CODEWITHSUNDEEP
timerlinux-kernelreal-time
The_Tim
The_Tim

Reputation: 21

Is it possible to modify hrtimer parameters from within a custom kernel module?

Is there a way to adjust an hrtimer's parameters (specifically I want to adjust min_delta_ns) from within a kernel module?

I'm writing a kernel module that has some outputs driven by an hrtimer. Here's a rough outline of the basic code:

#include <linux/hrtimer.h>
#include <linux/sched.h>

#define MAXRUNS 300000
#define PERIOD_IN_NS 100000

static struct hrtimer hr_timer;
static ktime_t ktime_period_ns;
static volatile int runcount = 0;

static int some_function(parameters) {
    ktime_period_ns= ktime_set( 0, PERIOD_IN_NS );
    hrtimer_init ( &hr_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL );
    htimer.function = timer_callback;
    hrtimer_start( &hr_timer, ktime_period_ns, HRTIMER_MODE_REL );

    return 0;
}

static enum hrtimer_restart timer_callback(struct hrtimer *timer)
{
        runcount++;

        if (runcount < MAXRUNS) {
            // do stuff
            hrtimer_forward_now(&hr_timer, ktime_period_ns);
            return HRTIMER_RESTART;
        } else {
            runcount = 0;
            return HRTIMER_NORESTART;
        }
}

When I run it with PERIOD_IN_NS of 100,000 or greater everything works great. However, if I drop that value to say, 50,000, the period of my clamps at around 90,000 (ish) and output becomes unpredictable.

I ran cat /proc/timer_list to get the details of my timers and here are the details what I believe is the relevant timer:

Tick Device: mode:     1
Per CPU device: 0
Clock Event Device: mxc_timer1
 max_delta_ns:   1431655752223
 min_delta_ns:   85000
 mult:           6442451
 shift:          31
 mode:           3
 next_event:     13571723000000 nsecs
 set_next_event: v2_set_next_event
 set_mode:       mxc_set_mode
 event_handler:  hrtimer_interrupt
 retries:        0

From what I've read about how hrtimer works, that min_delta_ns of 85000 means that I can't run interrupts with a period any smaller than 85,000 nanoseconds. I'd like to try to decrease that value to see if I can get my code to cycle any faster without detrimental effects to the system (I'm running this on Raspberry-Pi-like dev board called the HummingBoard).

It looks like this clock is being initially configured in my specific architecture's version of time.c (line 180), but I can't figure out how to access and modify the values outside of that context in my custom kernel module.

Is there a way to adjust the values of an hrtimer's parameters from within my kernel module?

Upvotes: 2

Views: 618

Answers (1)

CL.
CL.

Reputation: 180260

The min_delta_ns value describes a property of the hardware device. Even if you were able to change this value (which you cannot), the timer event would not actually arrive any faster.

Upvotes: 1

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