Poor performance

[klauweg]

I'm using libgpiod on bananaPi Zero M2.
libgpiod version is 2.2.1.
A simple library call like
gpiod_line_request_set_value(matrix, O_CLK, GPIOD_LINE_VALUE_ACTIVE);
needs about 3ms.
Setting a value using a line subset needs about 10ms.
This feels quite slow. Is that normal behaviour?
Unfortunately i'm not very experienced. So i played around with google-profiler and get this output:

     443  86.5%  86.5%      443  86.5% __libc_do_syscall
      16   3.1%  89.6%       28   5.5% gpiod_line_request_set_values_subset
      12   2.3%  92.0%       12   2.3% __GI___ioctl
      12   2.3%  94.3%       12   2.3% offset_to_bit (inline)
       9   1.8%  96.1%        9   1.8% gpiod_line_request_set_value
       4   0.8%  96.9%        4   0.8% _init@1b24
       4   0.8%  97.7%        4   0.8% gpiod_ioctl
       4   0.8%  98.4%        4   0.8% gpiod_line_mask_set_bit
       3   0.6%  99.0%        3   0.6% main
       2   0.4%  99.4%        3   0.6% gpiod_line_mask_assign_bit
       1   0.2%  99.6%        1   0.2% 0xbef9a5d8
       1   0.2%  99.8%        1   0.2% _init@7dc
       1   0.2% 100.0%        1   0.2% gpiod_line_mask_set_bit (inline)

So most of the time it is stuck with __libc_do_syscall?
How can this be related to libgpiod?
Any hints are welcome :-).

[Ablu]

@klauweg libgpiod is simply a library that wraps the Linux kernel API. That API is defined by syscalls, then then will eventually end up calling into your hardware-specific GPIO driver.

Your findings (though I do not fully understand exactly what you traced) point towards that driver being slow. So you might want to debug where exactly that time is spent in the kernel (FTRACE might be a good thing to look into as first step).

libgpiod itself is unlikely to be the culprit here.

[klauweg]

Ok, this ftrace thing looks very interesting.
I will investigate this further by myself.
Thank you for the tip!

[brgl]

Is this the SoC's controller or some external, I2C expander? Which driver is it using exactly?

[klauweg]

Thank you for reply.
It is the SoC controller.
(in my case using gpiochip0)

gpiodetect
gpiochip0 [1c20800.pinctrl] (224 lines)
gpiochip1 [1f02c00.pinctrl] (32 lines)

Of course i will provide more information if needed.

[brgl]

Yes, I still have no idea which driver you're using.

[klauweg]

Sorry for my ignorance. I'm new to this topic.
I assume this entry identifies the driver you are talking about:

[    1.644440] gpio gpiochip0: Static allocation of GPIO base is deprecated, use dynamic allocation.
[    1.647958] sun8i-h3-pinctrl 1c20800.pinctrl: initialized sunXi PIO driver