Re: [PATCH v7 3/3] leds: trigger: implement a tty trigger

[Johan Hovold <johan@kernel.org>]

On Tue, Jul 07, 2020 at 06:59:58PM +0200, Uwe Kleine-König wrote:
> Usage is as follows:
> 
> 	myled=ledname
> 	tty=ttyS0
> 
> 	echo tty > /sys/class/leds/$myled/trigger
> 	echo $tty > /sys/class/leds/$myled/ttyname
> 
> . When this new trigger is active it periodically checks the tty's
> statistics and when it changed since the last check the led is flashed
> once.
> 
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> ---
>  .../ABI/testing/sysfs-class-led-trigger-tty   |   6 +
>  drivers/leds/trigger/Kconfig                  |   7 +
>  drivers/leds/trigger/Makefile                 |   1 +
>  drivers/leds/trigger/ledtrig-tty.c            | 192 ++++++++++++++++++
>  4 files changed, 206 insertions(+)
>  create mode 100644 Documentation/ABI/testing/sysfs-class-led-trigger-tty
>  create mode 100644 drivers/leds/trigger/ledtrig-tty.c
> 
> diff --git a/Documentation/ABI/testing/sysfs-class-led-trigger-tty b/Documentation/ABI/testing/sysfs-class-led-trigger-tty
> new file mode 100644
> index 000000000000..5c53ce3ede36
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-class-led-trigger-tty
> @@ -0,0 +1,6 @@
> +What:		/sys/class/leds/<led>/ttyname
> +Date:		Jul 2020
> +KernelVersion:	5.8
> +Contact:	linux-leds@vger.kernel.org
> +Description:
> +		Specifies the tty device name of the triggering tty
> diff --git a/drivers/leds/trigger/Kconfig b/drivers/leds/trigger/Kconfig
> index ce9429ca6dde..40ff08c93f56 100644
> --- a/drivers/leds/trigger/Kconfig
> +++ b/drivers/leds/trigger/Kconfig
> @@ -144,4 +144,11 @@ config LEDS_TRIGGER_AUDIO
>  	  the audio mute and mic-mute changes.
>  	  If unsure, say N
>  
> +config LEDS_TRIGGER_TTY
> +	tristate "LED Trigger for TTY devices"
> +	depends on TTY
> +	help
> +	  This allows LEDs to be controlled by activity on ttys which includes
> +	  serial devices like /dev/ttyS0.
> +
>  endif # LEDS_TRIGGERS
> diff --git a/drivers/leds/trigger/Makefile b/drivers/leds/trigger/Makefile
> index 733a83e2a718..25c4db97cdd4 100644
> --- a/drivers/leds/trigger/Makefile
> +++ b/drivers/leds/trigger/Makefile
> @@ -15,3 +15,4 @@ obj-$(CONFIG_LEDS_TRIGGER_PANIC)	+= ledtrig-panic.o
>  obj-$(CONFIG_LEDS_TRIGGER_NETDEV)	+= ledtrig-netdev.o
>  obj-$(CONFIG_LEDS_TRIGGER_PATTERN)	+= ledtrig-pattern.o
>  obj-$(CONFIG_LEDS_TRIGGER_AUDIO)	+= ledtrig-audio.o
> +obj-$(CONFIG_LEDS_TRIGGER_TTY)		+= ledtrig-tty.o
> diff --git a/drivers/leds/trigger/ledtrig-tty.c b/drivers/leds/trigger/ledtrig-tty.c
> new file mode 100644
> index 000000000000..e44e2202fa34
> --- /dev/null
> +++ b/drivers/leds/trigger/ledtrig-tty.c
> @@ -0,0 +1,192 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/delay.h>
> +#include <linux/leds.h>
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +#include <linux/tty.h>
> +#include <uapi/linux/serial.h>
> +
> +struct ledtrig_tty_data {
> +	struct led_classdev *led_cdev;
> +	struct delayed_work dwork;
> +	struct mutex mutex;
> +	const char *ttyname;
> +	struct tty_struct *tty;
> +	int rx, tx;
> +};
> +
> +static void ledtrig_tty_halt(struct ledtrig_tty_data *trigger_data)
> +{
> +	cancel_delayed_work_sync(&trigger_data->dwork);
> +}

> +static ssize_t ttyname_store(struct device *dev,
> +			     struct device_attribute *attr, const char *buf,
> +			     size_t size)
> +{
> +	struct ledtrig_tty_data *trigger_data = led_trigger_get_drvdata(dev);
> +	char *ttyname;
> +	ssize_t ret = size;
> +
> +	ledtrig_tty_halt(trigger_data);
> +
> +	mutex_lock(&trigger_data->mutex);
> +
> +	if (size > 0 && buf[size - 1] == '\n')
> +		size -= 1;
> +
> +	if (size) {
> +		ttyname = kmemdup_nul(buf, size, GFP_KERNEL);
> +		if (!ttyname) {
> +			ret = -ENOMEM;
> +			goto out_unlock;
> +		}
> +	} else {
> +		ttyname = NULL;
> +	}
> +
> +	kfree(trigger_data->ttyname);
> +	tty_kref_put(trigger_data->tty);
> +	trigger_data->tty = NULL;
> +
> +	trigger_data->ttyname = ttyname;
> +
> +out_unlock:
> +	mutex_unlock(&trigger_data->mutex);
> +
> +	if (ttyname)
> +		ledtrig_tty_restart(trigger_data);
> +
> +	return ret;
> +}
> +static DEVICE_ATTR_RW(ttyname);
> +
> +static void ledtrig_tty_work(struct work_struct *work)
> +{
> +	struct ledtrig_tty_data *trigger_data =
> +		container_of(work, struct ledtrig_tty_data, dwork.work);
> +	struct serial_icounter_struct icount;
> +	int ret;
> +	bool firstrun = false;
> +
> +	mutex_lock(&trigger_data->mutex);
> +
> +	BUG_ON(!trigger_data->ttyname);
> +
> +	/* try to get the tty corresponding to $ttyname */
> +	if (!trigger_data->tty) {
> +		dev_t devno;
> +		struct tty_struct *tty;
> +		int ret;
> +
> +		firstrun = true;
> +
> +		ret = tty_dev_name_to_number(trigger_data->ttyname, &devno);
> +		if (ret < 0)
> +			/*
> +			 * A device with this name might appear later, so keep
> +			 * retrying.
> +			 */
> +			goto out;
> +
> +		tty = tty_kopen_shared(devno);
> +		if (IS_ERR(tty) || !tty)
> +			/* What to do? retry or abort */
> +			goto out;
> +
> +		trigger_data->tty = tty;
> +	}
> +
> +	ret = tty_get_icount(trigger_data->tty, &icount);
> +	if (ret)
> +		return;
> +
> +	while (firstrun ||
> +	       icount.rx != trigger_data->rx ||
> +	       icount.tx != trigger_data->tx) {
> +
> +		led_set_brightness(trigger_data->led_cdev, LED_ON);
> +
> +		msleep(100);
> +
> +		led_set_brightness(trigger_data->led_cdev, LED_OFF);
> +
> +		trigger_data->rx = icount.rx;
> +		trigger_data->tx = icount.tx;
> +		firstrun = false;
> +
> +		ret = tty_get_icount(trigger_data->tty, &icount);
> +		if (ret)
> +			return;
> +	}

Haven't looked at the latest proposal in detail, but this looks broken
as you can potentially loop indefinitely in a worker thread, and with no
way to stop the trigger (delayed work).

> +
> +out:
> +	mutex_unlock(&trigger_data->mutex);
> +	schedule_delayed_work(&trigger_data->dwork, msecs_to_jiffies(100));
> +}

Johan