Re: [PATCH] media: rc: pwm-ir-tx: use hrtimer for edge generation
From: Ivaylo Dimitrov
Date: Sat Oct 14 2023 - 03:24:25 EST
Hi,
On 13.10.23 г. 14:08 ч., Sean Young wrote:
On Thu, Oct 05, 2023 at 01:28:59PM +0300, Ivaylo Dimitrov wrote:
usleep_range() seems to suffer from scheduling latency of up to 400 us on
some platforms (like OMAP) which makes it unusable for IR pulses edge
timings. In the same time pwm_ir_trx() is called in a context with priority
which might not be suitable for real-time IR pulses generation.
Fix that by using hrtimer and a thread with sched_set_fifo() priority. That
way scheduling latency is compensated by the fact that PWM is controlled in
the thread after a completion is signalled in hrtimer function - we have
more or less the same latency for every edge. If completion comes earlier
than needed, we do udelay() till the exact time for the next edge. That
way pulse width generation is robust and precise and mostly independent of
the system load.
Tests on Nokia N900 showed that udelay() is called with up to 200 us in
worst cases, usually times are less that 100 us.
Cc the RT folks, I would like some input on this. How bad is this for
linux-rt or non-linux-rt for latency? Is this a problem, and if so is there
an #ifdef we can use to avoid it in certain builds?
I was rather thinking of conditionally enabling that only if the new API
pwm_can_sleep() (or whatever name you come-up in the end) is in-place
and returns true.
BTW, we can reduce the time we do udelay() to few us, by waiting for
completion twice for the first edge(that way compensating for the
hrtimer setup time), but I didn't do that on purpose, for the sake of
precision I would prefer to do usleep() a bit longer, than to have bad
edge timings.
But yeah, lets see what more experienced have to say.
Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@xxxxxxxxx>
---
drivers/media/rc/pwm-ir-tx.c | 122 ++++++++++++++++++++++++++++++++++++++++---
1 file changed, 115 insertions(+), 7 deletions(-)
diff --git a/drivers/media/rc/pwm-ir-tx.c b/drivers/media/rc/pwm-ir-tx.c
index 7732054c..cb6ce73 100644
--- a/drivers/media/rc/pwm-ir-tx.c
+++ b/drivers/media/rc/pwm-ir-tx.c
@@ -4,6 +4,7 @@
*/
#include <linux/kernel.h>
+#include <linux/kthread.h>
#include <linux/module.h>
#include <linux/pwm.h>
#include <linux/delay.h>
@@ -17,8 +18,16 @@
struct pwm_ir {
struct pwm_device *pwm;
+ struct hrtimer timer;
+ struct task_struct *tx_thread;
+ wait_queue_head_t tx_wq;
+ struct completion tx_done;
+ struct completion edge;
unsigned int carrier;
unsigned int duty_cycle;
+ unsigned int *txbuf;
+ unsigned int count;
+ unsigned int index;
};
static const struct of_device_id pwm_ir_of_match[] = {
@@ -48,12 +57,41 @@ static int pwm_ir_set_carrier(struct rc_dev *dev, u32 carrier)
return 0;
}
-static int pwm_ir_tx(struct rc_dev *dev, unsigned int *txbuf,
- unsigned int count)
+static enum hrtimer_restart pwm_ir_timer_cb(struct hrtimer *timer)
+{
+ struct pwm_ir *pwm_ir = container_of(timer, struct pwm_ir, timer);
+ ktime_t now;
+
+ /*
+ * If we happen to hit an odd latency spike, loop through the
+ * pulses until we catch up.
+ */
+ do {
+ u64 edge;
+
+ complete(&pwm_ir->edge);
+
+ if (pwm_ir->index >= pwm_ir->count)
+ return HRTIMER_NORESTART;
+
+ edge = US_TO_NS(pwm_ir->txbuf[pwm_ir->index]);
+ hrtimer_add_expires_ns(timer, edge);
+
+ pwm_ir->index++;
+
+ now = timer->base->get_time();
+
+ } while (hrtimer_get_expires_tv64(timer) < now);
+
+ return HRTIMER_RESTART;
+}
+
+static void _pwm_ir_tx(struct pwm_ir *pwm_ir)
{
- struct pwm_ir *pwm_ir = dev->priv;
struct pwm_device *pwm = pwm_ir->pwm;
struct pwm_state state;
+ unsigned int *txbuf = pwm_ir->txbuf;
+ unsigned int count = pwm_ir->count;
int i;
ktime_t edge;
long delta;
@@ -63,6 +101,8 @@ static int pwm_ir_tx(struct rc_dev *dev, unsigned int *txbuf,
state.period = DIV_ROUND_CLOSEST(NSEC_PER_SEC, pwm_ir->carrier);
pwm_set_relative_duty_cycle(&state, pwm_ir->duty_cycle, 100);
+ hrtimer_start(&pwm_ir->timer, 0, HRTIMER_MODE_REL);
+ wait_for_completion(&pwm_ir->edge);
edge = ktime_get();
for (i = 0; i < count; i++) {
@@ -70,14 +110,50 @@ static int pwm_ir_tx(struct rc_dev *dev, unsigned int *txbuf,
pwm_apply_state(pwm, &state);
edge = ktime_add_us(edge, txbuf[i]);
+ wait_for_completion(&pwm_ir->edge);
+
delta = ktime_us_delta(edge, ktime_get());
+
if (delta > 0)
- usleep_range(delta, delta + 10);
+ udelay(delta);
So you're sleeping if the timer arrives early, but what about if it arrives
late? No amount of sleeping will here there.
If it arrives late, nothing we can do, we just change the pwm state
immediately and move on, waiting for the next edge. Why shall we delay
more if we are already late?
}
state.enabled = false;
pwm_apply_state(pwm, &state);
+ pwm_ir->count = 0;
+}
+
+static int pwm_ir_thread(void *data)
+{
+ struct pwm_ir *pwm_ir = data;
+
+ for (;;) {
+ wait_event_idle(pwm_ir->tx_wq,
+ kthread_should_stop() || pwm_ir->count);
+
+ if (kthread_should_stop())
+ break;
+
+ _pwm_ir_tx(pwm_ir);
+ complete(&pwm_ir->tx_done);
+ }
+
+ return 0;
+}
Rather than creating a new thread, we could use re-use the process that
is doing the TX, i.e. put this in pwm_ir_tx() itself. That should work,
right?
It will work, definitely, but that would mean calling sched_set_fifo()
on the process/thread that is doing the TX, and I don't think this is a
good idea. I think this would open a DOS security hole, like, process
sending TX buffer and doing a busy loop will more or less freeze the
system for the duration of the TX buffer, no? Not necessarily a
malicious process, just a badly written one.
Unless I miss something and pwm_ir_tx() is executed in another context,
not in the userspace process doing write() (or ioctl() or whatever).
+
+static int pwm_ir_tx(struct rc_dev *dev, unsigned int *txbuf,
+ unsigned int count)
+{
+ struct pwm_ir *pwm_ir = dev->priv;
+
+ pwm_ir->txbuf = txbuf;
+ pwm_ir->count = count;
+ pwm_ir->index = 0;
+
+ wake_up(&pwm_ir->tx_wq);
+ wait_for_completion(&pwm_ir->tx_done);
+
return count;
}
@@ -91,12 +167,24 @@ static int pwm_ir_probe(struct platform_device *pdev)
if (!pwm_ir)
return -ENOMEM;
+ platform_set_drvdata(pdev, pwm_ir);
+
pwm_ir->pwm = devm_pwm_get(&pdev->dev, NULL);
if (IS_ERR(pwm_ir->pwm))
return PTR_ERR(pwm_ir->pwm);
- pwm_ir->carrier = 38000;
+ /* Use default, in case userspace does not set the carrier */
+ pwm_ir->carrier = DIV_ROUND_CLOSEST_ULL(pwm_get_period(pwm_ir->pwm),
+ NSEC_PER_SEC);
pwm_ir->duty_cycle = 50;
+ pwm_ir->count = 0;
+
+ init_waitqueue_head(&pwm_ir->tx_wq);
+ init_completion(&pwm_ir->edge);
+ init_completion(&pwm_ir->tx_done);
+
+ hrtimer_init(&pwm_ir->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+ pwm_ir->timer.function = pwm_ir_timer_cb;
rcdev = devm_rc_allocate_device(&pdev->dev, RC_DRIVER_IR_RAW_TX);
if (!rcdev)
@@ -110,14 +198,34 @@ static int pwm_ir_probe(struct platform_device *pdev)
rcdev->s_tx_carrier = pwm_ir_set_carrier;
rc = devm_rc_register_device(&pdev->dev, rcdev);
- if (rc < 0)
+ if (rc < 0) {
dev_err(&pdev->dev, "failed to register rc device\n");
+ return rc;
+ }
+
+ pwm_ir->tx_thread = kthread_create(pwm_ir_thread, pwm_ir, "%s/tx",
+ dev_name(&pdev->dev));
+ if (IS_ERR(pwm_ir->tx_thread))
+ return PTR_ERR(pwm_ir->tx_thread);
- return rc;
+ sched_set_fifo(pwm_ir->tx_thread);
+ wake_up_process(pwm_ir->tx_thread);
+
+ return 0;
+}
+
+static int pwm_ir_remove(struct platform_device *pdev)
+{
+ struct pwm_ir *pwm_ir = platform_get_drvdata(pdev);
+
+ kthread_stop(pwm_ir->tx_thread);
+
+ return 0;
}
static struct platform_driver pwm_ir_driver = {
.probe = pwm_ir_probe,
+ .remove = pwm_ir_remove,
.driver = {
.name = DRIVER_NAME,
.of_match_table = pwm_ir_of_match,
--
1.9.1