Re: [PATCH v5 7/7] tty: i3c: add TTY over I3C master support

From: Jiri Slaby <jirislaby@kernel.org>
To: Frank Li <Frank.Li@nxp.com>, miquel.raynal@bootlin.com
Cc: alexandre.belloni@bootlin.com, conor.culhane@silvaco.com,
	imx@lists.linux.dev, joe@perches.com,
	linux-i3c@lists.infradead.org, linux-kernel@vger.kernel.org,
	zbigniew.lukwinski@linux.intel.com, gregkh@linuxfoundation.org,
	linux-serial@vger.kernel.org
Subject: Re: [PATCH v5 7/7] tty: i3c: add TTY over I3C master support
Date: Tue, 5 Dec 2023 10:56:09 +0100	[thread overview]
Message-ID: <2c3824d7-0960-4de7-8fae-01478fdef8fe@kernel.org> (raw)
In-Reply-To: <20231130224408.3591288-8-Frank.Li@nxp.com>

On 30. 11. 23, 23:44, Frank Li wrote:
> In typical embedded Linux systems, UART consoles require at least two pins,
> TX and RX. In scenarios where I2C/I3C devices like sensors or PMICs are
> present, we can save these two pins by using this driver. Pins is crucial
> resources, especially in small chip packages.
> 
> This introduces support for using the I3C bus to transfer console tty data,
> effectively replacing the need for dedicated UART pins. This not only
> conserves valuable pin resources but also facilitates testing of I3C's
> advanced features, including early termination, in-band interrupt (IBI)
> support, and the creation of more complex data patterns. Additionally,
> it aids in identifying and addressing issues within the I3C controller
> driver.
> 
> Signed-off-by: Frank Li <Frank.Li@nxp.com>
...
> --- a/drivers/tty/Kconfig
> +++ b/drivers/tty/Kconfig
> @@ -412,6 +412,19 @@ config RPMSG_TTY
>   	  To compile this driver as a module, choose M here: the module will be
>   	  called rpmsg_tty.
>   
> +config I3C_TTY
> +	tristate "TTY over I3C"
> +	depends on I3C
> +	help
> +	  Select this options if you'd like use TTY over I3C master controller

this option
to use
add a period to the end.

> +
> +	  This makes it possible for user-space programs to send and receive
> +	  data as a standard tty protocol. I3C provide relatively higher data
> +	  transfer rate and less pin numbers, SDA/SCL are shared with other
> +	  devices.
> +
> +	  If unsure, say N
> +
>   endif # TTY
>   
>   source "drivers/tty/serdev/Kconfig"
...
> --- /dev/null
> +++ b/drivers/tty/i3c_tty.c
> @@ -0,0 +1,443 @@
...
> +struct ttyi3c_port {
> +	struct tty_port port;
> +	int minor;
> +	spinlock_t xlock; /* protect xmit */
> +	char tx_buff[I3C_TTY_TRANS_SIZE];
> +	char rx_buff[I3C_TTY_TRANS_SIZE];

These should be u8 as per the other changes throughout the tty layer.

> +	struct i3c_device *i3cdev;
> +	struct work_struct txwork;
> +	struct work_struct rxwork;
> +	struct completion txcomplete;
> +	unsigned long status;
> +	int buf_overrun;

Can this be ever negative?

> +};
> +
> +struct workqueue_struct *workqueue;

Is this related:
     Still below items not be fixed (according to Jiri Slaby's comments)
     - rxwork thread: need trigger from two position.
     - common thread queue: need some suggestion
?

As I don't remember, could you elaborate again why you need your own 
workqueue? You need to do it in the commit log anyway.

...
> +static ssize_t i3c_write(struct tty_struct *tty, const unsigned char *buf, size_t count)
> +{
> +	struct ttyi3c_port *sport = tty->driver_data;
> +	unsigned long flags;
> +	bool is_empty;
> +	int ret;
> +
> +	spin_lock_irqsave(&sport->xlock, flags);
> +	ret = kfifo_in(&sport->port.xmit_fifo, buf, count);
> +	is_empty = kfifo_is_empty(&sport->port.xmit_fifo);
> +	spin_unlock_irqrestore(&sport->xlock, flags);
> +
> +	if (!is_empty)
> +		queue_work(workqueue, &sport->txwork);
> +
> +	return ret;
> +}
> +
> +static int i3c_put_char(struct tty_struct *tty, unsigned char ch)
> +{
> +	struct ttyi3c_port *sport = tty->driver_data;
> +	unsigned long flags;
> +	int ret = 0;

Unneeded initialization.

> +
> +	spin_lock_irqsave(&sport->xlock, flags);
> +	ret = kfifo_put(&sport->port.xmit_fifo, ch);
> +	spin_unlock_irqrestore(&sport->xlock, flags);
> +
> +	return ret;
> +}
...
> +static void tty_i3c_rxwork(struct work_struct *work)
> +{
> +	struct ttyi3c_port *sport = container_of(work, struct ttyi3c_port, rxwork);
> +	struct i3c_priv_xfer xfers;
> +	int retry = I3C_TTY_RETRY;

Likely, should be unsigned.

> +	u16 status = BIT(0);
> +	int ret;
> +
> +	memset(&xfers, 0, sizeof(xfers));
> +	xfers.data.in = sport->rx_buff;
> +	xfers.len = I3C_TTY_TRANS_SIZE;
> +	xfers.rnw = 1;
> +
> +	do {
> +		if (test_bit(I3C_TTY_RX_STOP, &sport->status))
> +			break;
> +
> +		i3c_device_do_priv_xfers(sport->i3cdev, &xfers, 1);
> +
> +		if (xfers.actual_len) {
> +			ret = tty_insert_flip_string(&sport->port, sport->rx_buff,
> +						     xfers.actual_len);
> +			if (ret < xfers.actual_len)
> +				sport->buf_overrun++;
> +
> +			retry = I3C_TTY_RETRY;
> +			continue;
> +		}
> +
> +		status = BIT(0);
> +		i3c_device_getstatus_format1(sport->i3cdev, &status);
> +		/*
> +		 * Target side needs some time to fill data into fifo. Target side may not
> +		 * have hardware update status in real time. Software update status always
> +		 * needs some delays.
> +		 *
> +		 * Generally, target side have circular buffer in memory, it will be moved
> +		 * into FIFO by CPU or DMA. 'status' just show if circular buffer empty. But
> +		 * there are gap, especially CPU have not response irq to fill FIFO in time.
> +		 * So xfers.actual will be zero, wait for little time to avoid flood
> +		 * transfer in i3c bus.
> +		 */
> +		usleep_range(I3C_TTY_YIELD_US, 10 * I3C_TTY_YIELD_US);
> +		retry--;
> +
> +	} while (retry && (status & BIT(0)));
> +
> +	tty_flip_buffer_push(&sport->port);
> +}
> +
> +static void tty_i3c_txwork(struct work_struct *work)
> +{
> +	struct ttyi3c_port *sport = container_of(work, struct ttyi3c_port, txwork);
> +	struct i3c_priv_xfer xfers;
> +	int retry = I3C_TTY_RETRY;

Detto.

> +	unsigned long flags;
> +	int ret;
> +
> +	xfers.rnw = 0;
> +	xfers.data.out = sport->tx_buff;
> +
> +	while (!kfifo_is_empty(&sport->port.xmit_fifo) && retry) {
> +		xfers.len = kfifo_len(&sport->port.xmit_fifo);
> +		xfers.len = min_t(u16, I3C_TTY_TRANS_SIZE, xfers.len);
> +
> +		xfers.len = kfifo_out_peek(&sport->port.xmit_fifo, sport->tx_buff, xfers.len);

Can this simply be:
xfers.len = kfifo_out_peek(&sport->port.xmit_fifo, sport->tx_buff, 
I3C_TTY_TRANS_SIZE);
?

> +
> +		ret = i3c_device_do_priv_xfers(sport->i3cdev, &xfers, 1);
> +		if (ret) {
> +			/*
> +			 * Target side may not move data out of FIFO. delay can't resolve problem,
> +			 * just reduce some possiblity. Target can't end I3C SDR mode write
> +			 * transfer, discard data is reasonable when FIFO overrun.
> +			 */
> +			usleep_range(I3C_TTY_YIELD_US, 10 * I3C_TTY_YIELD_US);
> +			retry--;
> +		} else {
> +			retry = I3C_TTY_RETRY;
> +			ret = kfifo_out(&sport->port.xmit_fifo, sport->tx_buff, xfers.len);

Just to make sure: xfers.len is nor overwritten by 
i3c_device_do_priv_xfers(), right?

> +		}
> +	}
> +
> +	spin_lock_irqsave(&sport->xlock, flags);

Why do you take the lock here, but not during the kfifo operations above?

> +	if (kfifo_is_empty(&sport->port.xmit_fifo))
> +		complete(&sport->txcomplete);
> +	spin_unlock_irqrestore(&sport->xlock, flags);
> +}

> +static int __init i3c_tty_init(void)
> +{
> +	int ret;
> +
> +	i3c_tty_driver = tty_alloc_driver(I3C_TTY_MINORS,
> +					  TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV);
> +
> +	if (IS_ERR(i3c_tty_driver))
> +		return PTR_ERR(i3c_tty_driver);
> +
> +	i3c_tty_driver->driver_name = "ttyI3C";
> +	i3c_tty_driver->name = "ttyI3C";
> +	i3c_tty_driver->minor_start = 0,
> +	i3c_tty_driver->type = TTY_DRIVER_TYPE_SERIAL,
> +	i3c_tty_driver->subtype = SERIAL_TYPE_NORMAL,
> +	i3c_tty_driver->init_termios = tty_std_termios;
> +	i3c_tty_driver->init_termios.c_cflag = B9600 | CS8 | CREAD | HUPCL |
> +					       CLOCAL;
> +	i3c_tty_driver->init_termios.c_lflag = 0;
> +
> +	tty_set_operations(i3c_tty_driver, &i3c_tty_ops);
> +
> +	ret = tty_register_driver(i3c_tty_driver);
> +	if (ret)
> +		goto err_tty_register_driver;
> +
> +	ret = i3c_driver_register(&i3c_driver);
> +	if (ret)
> +		goto err_i3c_driver_register;
> +
> +	workqueue = alloc_workqueue("ttyI3C", 0, 0);

Can it happen that you already queue something on this wq, while not 
allocated yet? I mean: should this be done first in i3c_tty_init()?

> +	if (!workqueue) {
> +		ret = PTR_ERR(workqueue);
> +		goto err_alloc_workqueue;
> +	}
> +
> +	return 0;
> +
> +err_alloc_workqueue:
> +	i3c_driver_unregister(&i3c_driver);
> +
> +err_i3c_driver_register:
> +	tty_unregister_driver(i3c_tty_driver);
> +
> +err_tty_register_driver:
> +	tty_driver_kref_put(i3c_tty_driver);
> +
> +	return ret;
> +}
> +
> +static void __exit i3c_tty_exit(void)
> +{
> +	i3c_driver_unregister(&i3c_driver);
> +	tty_unregister_driver(i3c_tty_driver);
> +	tty_driver_kref_put(i3c_tty_driver);
> +	idr_destroy(&i3c_tty_minors);

What about the wq?

> +}

regards,
-- 
js
suse labs