Periodic task in a Linux kernel module
-
25-10-2019 - |
Question
Currently I am developing GPIO kernel module for friendlyarm Linux 2.6.32.2 (mini2440). I am from electronics background and new to Linux.
The kernel module loaded at start-up and the related device file is located in /dev
as gpiofreq
.
At first time writing to device file, GPIO pin toggles continuously at 50kHz. At second time writing it stop toggling. At third time, it starts again, and so on.
I have wrote separate kernel module to generate freq. but CPU freezes after writing device file at first time. The terminal prompt is shown but I can not run any command afterwards.
Here is the code-snippet:
//calling function which generates continuous freq at gpio
static int send_freq(void *arg)
{
set_current_state(TASK_INTERRUPTIBLE);
for(;;)
{
gpio_set_value(192,1);
udelay(10);
gpio_set_value(192,0);
udelay(10);
}
return 0;
}
Here is the device write code, which start or stop with any data written to device file.
if(toggle==0)
{
printk("Starting Freq.\n");
task=kthread_run(&send_freq,(void *)freq,"START");
toggle=1;
}
else
{
printk("Operation Terminated.\n");
i = kthread_stop(task);
toggle=0;
}
Solution
You are doing an infinite loop in a kernel thread, there is no room for anything else to happen, except IRQ and maybe other kernel thread.
What you could do is either
program a timer on your hardware and do your pin toggling in an interrupt
replace udelay with usleep_range
I suggest doing thing progressively, and starting in the kHz range with usleep_range, and eventually moving to cust om timer + ISR
in either case, you will probably have a lot of jitter, and doing such gpio toggling may be a good idea on a DSP or a PIC, but is a waste of resources on ARM + Linux, unless you are hardware assisted with pwm capable gpio engine.