Re: [PATCH 2/2] w1: add UART w1 bus driver

[Krzysztof Kozlowski]

On 17/12/2023 13:20, Christoph Winklhofer wrote:
> 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.
> 
> The 1-wire timing pattern and the corresponding UART baud-rate with the
> interpretation of the transferred bytes are described in the document:
> 
> Link: https://www.analog.com/en/technical-articles/using-a-uart-to-implement-a-1wire-bus-master.html
> 
> In short, the UART peripheral must support full-duplex and operate in
> open-drain mode. The timing patterns are generated by a specific
> combination of baud-rate and transmitted byte, which corresponds to a
> 1-Wire read bit, write bit or reset.
> 
> Signed-off-by: Christoph Winklhofer <cj.winklhofer@xxxxxxxxx>
> ---
>  Documentation/w1/masters/index.rst   |   1 +
>  Documentation/w1/masters/w1-uart.rst |  48 +++++++
>  drivers/w1/masters/Kconfig           |  10 ++
>  drivers/w1/masters/Makefile          |   1 +
>  drivers/w1/masters/w1-uart.c         | 189 +++++++++++++++++++++++++++
>  5 files changed, 249 insertions(+)
>  create mode 100644 Documentation/w1/masters/w1-uart.rst
>  create mode 100644 drivers/w1/masters/w1-uart.c
> 
> diff --git a/Documentation/w1/masters/index.rst b/Documentation/w1/masters/index.rst
> index 4442a98850ad..cc40189909fd 100644
> --- a/Documentation/w1/masters/index.rst
> +++ b/Documentation/w1/masters/index.rst
> @@ -12,3 +12,4 @@
>     mxc-w1
>     omap-hdq
>     w1-gpio
> +   w1-uart
> diff --git a/Documentation/w1/masters/w1-uart.rst b/Documentation/w1/masters/w1-uart.rst
> new file mode 100644
> index 000000000000..0f546cf77f13
> --- /dev/null
> +++ b/Documentation/w1/masters/w1-uart.rst
> @@ -0,0 +1,48 @@
> +.. SPDX-License-Identifier: GPL-2.0-or-later
> +
> +=====================
> +Kernel driver w1-uart
> +=====================
> +
> +Author: Christoph Winklhofer <cj.winklhofer@xxxxxxxxx>
> +
> +
> +Description
> +-----------
> +
> +UART 1-wire bus driver. The driver utilizes the UART interface via the
> +Serial Device Bus to create the 1-Wire timing patterns as described in
> +the document `"Using a UART to Implement a 1-Wire Bus Master"`_.
> +
> +.. _"Using a UART to Implement a 1-Wire Bus Master": https://www.analog.com/en/technical-articles/using-a-uart-to-implement-a-1wire-bus-master.html
> +
> +In short, the UART peripheral must support full-duplex and operate in
> +open-drain mode. The timing patterns are generated by a specific
> +combination of baud-rate and transmitted byte, which corresponds to a
> +1-Wire read bit, write bit or reset pulse.
> +
> +For instance the timing pattern for a 1-Wire reset and presence detect uses
> +the baud-rate 9600, i.e. 104.2 us per bit. The transmitted byte 0xf0 over
> +UART (least significant bit first, start-bit low) sets the reset low time
> +for 1-Wire to 521 us. A present 1-Wire device changes the received byte by
> +pulling the line low, which is used by the driver to evaluate the result of
> +the 1-Wire operation.
> +
> +Similar for a 1-Wire read bit or write bit, which uses the baud-rate
> +115200, i.e. 8.7 us per bit. The transmitted byte 0x00 is used for a
> +Write-0 operation and the byte 0xff for Read-0, Read-1 and Write-1.
> +
> +
> +Usage
> +-----
> +
> +Specify the UART 1-wire bus in the device tree by adding the single child
> +onewire to the serial node (e.g. uart0). For example:
> +::
> +
> +  @uart0 {
> +    ...
> +    onewire {
> +      compatible = "w1-uart";
> +    };
> +  };
> diff --git a/drivers/w1/masters/Kconfig b/drivers/w1/masters/Kconfig
> index 513c0b114337..e6049a75b35b 100644
> --- a/drivers/w1/masters/Kconfig
> +++ b/drivers/w1/masters/Kconfig
> @@ -78,5 +78,15 @@ config W1_MASTER_SGI
>  	  This support is also available as a module.  If so, the module
>  	  will be called sgi_w1.
>  
> +config W1_MASTER_UART
> +	tristate "UART 1-wire driver"
> +	depends on SERIAL_DEV_BUS
> +	help
> +	  Say Y here if you want to communicate with your 1-wire devices using
> +	  UART interface.
> +
> +	  This support is also available as a module.  If so, the module
> +	  will be called w1-uart.
> +
>  endmenu
>  
> diff --git a/drivers/w1/masters/Makefile b/drivers/w1/masters/Makefile
> index 6c5a21f9b88c..227f80987e69 100644
> --- a/drivers/w1/masters/Makefile
> +++ b/drivers/w1/masters/Makefile
> @@ -12,3 +12,4 @@ obj-$(CONFIG_W1_MASTER_MXC)		+= mxc_w1.o
>  obj-$(CONFIG_W1_MASTER_GPIO)		+= w1-gpio.o
>  obj-$(CONFIG_HDQ_MASTER_OMAP)		+= omap_hdq.o
>  obj-$(CONFIG_W1_MASTER_SGI)		+= sgi_w1.o
> +obj-$(CONFIG_W1_MASTER_UART)		+= w1-uart.o
> diff --git a/drivers/w1/masters/w1-uart.c b/drivers/w1/masters/w1-uart.c
> new file mode 100644
> index 000000000000..e0da8ebfef58
> --- /dev/null
> +++ b/drivers/w1/masters/w1-uart.c
> @@ -0,0 +1,189 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * w1-uart - UART 1-Wire bus driver
> + *
> + * Uses the UART interface (via Serial Device Bus) to create the 1-Wire
> + * timing patterns. Implements the following 1-Wire master interface:
> + *
> + * - reset_bus: baud-rate 9600 (bit time 104.2 us) and tx-byte 0xf0
> + *
> + * - touch_bit: baud-rate 115200 (bit time 8.7 us) and tx-byte 0x00
> + *              (Write-0) or 0xff (Read-0, Read-1, Write-1)
> + *
> + * Author: Christoph Winklhofer <cj.winklhofer@xxxxxxxxx>
> + */
> +
> +#include <linux/atomic.h>
> +#include <linux/completion.h>
> +#include <linux/jiffies.h>
> +#include <linux/module.h>
> +#include <linux/of.h>
> +#include <linux/serdev.h>
> +#include <linux/w1.h>
> +
> +#define W1_UART_TIMEOUT msecs_to_jiffies(500)
> +
> +struct w1_uart_device {
> +	struct serdev_device *serdev;
> +	struct w1_bus_master bus;
> +
> +	struct completion buf_received;
> +	atomic_t buf;
> +
> +	unsigned int baudrate;
> +};
> +
> +/*
> + * 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,
> +				unsigned int baudrate, unsigned char tx_byte,
> +				unsigned char *rx_byte)
> +{
> +	struct serdev_device *serdev = w1dev->serdev;
> +	struct device *dev = &serdev->dev;
> +	int ret;
> +
> +	serdev_device_write_flush(serdev);
> +	if (w1dev->baudrate != baudrate) {
> +		w1dev->baudrate = serdev_device_set_baudrate(serdev, baudrate);
> +		if (w1dev->baudrate != baudrate) {
> +			dev_err(dev, "set baudrate failed\n");
> +			return -1;

Use errno values when applicable (e.g. for internal parts).

> +		}
> +	}
> +
> +	reinit_completion(&w1dev->buf_received);
> +	ret = serdev_device_write_buf(serdev, &tx_byte, 1);
> +	if (ret != 1) {
> +		dev_err(dev, "write failed\n");

This is going to spam the logs. Check existing usages - nothing prints
errors on every write or read.

> +		return -1;
> +	}
> +	ret = wait_for_completion_interruptible_timeout(&w1dev->buf_received,
> +							W1_UART_TIMEOUT);
> +	if (ret <= 0) {
> +		dev_err(dev, "receive failed\n");

Also problem of possible flood.

> +		return -1;
> +	}
> +
> +	*rx_byte = (unsigned char)atomic_read(&w1dev->buf);
> +	return 0;
> +}
> +
> +static int w1_uart_serdev_receive_buf(struct serdev_device *serdev,
> +				      const unsigned char *buf, size_t count)
> +{
> +	struct w1_uart_device *w1dev = serdev_device_get_drvdata(serdev);
> +
> +	if (count > 0) {
> +		atomic_set(&w1dev->buf, buf[count - 1]);

I fail to understand why you use atomic here. It does not solve any
locking problems - it's still there. Load and store tearing? But is it
ever possible for one byte stores and loads? Doesn't C standard
guarantee that byte writes are atomic?


I also do not understand why you store only last byte, ignoring all the
rest.

> +		complete(&w1dev->buf_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:
> + * The baud-rate 9600 (104.2 us per bit) and the byte 0xf0 generates a
> + * reset low time of 521 us (start-bit low). 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;
> +	unsigned char buf;

This is not buffer, just a value.

> +	int ret;
> +
> +	ret = w1_uart_serdev_tx_rx(w1dev, 9600, 0xf0, &buf);

Why the rate is fixed? What if all devices support different baud rates?

> +	if (ret < 0)
> +		return -1;
> +
> +	/* Device present (0) or no device (1) */
> +	return buf != 0xf0 ? 0 : 1;
> +}
> +
> +/*
> + * 1-Wire read and write cycle:
> + * The baud-rate 115200 (8.7 us per bit) and the byte 0xff generates the
> + * timing pattern for Read-0, Read-1 and Write-1 operation, with a low
> + * time of 8.7 us (start-bit low). 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;
> +	unsigned char buf;

One character is not a buffer.

> +	int ret;
> +
> +	ret = w1_uart_serdev_tx_rx(w1dev, 115200, bit ? 0xff : 0x00, &buf);

Why the rate is fixed? What if all devices support different baud rates?

> +	/* return inactive bus state on error */
> +	if (ret < 0)
> +		return 1;
> +
> +	return buf == 0xff ? 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(struct w1_uart_device), GFP_KERNEL);

sizeof(*)

> +	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;
> +
> +	serdev_device_set_drvdata(serdev, w1dev);
> +	serdev_device_set_client_ops(serdev, &w1_uart_serdev_ops);
> +
> +	init_completion(&w1dev->buf_received);
> +
> +	ret = devm_serdev_device_open(dev, serdev);
> +	if (ret < 0)
> +		return ret;
> +	serdev_device_set_parity(serdev, SERDEV_PARITY_NONE);
> +	serdev_device_set_flow_control(serdev, false);
> +
> +	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);
> +
> +	complete(&w1dev->buf_received);

This is odd - w1_uart_serdev_tx_rx() will read data on driver removal.
You should make sure that on removal any reads do not follow usual path.

> +	w1_remove_master_device(&w1dev->bus);
> +}
> +
> +#ifdef CONFIG_OF

Drop

> +static const struct of_device_id w1_uart_of_match[] = {
> +	{ .compatible = "w1-uart" },
> +	{},
> +};
> +MODULE_DEVICE_TABLE(of, w1_uart_of_match);
> +#endif
> +
> +static struct serdev_device_driver w1_uart_driver = {
> +	.driver	= {
> +		.name		= "w1-uart",
> +		.of_match_table	= of_match_ptr(w1_uart_of_match),

Drop of_match_ptr

> +	},
> +	.probe	= w1_uart_probe,
> +	.remove	= w1_uart_remove,
> +};

Best regards,
Krzysztof