linux-kernel - Re: [PATCH v9 3/3] leds: trigger: implement a tty trigger

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Message-ID: <20201020123609.GD19856@duo.ucw.cz>
Date:   Tue, 20 Oct 2020 14:36:09 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Dan Murphy <dmurphy@...com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, kernel@...gutronix.de,
        linux-kernel@...r.kernel.org, Johan Hovold <johan@...nel.org>,
        linux-leds@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v9 3/3] leds: trigger: implement a tty trigger

Hi!

> 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@...gutronix.de>
> ---

> +static ssize_t ttyname_show(struct device *dev,
> +			    struct device_attribute *attr, char *buf)
> +{
> +	struct ledtrig_tty_data *trigger_data = led_trigger_get_drvdata(dev);
> +	ssize_t len = 0;

Unused value. Not sure if some checker will complain.

> +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;

If this happens, you return error to the user and
trigger_data->ttyname remains with the old value, but trigger is now
stopped. That is not exactly consistent.


> +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;
> +
> +	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;
> +
> +		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;

Poll every 100 msec... Hmm.... Okay, I guess?

> +	ret = tty_get_icount(trigger_data->tty, &icount);
> +	if (ret) {
> +		mutex_unlock(&trigger_data->mutex);
> +		dev_info(trigger_data->tty->dev, "Failed to get icount, stopped polling\n");
> +		mutex_unlock(&trigger_data->mutex);

Eh?

> +		return;
> +	}
> +
> +	if (icount.rx != trigger_data->rx ||
> +	    icount.tx != trigger_data->tx) {
> +		led_set_brightness(trigger_data->led_cdev, LED_ON);

Are you sure about LED_ON here? It should use current brightness
selected by brightness file...

> +static int ledtrig_tty_activate(struct led_classdev *led_cdev)
> +{
> +	struct ledtrig_tty_data *trigger_data;
> +
> +	trigger_data = kzalloc(sizeof(*trigger_data), GFP_KERNEL);
> +	if (!trigger_data)
> +		return -ENOMEM;
> +
> +	led_set_trigger_data(led_cdev, trigger_data);
> +
> +	INIT_DELAYED_WORK(&trigger_data->dwork, ledtrig_tty_work);
> +	trigger_data->led_cdev = led_cdev;
> +	mutex_init(&trigger_data->mutex);

How is this protected from concurrent access from sysfs?

Best regards,
							Pavel

-- 
http://www.livejournal.com/~pavelmachek

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