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SubjectRE: [PATCH] media: rc: gpio-ir-recv: add QoS support for cpuidle system
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Hi Sean,

Thanks a lot for your review.

> -----Original Message-----
> From: Sean Young <sean@mess.org>
> Sent: 2020年9月15日 17:34
> To: Joakim Zhang <qiangqing.zhang@nxp.com>
> Cc: mchehab@kernel.org; linux-media@vger.kernel.org;
> linux-kernel@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>
> Subject: Re: [PATCH] media: rc: gpio-ir-recv: add QoS support for cpuidle
> system
>
>
> Hi Joakim,
>
> Thanks for your patch, I think it looks good in principle but needs a few small
> fixes.
>
> On Tue, Sep 15, 2020 at 11:02:02PM +0800, Joakim Zhang wrote:
> > GPIO IR receive is much rely on interrupt response, uneven interrupt
> > latency will lead to incorrect timing, so the decoder fails to decode
> > it. The issue is particularly acute on systems which supports cpuidle,
> > dynamically disable and enable cpuidle can solve this problem to a
> > great extent.
>
> This is the first soc to be affected by this problem, and gpio-ir-recv has been
> used on my systems. For example, the raspberry pi has cpu idle enabled and
> does not suffer from this problem. There are many more; this driver has been
> used on many arm devices, which will have cpuidle enabled.
I have not noticed raspberry pi enabled cpu idle in Linux, could you point me where it is? Then I can have a look, whether it implements multiple idle or not, to see why it makes difference.


> >
> > However, there is a downside to this approach, the measurement of
> > header on the first frame may incorrect. Test on i.MX8M serials, when
> > enable cpuidle, interrupt latency could be about 500us.
> >
> > With this patch:
> > 1. has no side effect on non-cpuidle system.
> > 2. latency is still much longer for the first gpio interrupt on
> > cpuidle system, so the first frame may not be decoded. Generally, RC
> > would transmit multiple frames at once press, we can sacrifice the first
> frame.
> >
> > Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
> > ---
> > drivers/media/rc/gpio-ir-recv.c | 49
> > ++++++++++++++++++++++++++++++++-
> > 1 file changed, 48 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/media/rc/gpio-ir-recv.c
> > b/drivers/media/rc/gpio-ir-recv.c index a20413008c3c..42c942ce98cd
> > 100644
> > --- a/drivers/media/rc/gpio-ir-recv.c
> > +++ b/drivers/media/rc/gpio-ir-recv.c
> > @@ -11,6 +11,8 @@
> > #include <linux/of.h>
> > #include <linux/of_gpio.h>
> > #include <linux/platform_device.h>
> > +#include <linux/pm_runtime.h>
> > +#include <linux/pm_qos.h>
> > #include <linux/irq.h>
> > #include <media/rc-core.h>
> >
> > @@ -20,17 +22,36 @@ struct gpio_rc_dev {
> > struct rc_dev *rcdev;
> > struct gpio_desc *gpiod;
> > int irq;
> > + struct pm_qos_request qos;
> > };
> >
> > static irqreturn_t gpio_ir_recv_irq(int irq, void *dev_id) {
> > - int val;
> > + int ret, val;
> > struct gpio_rc_dev *gpio_dev = dev_id;
> > + struct device *dev = gpio_dev->rcdev->dev.parent;
> > +
> > + /*
> > + * For cpuidle system:
>
> For some cpuidle systems, not all. This is why this feature needs a device tree
> option for enabling. Otherwise, it will negatively affect power usage on e.g.
> raspberry pi.
Yes, you are right. As you said, some cpu idle systems may not suffer such case, I will add a device tree property.


> > + * Respond to interrupt taking more latency when cpu in idle.
> > + * Invoke asynchronous pm runtime get from interrupt context,
> > + * this may introduce a millisecond delay to call resume callback,
> > + * where to disable cpuilde.
> > + *
> > + * Two issues lead to fail to decode first frame, one is latency to
> > + * respond interupt, another is delay introduced by async api.
> > + */
> > + ret = pm_runtime_get(dev);
> > + if (ret < 0)
> > + return IRQ_NONE;
>
> If we end up here, we also abandon sending the IR to rc-core (below). I don't
> think it should do that. It should call ir_raw_event_store_edge() always even if
> it can't do the pm things.
Make sense, I will remove this error check.


> >
> > val = gpiod_get_value(gpio_dev->gpiod);
> > if (val >= 0)
> > ir_raw_event_store_edge(gpio_dev->rcdev, val == 1);
> >
> > + pm_runtime_mark_last_busy(dev);
> > + pm_runtime_put_autosuspend(dev);
> > +
> > return IRQ_HANDLED;
> > }
> >
> > @@ -92,6 +113,12 @@ static int gpio_ir_recv_probe(struct
> > platform_device *pdev)
> >
> > platform_set_drvdata(pdev, gpio_dev);
> >
> > +
> > + pm_runtime_set_autosuspend_delay(dev, (rcdev->timeout / 1000 /
> > +1000));
>
> rcdev->timeout is in microseconds (since very recently), so this is wrong.
What this meaning, is that rcdev->timeout woud change the unit, to be microseconds, not nanoseconds any more ?


> Also, the timeout can be changed using the LIRC_SET_REC_TIMEOUT ioctl
> (using ir-ctl -t in userspace). The autosuspend delay should be updated when
> this happens. This can be done by implementing the s_timeout rcdev function.
Make sense, could you point me where s_timeout funcition has implemented? So that I can refer to it, I am not familiar with this.


BRs,
Joakim
> > + pm_runtime_use_autosuspend(dev);
> > + pm_runtime_set_suspended(dev);
> > + pm_runtime_enable(dev);
> > +
> > return devm_request_irq(dev, gpio_dev->irq, gpio_ir_recv_irq,
> > IRQF_TRIGGER_FALLING | IRQF_TRIGGER_RISING,
> > "gpio-ir-recv-irq", gpio_dev);
> > @@ -122,9 +149,29 @@ static int gpio_ir_recv_resume(struct device *dev)
> > return 0;
> > }
> >
> > +static int gpio_ir_recv_runtime_suspend(struct device *dev) {
> > + struct gpio_rc_dev *gpio_dev = dev_get_drvdata(dev);
> > +
> > + cpu_latency_qos_remove_request(&gpio_dev->qos);
> > +
> > + return 0;
> > +}
> > +
> > +static int gpio_ir_recv_runtime_resume(struct device *dev) {
> > + struct gpio_rc_dev *gpio_dev = dev_get_drvdata(dev);
> > +
> > + cpu_latency_qos_add_request(&gpio_dev->qos, 0);
> > +
> > + return 0;
> > +}
> > +
> > static const struct dev_pm_ops gpio_ir_recv_pm_ops = {
> > .suspend = gpio_ir_recv_suspend,
> > .resume = gpio_ir_recv_resume,
> > + .runtime_suspend = gpio_ir_recv_runtime_suspend,
> > + .runtime_resume = gpio_ir_recv_runtime_resume,
> > };
> > #endif
> >
> > --
> > 2.17.1
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