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SubjectRe: [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.

>
> 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.

> + * 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.

>
> 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.
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.

> + 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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