User-Space Vs Kernel-Space GPIO Speed Test

How accurate is the Linux kernel speed to handle a GPIO pin? what's the smallest time we can indeed achieve, but at same time stable, which allow us control from an user-space application or kernel-space driver a GPIO pin? Below you can see a test made with Broadcom BCM2835 SoC (Raspberry Pi / Raspbian) running at 700 Mhz.

Using the function "usleep()" in the user-space C application and high resolution timers (hrtimer) in the kernel-space driver, Linux kernel 3.12.32 with and without the real-time patch (RT). First without touch the system (Idle) and then with the 'top' tool running while keyboard 'down' button pressed.

The table below show the results, 'Programmed Time' is the smallest time found after some tests as the stable/usable, that start makes sense with the value supplied in the functions: usleep() and hrtimer.

	RT patch	Programmed Time(uS)	Result Idle(uS)	Variation Idle(uS)	Result Top(uS)	Variation Top(uS)
user-space	no	100	94-104	10	110-134	24
user-space	yes	100	94-108	14	124-160	36
kernel-space	no	20	19-20	1	28-25	7
kernel-space	yes	40	39-42	3	36-50	14

What happens with usleep(1) ?

The minimal value for usleep() function is 1, that give us 1 uS, right? wrong! look what happens:

As we can see, there are a 'delay' of 76 uS. This explain why usleep(100) give us not 100 uS, but 176 uS, look:

So, if you want a 100 uS time, you need use usleep(100-76).

What happens with hrtimer set to 1 nanosecond?

Instability! nothing more than noise and random times! 20 uS was the smallest value found as stable/usable without the RT patch:

And with the RT patch, 40 uS:

The idea of the Linux kernel RT patch is improve the performance of user-space applications, "giving more CPU time", applications run on user-space, so where this 'extra' CPU time came from? as we could see, with the RT patch the kernel-space time increased from 20 uS to 40 uS, giving us the answer. The best scenario for GPIO speed, is the Linux kernel without the RT patch and from kernel-space, achieving 20 uS as the smallest, controllable and stable time.