm and explicitely set OBE to high, but the behaviour is the same - sometimes it work - sometimes not!

Do you have an advice for me?

Thanks in advance,

Johannes

Re: Vybrid GPIO on /sys/class/gpio does not switch properly

falstaff — Mon, 02 Mar 2015 10:20:28 GMT

Hi Johannes,

What Kernel version do you exactly use? The Mainline GPIO/Pinctrl driver behaves slightly different then the 3.0/3.13 Timesys one. Also, how do you debug exactly? At least on current kernels, it looks like the value write could sleep. Do you break the code execution after the write?

Linux/drivers/gpio/gpiolib-sysfs.c - Linux Cross Reference - Free Electrons

What polarity do you use? (see active_low file). However, I don't think this is the issue since you do not change it and hence it should be the same issue when using sysfs from the shell.

Stefan

Re: Vybrid GPIO on /sys/class/gpio does not switch properly

johannestraxler — Mon, 02 Mar 2015 11:44:17 GMT

Hi Stefan,

I'm using

Linux version 3.0.15-ts-armv7l

gcc version 4.8.3 (Timesys 20140811) )

PREEMPT

In this case I didn't have to debug, it was enough to write a shell script, which toggles the pin and verified it with an oscilloscope. The result on the scope was that the pin did not toggle every time as expected, but it stay for one complete period low or high.

ACTIVE_LOW is 0.

Re: Vybrid GPIO on /sys/class/gpio does not switch properly

falstaff — Mon, 02 Mar 2015 13:16:16 GMT

Hi Johannes,

In the initial post you stated that it worked in Bash. So it does not work when using a loop in a bash script? How do you know whether it toggled the pin or whether it just had a glitch timewise (latency)? We recently used a Python script on one of our modules and got a pretty decent square wave, however we run it on a newer kernel.org Linux kernel:

http://developer.toradex.com/knowledge-base/python-in-linux

Stefan

Re: Re: Vybrid GPIO on /sys/class/gpio does not switch properly

johannestraxler — Mon, 02 Mar 2015 14:08:25 GMT

Hi Stefan,

in my initial post I mentioned that I just used the shell to check the status of the the GPIO file parallel to the C-Program. Let me explain again in detail:

1. Set GPIO to low in C-Program: write (gpio_fd, "0", 1);

2. Checked with scope: the pin is still HIGH!

3. Checked File-Status in shell: "cat /sys/class/gpio/gpio113/value" - returns "0", so even linux thinks that the pin is low, but it is actually high, because the scope doesn't lie! :smileywink:

4. Tried to explicitely set the pin to LOW: "echo 0 > /sys/class/gpio/gpio113/value" - suddenly the pin goes to LOW.

But this behaviour is not really reproducable, it happens stochastic.

In my second post I tried to set the GPIO from shell only and did this in a loop. Also here it happens, so it is not related to the C code.

I tried the loop with different intervals (100ms/500ms) which are quite long, so I could easily measure it with a scope.

Re: Re: Vybrid GPIO on /sys/class/gpio does not switch properly

falstaff — Mon, 02 Mar 2015 17:42:24 GMT

Hi Johannes,

The reason the kernel does not see the value of the outside world is that the Input buffer is explicitly disabled in the old 3.0 kernel, when direction is "output":

http://git.toradex.com/cgit/linux-toradex.git/tree/arch/arm/plat-mxc/gpio-mvf.c?h=colibri_vf#n171

The driver upstream kernel driver explicitly does not change the input buffer, but leaves it up to the initial pinmux configuration whether the input buffer should be on or not (in our device trees we enable the input buffers for GPIO's):

http://git.toradex.com/cgit/linux-toradex.git/tree/drivers/pinctrl/freescale/pinctrl-imx.c?h=toradex_vf_3.18#n337

However, this does not explain the stochastic behavior. Honestly, I currently don't know how why this happens.

Stefan

Re: Re: Vybrid GPIO on /sys/class/gpio does not switch properly

johannestraxler — Tue, 03 Mar 2015 09:32:17 GMT

Dear Stefan,

thanks for your answer! Maybe I can modify the driver to be able to read the input buffer, so that I can set the output as long as the correct value is available, but this is just a workaround.

@Timesys Support: Can you check this issue and give feedback?

Thanks,

Johannes

Re: Vybrid GPIO on /sys/class/gpio does not switch properly

karina_valencia — Fri, 06 Mar 2015 18:52:24 GMT

timesyssupport can you share your comments about this case?

Re: Vybrid GPIO on /sys/class/gpio does not switch properly

karina_valencia — Mon, 09 Mar 2015 21:54:09 GMT

Timesys Support do you have an update?

Re: Vybrid GPIO on /sys/class/gpio does not switch properly

karina_valencia — Fri, 13 Mar 2015 15:00:51 GMT

reminder

Timesys Support do you have an update?

Re: Vybrid GPIO on /sys/class/gpio does not switch properly

timesyssupport — Tue, 17 Mar 2015 18:33:56 GMT

Hello Johannes,

Our apologies for the delay in replying; our ping notifications have been silently bouncing on our end.

Our Vybrid kernel maintainers are reviewing, but as we have moved our Vybrid support to a 3.13 kernel, futher developments on the 3.0 kernel would need to be addressed as an engineering engagement.

Regards,

Timesys Support

Re: Vybrid GPIO on /sys/class/gpio does not switch properly

johannestraxler — Wed, 18 Mar 2015 06:32:38 GMT

Dear Timesys,

Unfortunately it is currently not possible to build a 3.13 Kernel for "PHYTEC phyCORE Vybrid Development Kit/pcm052" in your build factory. It is no problem for us to switch to a newer kernel!

Thanks,

Johannes

Re: Vybrid GPIO on /sys/class/gpio does not switch properly

timesyssupport — Thu, 19 Mar 2015 11:22:27 GMT

Hello Johannes,

We understand that Phytec is working to update their kernel support to the 3.13 release; until they have validated the kernel support, we will only be able to provide the 3.0 kernel. If you have a pressing need for this kernel release, we would suggest contacting Phytec to determine their roadmap.

Thank you,

Timesys Support