Re: [PATCH v5 3/3] w1: add UART w1 bus driver

[Krzysztof Kozlowski <krzk@kernel.org>]

On 26/01/2024 16:42, Christoph Winklhofer via B4 Relay wrote:
> From: Christoph Winklhofer <cj.winklhofer@gmail.com>
> 
> Add a UART 1-Wire bus driver. The driver utilizes the UART interface via
> the Serial Device Bus to create the 1-Wire timing patterns. The driver
> was tested on a "Raspberry Pi 3B" with a DS18B20 and on a "Variscite
> DART-6UL" with a DS18S20 temperature sensor.
> 

...

> + * struct w1_uart_config - configuration for 1-Wire operation
> + *
> + * @baudrate: baud-rate returned from serdev
> + * @delay_us: delay to complete a 1-Wire cycle (in us)
> + * @tx_byte: byte to generate 1-Wire timing pattern
> + */
> +struct w1_uart_config {
> +	unsigned int baudrate;
> +	unsigned int delay_us;
> +	u8 tx_byte;
> +};
> +
> +struct w1_uart_device {
> +	struct serdev_device *serdev;
> +	struct w1_bus_master bus;
> +
> +	struct w1_uart_config cfg_reset;
> +	struct w1_uart_config cfg_touch_0;
> +	struct w1_uart_config cfg_touch_1;
> +
> +	struct completion rx_byte_received;
> +	int rx_err;
> +	u8 rx_byte;
> +

Missing documentation of mutex scope. What does it protect?

> +	struct mutex mutex;
> +};
> +

...

> +/*
> + * Send one byte (tx_byte) and read one byte (rx_byte) via serdev.
> + */
> +static int w1_uart_serdev_tx_rx(struct w1_uart_device *w1dev,
> +				const struct w1_uart_config *w1cfg, u8 *rx_byte)
> +{
> +	struct serdev_device *serdev = w1dev->serdev;
> +	int ret;
> +
> +	serdev_device_write_flush(serdev);
> +	serdev_device_set_baudrate(serdev, w1cfg->baudrate);
> +
> +	/* write and immediately read one byte */
> +	reinit_completion(&w1dev->rx_byte_received);
> +	ret = serdev_device_write_buf(serdev, &w1cfg->tx_byte, 1);
> +	if (ret != 1)
> +		return -EIO;
> +	ret = wait_for_completion_interruptible_timeout(
> +		&w1dev->rx_byte_received, W1_UART_TIMEOUT);
> +	if (ret <= 0)
> +		return -EIO;
> +
> +	/* locking could fail during driver remove or when serdev is

It's not netdev, so:
/*
 *

> +	 * unexpectedly in the receive callback.
> +	 */
> +	if (!mutex_trylock(&w1dev->mutex))
> +		return -EIO;
> +
> +	ret = w1dev->rx_err;
> +	if (ret == 0)
> +		*rx_byte = w1dev->rx_byte;
> +
> +	if (w1cfg->delay_us > 0)
> +		fsleep(w1cfg->delay_us);
> +
> +	mutex_unlock(&w1dev->mutex);
> +
> +	return ret;
> +}
> +
> +static ssize_t w1_uart_serdev_receive_buf(struct serdev_device *serdev,
> +					  const u8 *buf, size_t count)
> +{
> +	struct w1_uart_device *w1dev = serdev_device_get_drvdata(serdev);
> +
> +	mutex_lock(&w1dev->mutex);
> +
> +	/* sent a single byte and receive one single byte */
> +	if (count == 1) {
> +		w1dev->rx_byte = buf[0];
> +		w1dev->rx_err = 0;
> +	} else {
> +		w1dev->rx_err = -EIO;
> +	}
> +
> +	mutex_unlock(&w1dev->mutex);
> +	complete(&w1dev->rx_byte_received);
> +
> +	return count;
> +}
> +
> +static const struct serdev_device_ops w1_uart_serdev_ops = {
> +	.receive_buf = w1_uart_serdev_receive_buf,
> +	.write_wakeup = serdev_device_write_wakeup,
> +};
> +
> +/*
> + * 1-wire reset and presence detect: A present slave will manipulate
> + * the received byte by pulling the 1-Wire low.
> + */
> +static u8 w1_uart_reset_bus(void *data)
> +{
> +	struct w1_uart_device *w1dev = data;
> +	const struct w1_uart_config *w1cfg = &w1dev->cfg_reset;
> +	int ret;
> +	u8 val;
> +
> +	ret = w1_uart_serdev_tx_rx(w1dev, w1cfg, &val);
> +	if (ret < 0)
> +		return -1;
> +
> +	/* Device present (0) or no device (1) */
> +	return val != w1cfg->tx_byte ? 0 : 1;
> +}
> +
> +/*
> + * 1-Wire read and write cycle: Only the read-0 manipulates the
> + * received byte, all others left the line untouched.
> + */
> +static u8 w1_uart_touch_bit(void *data, u8 bit)
> +{
> +	struct w1_uart_device *w1dev = data;
> +	const struct w1_uart_config *w1cfg = bit ? &w1dev->cfg_touch_1 :
> +						   &w1dev->cfg_touch_0;
> +	int ret;
> +	u8 val;
> +
> +	ret = w1_uart_serdev_tx_rx(w1dev, w1cfg, &val);
> +
> +	/* return inactive bus state on error */
> +	if (ret < 0)
> +		return 1;
> +
> +	return val == w1cfg->tx_byte ? 1 : 0;
> +}
> +
> +static int w1_uart_probe(struct serdev_device *serdev)
> +{
> +	struct device *dev = &serdev->dev;
> +	struct w1_uart_device *w1dev;
> +	int ret;
> +
> +	w1dev = devm_kzalloc(dev, sizeof(*w1dev), GFP_KERNEL);
> +	if (!w1dev)
> +		return -ENOMEM;
> +	w1dev->bus.data = w1dev;
> +	w1dev->bus.reset_bus = w1_uart_reset_bus;
> +	w1dev->bus.touch_bit = w1_uart_touch_bit;
> +	w1dev->serdev = serdev;
> +
> +	init_completion(&w1dev->rx_byte_received);
> +	mutex_init(&w1dev->mutex);
> +
> +	ret = w1_uart_serdev_open(w1dev);
> +	if (ret < 0)
> +		return ret;
> +	serdev_device_set_drvdata(serdev, w1dev);
> +	serdev_device_set_client_ops(serdev, &w1_uart_serdev_ops);
> +
> +	return w1_add_master_device(&w1dev->bus);
> +}
> +
> +static void w1_uart_remove(struct serdev_device *serdev)
> +{
> +	struct w1_uart_device *w1dev = serdev_device_get_drvdata(serdev);
> +
> +	mutex_lock(&w1dev->mutex);
> +
> +	w1_remove_master_device(&w1dev->bus);
> +
> +	mutex_unlock(&w1dev->mutex);

This is still suspicious. You do not have serdev_device_close and you
want to protect from concurrent access but it looks insufficient.

This code assumes that:

w1_uart_remove()
  <-- here concurrent read/write might start
  mutex_lock()
  w1_remove_master_device()
  mutex_unlock()
  <-- now w1_uart_serdev_tx_rx() or w1_uart_serdev_receive_buf() can be
executed, but device is removed. So what's the point of the mutex here?

What exactly is protected by the mutex? So far it looks like only some
contents of w1dev, but it does not matter, because it that memory is
still valid at this point.

After describing what is protected we can think whether it is really
protected...


> 

Best regards,
Krzysztof