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Date:   Tue, 26 Jan 2021 21:16:04 +0530
From:   Manivannan Sadhasivam <mani@kernel.org>
To:     Johan Hovold <johan@kernel.org>
Cc:     gregkh@linuxfoundation.org, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org, patong.mxl@gmail.com,
        linus.walleij@linaro.org, mchehab+huawei@kernel.org,
        angelo.dureghello@timesys.com
Subject: Re: [PATCH v5 1/3] usb: serial: Add MaxLinear/Exar USB to Serial
 driver
Message-ID: <20210126154604.GC29751@thinkpad>
References: <20201122170822.21715-1-mani@kernel.org>
 <20201122170822.21715-2-mani@kernel.org>
 <YAlVLOqzx8otPgOg@hovoldconsulting.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <YAlVLOqzx8otPgOg@hovoldconsulting.com>
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On Thu, Jan 21, 2021 at 11:19:24AM +0100, Johan Hovold wrote:
> On Sun, Nov 22, 2020 at 10:38:20PM +0530, Manivannan Sadhasivam wrote:
> > Add support for MaxLinear/Exar USB to Serial converters. This driver
> > only supports XR21V141X series but it can be extended to other series
> > from Exar as well in future.
> 
> There are still a few issues with this driver, but I really don't want
> to have to review it again in a couple of months so I've fixed it up
> myself this time.
> 
> The trivial stuff I folded into this patch, and I'll submit a follow-on
> series for the rest.
> 

Many thanks for doing this! These days it is really difficult to find time for
spare time stuffs.

And all of your fixes makes sense to me.

Thanks,
Mani

> > This driver is inspired from the initial one submitted by Patong Yang:
> > 
> > https://lore.kernel.org/linux-usb/20180404070634.nhspvmxcjwfgjkcv@advantechmxl-desktop
> 
> Using /r/ is shorter.
> 
> > While the initial driver was a custom tty USB driver exposing whole
> > new serial interface ttyXRUSBn, this version is completely based on USB
> > serial core thus exposing the interfaces as ttyUSBn. This will avoid
> > the overhead of exposing a new USB serial interface which the userspace
> > tools are unaware of.
> 
> I added a comment about the two driver modes here (ACM and "custom").
> 
> > Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
> > ---
> >  drivers/usb/serial/Kconfig     |   9 +
> >  drivers/usb/serial/Makefile    |   1 +
> >  drivers/usb/serial/xr_serial.c | 599 +++++++++++++++++++++++++++++++++
> >  3 files changed, 609 insertions(+)
> >  create mode 100644 drivers/usb/serial/xr_serial.c
> > 
> > diff --git a/drivers/usb/serial/Kconfig b/drivers/usb/serial/Kconfig
> > index 4007fa25a8ff..32595acb1d1d 100644
> > --- a/drivers/usb/serial/Kconfig
> > +++ b/drivers/usb/serial/Kconfig
> > @@ -644,6 +644,15 @@ config USB_SERIAL_UPD78F0730
> >  	  To compile this driver as a module, choose M here: the
> >  	  module will be called upd78f0730.
> >  
> > +config USB_SERIAL_XR
> > +	tristate "USB MaxLinear/Exar USB to Serial driver"
> > +	help
> > +	  Say Y here if you want to use MaxLinear/Exar USB to Serial converter
> > +	  devices.
> > +
> > +	  To compile this driver as a module, choose M here: the
> > +	  module will be called xr_serial.
> > +
> >  config USB_SERIAL_DEBUG
> >  	tristate "USB Debugging Device"
> >  	help
> > diff --git a/drivers/usb/serial/Makefile b/drivers/usb/serial/Makefile
> > index 2d491e434f11..4f69c2a3aff3 100644
> > --- a/drivers/usb/serial/Makefile
> > +++ b/drivers/usb/serial/Makefile
> > @@ -62,4 +62,5 @@ obj-$(CONFIG_USB_SERIAL_VISOR)			+= visor.o
> >  obj-$(CONFIG_USB_SERIAL_WISHBONE)		+= wishbone-serial.o
> >  obj-$(CONFIG_USB_SERIAL_WHITEHEAT)		+= whiteheat.o
> >  obj-$(CONFIG_USB_SERIAL_XIRCOM)			+= keyspan_pda.o
> > +obj-$(CONFIG_USB_SERIAL_XR)			+= xr_serial.o
> >  obj-$(CONFIG_USB_SERIAL_XSENS_MT)		+= xsens_mt.o
> > diff --git a/drivers/usb/serial/xr_serial.c b/drivers/usb/serial/xr_serial.c
> > new file mode 100644
> > index 000000000000..e166916554d6
> > --- /dev/null
> > +++ b/drivers/usb/serial/xr_serial.c
> > @@ -0,0 +1,599 @@
> > +// SPDX-License-Identifier: GPL-2.0+
> > +/*
> > + * MaxLinear/Exar USB to Serial driver
> > + *
> > + * Based on the initial driver written by Patong Yang:
> > + * https://lore.kernel.org/linux-usb/20180404070634.nhspvmxcjwfgjkcv@advantechmxl-desktop
> > + *
> > + * Copyright (c) 2018 Patong Yang <patong.mxl@gmail.com>
> > + * Copyright (c) 2020 Manivannan Sadhasivam <mani@kernel.org>
> 
> I moved your copyright notice under "MaxLinear..." so that it's clear
> who claims what copyright.
> 
> > + */
> > +
> > +#include <linux/kernel.h>
> > +#include <linux/module.h>
> > +#include <linux/slab.h>
> > +#include <linux/tty.h>
> > +#include <linux/usb.h>
> > +#include <linux/usb/serial.h>
> > +
> > +struct xr_txrx_clk_mask {
> > +	u16 tx;
> > +	u16 rx0;
> > +	u16 rx1;
> > +};
> > +
> > +#define XR_INT_OSC_HZ			48000000U
> > +#define XR21V141X_MIN_SPEED		46U
> > +#define XR21V141X_MAX_SPEED		XR_INT_OSC_HZ
> > +
> > +/* USB Requests */
> > +#define XR21V141X_SET_REQ		0
> > +#define XR21V141X_GET_REQ		1
> > +
> > +#define XR21V141X_CLOCK_DIVISOR_0	0x04
> > +#define XR21V141X_CLOCK_DIVISOR_1	0x05
> > +#define XR21V141X_CLOCK_DIVISOR_2	0x06
> > +#define XR21V141X_TX_CLOCK_MASK_0	0x07
> > +#define XR21V141X_TX_CLOCK_MASK_1	0x08
> > +#define XR21V141X_RX_CLOCK_MASK_0	0x09
> > +#define XR21V141X_RX_CLOCK_MASK_1	0x0a
> > +
> > +/* XR21V141X register blocks */
> > +#define XR21V141X_UART_REG_BLOCK	0
> > +#define XR21V141X_UM_REG_BLOCK		4
> > +#define XR21V141X_UART_CUSTOM_BLOCK	0x66
> > +
> > +/* XR21V141X UART Manager Registers */
> > +#define XR21V141X_UM_FIFO_ENABLE_REG	0x10
> > +#define XR21V141X_UM_ENABLE_TX_FIFO	0x01
> > +#define XR21V141X_UM_ENABLE_RX_FIFO	0x02
> > +
> > +#define XR21V141X_UM_RX_FIFO_RESET	0x18
> > +#define XR21V141X_UM_TX_FIFO_RESET	0x1c
> > +
> > +#define XR21V141X_UART_ENABLE_TX	0x1
> > +#define XR21V141X_UART_ENABLE_RX	0x2
> > +
> > +#define XR21V141X_UART_MODE_RI		BIT(0)
> > +#define XR21V141X_UART_MODE_CD		BIT(1)
> > +#define XR21V141X_UART_MODE_DSR		BIT(2)
> > +#define XR21V141X_UART_MODE_DTR		BIT(3)
> > +#define XR21V141X_UART_MODE_CTS		BIT(4)
> > +#define XR21V141X_UART_MODE_RTS		BIT(5)
> > +
> > +#define XR21V141X_UART_BREAK_ON		0xff
> > +#define XR21V141X_UART_BREAK_OFF	0
> > +
> > +#define XR21V141X_UART_DATA_MASK	GENMASK(3, 0)
> > +#define XR21V141X_UART_DATA_7		0x7
> > +#define XR21V141X_UART_DATA_8		0x8
> > +
> > +#define XR21V141X_UART_PARITY_MASK	GENMASK(6, 4)
> > +#define XR21V141X_UART_PARITY_SHIFT	0x4
> > +#define XR21V141X_UART_PARITY_NONE	0x0
> > +#define XR21V141X_UART_PARITY_ODD	0x1
> > +#define XR21V141X_UART_PARITY_EVEN	0x2
> > +#define XR21V141X_UART_PARITY_MARK	0x3
> > +#define XR21V141X_UART_PARITY_SPACE	0x4
> > +
> > +#define XR21V141X_UART_STOP_MASK	BIT(7)
> > +#define XR21V141X_UART_STOP_SHIFT	0x7
> > +#define XR21V141X_UART_STOP_1		0x0
> > +#define XR21V141X_UART_STOP_2		0x1
> > +
> > +#define XR21V141X_UART_FLOW_MODE_NONE	0x0
> > +#define XR21V141X_UART_FLOW_MODE_HW	0x1
> > +#define XR21V141X_UART_FLOW_MODE_SW	0x2
> > +
> > +#define XR21V141X_UART_MODE_GPIO_MASK	GENMASK(2, 0)
> > +#define XR21V141X_UART_MODE_RTS_CTS	0x1
> > +#define XR21V141X_UART_MODE_DTR_DSR	0x2
> > +#define XR21V141X_UART_MODE_RS485	0x3
> > +#define XR21V141X_UART_MODE_RS485_ADDR	0x4
> > +
> > +#define XR21V141X_REG_ENABLE		0x03
> > +#define XR21V141X_REG_FORMAT		0x0b
> > +#define XR21V141X_REG_FLOW_CTRL		0x0c
> > +#define XR21V141X_REG_XON_CHAR		0x10
> > +#define XR21V141X_REG_XOFF_CHAR		0x11
> > +#define XR21V141X_REG_LOOPBACK		0x12
> > +#define XR21V141X_REG_TX_BREAK		0x14
> > +#define XR21V141X_REG_RS845_DELAY	0x15
> > +#define XR21V141X_REG_GPIO_MODE		0x1a
> > +#define XR21V141X_REG_GPIO_DIR		0x1b
> > +#define XR21V141X_REG_GPIO_INT_MASK	0x1c
> > +#define XR21V141X_REG_GPIO_SET		0x1d
> > +#define XR21V141X_REG_GPIO_CLR		0x1e
> > +#define XR21V141X_REG_GPIO_STATUS	0x1f
> > +
> > +static int xr_set_reg(struct usb_serial_port *port, u8 block, u8 reg,
> > +		      u8 val)
> 
> No need for line breaks.
> 
> > +{
> > +	struct usb_serial *serial = port->serial;
> > +	int ret;
> > +
> > +	ret = usb_control_msg(serial->dev,
> > +			      usb_sndctrlpipe(serial->dev, 0),
> > +			      XR21V141X_SET_REQ,
> > +			      USB_DIR_OUT | USB_TYPE_VENDOR, val,
> 
> The request-type should include USB_RECIP_DEVICE (0) as well.
> 
> > +			      reg | (block << 8), NULL, 0,
> > +			      USB_CTRL_SET_TIMEOUT);
> > +	if (ret < 0) {
> > +		dev_err(&port->dev, "Failed to set reg 0x%02x: %d\n", reg, ret);
> > +		return ret;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static int xr_get_reg(struct usb_serial_port *port, u8 block, u8 reg,
> > +		      u8 *val)
> > +{
> > +	struct usb_serial *serial = port->serial;
> > +	u8 *dmabuf;
> > +	int ret;
> > +
> > +	dmabuf = kmalloc(1, GFP_KERNEL);
> > +	if (!dmabuf)
> > +		return -ENOMEM;
> > +
> > +	ret = usb_control_msg(serial->dev,
> > +			      usb_rcvctrlpipe(serial->dev, 0),
> > +			      XR21V141X_GET_REQ,
> > +			      USB_DIR_IN | USB_TYPE_VENDOR, 0,
> > +			      reg | (block << 8), dmabuf, 1,
> > +			      USB_CTRL_GET_TIMEOUT);
> > +	if (ret == 1) {
> > +		*val = *dmabuf;
> > +		ret = 0;
> > +	} else {
> > +		dev_err(&port->dev, "Failed to get reg 0x%02x: %d\n", reg, ret);
> > +		if (ret >= 0)
> > +			ret = -EIO;
> > +	}
> > +
> > +	kfree(dmabuf);
> > +
> > +	return ret;
> > +}
> > +
> > +static int xr_set_reg_uart(struct usb_serial_port *port, u8 reg, u8 val)
> > +{
> > +	return xr_set_reg(port, XR21V141X_UART_REG_BLOCK, reg, val);
> > +}
> > +
> > +static int xr_get_reg_uart(struct usb_serial_port *port, u8 reg, u8 *val)
> > +{
> > +	return xr_get_reg(port, XR21V141X_UART_REG_BLOCK, reg, val);
> > +}
> > +
> > +static int xr_set_reg_um(struct usb_serial_port *port, u8 reg, u8 val)
> > +{
> > +	return xr_set_reg(port, XR21V141X_UM_REG_BLOCK, reg, val);
> > +}
> > +
> > +/* Tx and Rx clock mask values obtained from section 3.3.4 of datasheet */
> > +static const struct xr_txrx_clk_mask xr21v141x_txrx_clk_masks[] = {
> > +	{ 0x000, 0x000, 0x000 },
> > +	{ 0x000, 0x000, 0x000 },
> > +	{ 0x100, 0x000, 0x100 },
> > +	{ 0x020, 0x400, 0x020 },
> > +	{ 0x010, 0x100, 0x010 },
> > +	{ 0x208, 0x040, 0x208 },
> > +	{ 0x104, 0x820, 0x108 },
> > +	{ 0x844, 0x210, 0x884 },
> > +	{ 0x444, 0x110, 0x444 },
> > +	{ 0x122, 0x888, 0x224 },
> > +	{ 0x912, 0x448, 0x924 },
> > +	{ 0x492, 0x248, 0x492 },
> > +	{ 0x252, 0x928, 0x292 },
> > +	{ 0x94a, 0x4a4, 0xa52 },
> > +	{ 0x52a, 0xaa4, 0x54a },
> > +	{ 0xaaa, 0x954, 0x4aa },
> > +	{ 0xaaa, 0x554, 0xaaa },
> > +	{ 0x555, 0xad4, 0x5aa },
> > +	{ 0xb55, 0xab4, 0x55a },
> > +	{ 0x6b5, 0x5ac, 0xb56 },
> > +	{ 0x5b5, 0xd6c, 0x6d6 },
> > +	{ 0xb6d, 0xb6a, 0xdb6 },
> > +	{ 0x76d, 0x6da, 0xbb6 },
> > +	{ 0xedd, 0xdda, 0x76e },
> > +	{ 0xddd, 0xbba, 0xeee },
> > +	{ 0x7bb, 0xf7a, 0xdde },
> > +	{ 0xf7b, 0xef6, 0x7de },
> > +	{ 0xdf7, 0xbf6, 0xf7e },
> > +	{ 0x7f7, 0xfee, 0xefe },
> > +	{ 0xfdf, 0xfbe, 0x7fe },
> > +	{ 0xf7f, 0xefe, 0xffe },
> > +	{ 0xfff, 0xffe, 0xffd },
> > +};
> > +
> > +static int xr_set_baudrate(struct tty_struct *tty,
> > +			   struct usb_serial_port *port)
> > +{
> > +	u32 divisor, baud, idx;
> > +	u16 tx_mask, rx_mask;
> > +	int ret;
> > +
> > +	baud = clamp(tty->termios.c_ospeed, XR21V141X_MIN_SPEED,
> > +		     XR21V141X_MAX_SPEED);
> 
> You shouldn't report back anything else but B0 if that's requested, the
> actual rate can be left unchanged.
> 
> > +	divisor = XR_INT_OSC_HZ / baud;
> > +	idx = ((32 * XR_INT_OSC_HZ) / baud) & 0x1f;
> > +	tx_mask = xr21v141x_txrx_clk_masks[idx].tx;
> > +
> > +	if (divisor & 1)
> 
> Nit: 0x01 since bitmask.
> 
> > +		rx_mask = xr21v141x_txrx_clk_masks[idx].rx1;
> > +	else
> > +		rx_mask = xr21v141x_txrx_clk_masks[idx].rx0;
> > +
> > +	dev_dbg(&port->dev, "Setting baud rate: %u\n", baud);
> > +	/*
> > +	 * XR21V141X uses fractional baud rate generator with 48MHz internal
> > +	 * oscillator and 19-bit programmable divisor. So theoretically it can
> > +	 * generate most commonly used baud rates with high accuracy.
> > +	 */
> > +	ret = xr_set_reg_uart(port, XR21V141X_CLOCK_DIVISOR_0, (divisor & 0xff));
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = xr_set_reg_uart(port, XR21V141X_CLOCK_DIVISOR_1, ((divisor >>  8) & 0xff));
> 
> I broke these long lines again.
> 
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = xr_set_reg_uart(port, XR21V141X_CLOCK_DIVISOR_2, ((divisor >> 16) & 0xff));
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = xr_set_reg_uart(port, XR21V141X_TX_CLOCK_MASK_0, (tx_mask & 0xff));
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = xr_set_reg_uart(port, XR21V141X_TX_CLOCK_MASK_1, ((tx_mask >>  8) & 0xff));
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = xr_set_reg_uart(port, XR21V141X_RX_CLOCK_MASK_0, (rx_mask & 0xff));
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = xr_set_reg_uart(port, XR21V141X_RX_CLOCK_MASK_1, ((rx_mask >>  8) & 0xff));
> > +
> > +	tty_encode_baud_rate(tty, baud, baud);
> > +
> > +	return ret;
> > +}
> > +
> > +/*
> > + * According to datasheet, below is the recommended sequence for enabling UART
> > + * module in XR21V141X:
> > + *
> > + * Enable Tx FIFO
> > + * Enable Tx and Rx
> > + * Enable Rx FIFO
> > + */
> > +static int xr_uart_enable(struct usb_serial_port *port)
> > +{
> > +	int ret;
> > +
> > +	ret = xr_set_reg_um(port, XR21V141X_UM_FIFO_ENABLE_REG, XR21V141X_UM_ENABLE_TX_FIFO);
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = xr_set_reg_uart(port, XR21V141X_REG_ENABLE,
> > +			      XR21V141X_UART_ENABLE_TX | XR21V141X_UART_ENABLE_RX);
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = xr_set_reg_um(port, XR21V141X_UM_FIFO_ENABLE_REG,
> > +			    XR21V141X_UM_ENABLE_TX_FIFO | XR21V141X_UM_ENABLE_RX_FIFO);
> > +
> > +	if (ret)
> > +		xr_set_reg_uart(port, XR21V141X_REG_ENABLE, 0);
> > +
> > +	return ret;
> > +}
> > +
> > +static int xr_uart_disable(struct usb_serial_port *port)
> > +{
> > +	int ret;
> > +
> > +	ret = xr_set_reg_uart(port, XR21V141X_REG_ENABLE, 0);
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = xr_set_reg_um(port, XR21V141X_UM_FIFO_ENABLE_REG, 0);
> > +
> > +	return ret;
> > +}
> > +
> > +static void xr_set_flow_mode(struct tty_struct *tty,
> > +			     struct usb_serial_port *port)
> > +{
> > +	unsigned int cflag = tty->termios.c_cflag;
> > +	int ret;
> > +	u8 flow, gpio_mode;
> > +
> > +	ret = xr_get_reg_uart(port, XR21V141X_REG_GPIO_MODE, &gpio_mode);
> > +	if (ret)
> > +		return;
> > +
> > +	if (cflag & CRTSCTS) {
> 
> C_CRTSCTS(tty)
> 
> > +		dev_dbg(&port->dev, "Enabling hardware flow ctrl\n");
> > +
> > +		/*
> > +		 * RTS/CTS is the default flow control mode, so set GPIO mode
> > +		 * for controlling the pins manually by default.
> > +		 */
> > +		gpio_mode &= ~XR21V141X_UART_MODE_GPIO_MASK;
> 
> This needs to be done before the conditional so that GPIO mode is again
> selected when disabling flow control, otherwise your break RTS control.
> 
> Also you never configure DTR/RTS as outputs despite all pins being
> inputs by default according to the datasheet. Effectively, DTR control
> is currently broken and RTS can only be used for automatic flow control.
> 
> Did you not test these signals separately?
> 
> > +		gpio_mode |= XR21V141X_UART_MODE_RTS_CTS;
> > +		flow = XR21V141X_UART_FLOW_MODE_HW;
> > +	} else if (I_IXON(tty)) {
> > +		u8 start_char = START_CHAR(tty);
> > +		u8 stop_char = STOP_CHAR(tty);
> > +
> > +		dev_dbg(&port->dev, "Enabling sw flow ctrl\n");
> > +		flow = XR21V141X_UART_FLOW_MODE_SW;
> > +
> > +		xr_set_reg_uart(port, XR21V141X_REG_XON_CHAR, start_char);
> > +		xr_set_reg_uart(port, XR21V141X_REG_XOFF_CHAR, stop_char);
> > +	} else {
> > +		dev_dbg(&port->dev, "Disabling flow ctrl\n");
> > +		flow = XR21V141X_UART_FLOW_MODE_NONE;
> > +	}
> > +
> > +	/*
> > +	 * As per the datasheet, UART needs to be disabled while writing to
> > +	 * FLOW_CONTROL register.
> > +	 */
> > +	xr_uart_disable(port);
> > +	xr_set_reg_uart(port, XR21V141X_REG_FLOW_CTRL, flow);
> > +	xr_uart_enable(port);
> > +
> > +	xr_set_reg_uart(port, XR21V141X_REG_GPIO_MODE, gpio_mode);
> > +}
> > +
> > +static int xr_tiocmset_port(struct usb_serial_port *port,
> > +			    unsigned int set, unsigned int clear)
> > +{
> > +	u8 gpio_set = 0;
> > +	u8 gpio_clr = 0;
> > +	int ret = 0;
> > +
> > +	/* Modem control pins are active low, so set & clr are swapped */
> > +	if (set & TIOCM_RTS)
> > +		gpio_clr |= XR21V141X_UART_MODE_RTS;
> > +	if (set & TIOCM_DTR)
> > +		gpio_clr |= XR21V141X_UART_MODE_DTR;
> > +	if (clear & TIOCM_RTS)
> > +		gpio_set |= XR21V141X_UART_MODE_RTS;
> > +	if (clear & TIOCM_DTR)
> > +		gpio_set |= XR21V141X_UART_MODE_DTR;
> > +
> > +	/* Writing '0' to gpio_{set/clr} bits has no effect, so no need to do */
> > +	if (gpio_clr)
> > +		ret = xr_set_reg_uart(port, XR21V141X_REG_GPIO_CLR, gpio_clr);
> > +
> > +	if (gpio_set)
> > +		ret = xr_set_reg_uart(port, XR21V141X_REG_GPIO_SET, gpio_set);
> > +
> > +	return ret;
> > +}
> > +
> > +static int xr_tiocmset(struct tty_struct *tty,
> > +		       unsigned int set, unsigned int clear)
> > +{
> > +	struct usb_serial_port *port = tty->driver_data;
> > +
> > +	return xr_tiocmset_port(port, set, clear);
> > +}
> > +
> > +static void xr_dtr_rts(struct usb_serial_port *port, int on)
> > +{
> > +	if (on)
> > +		xr_tiocmset_port(port, TIOCM_DTR | TIOCM_RTS, 0);
> > +	else
> > +		xr_tiocmset_port(port, 0, TIOCM_DTR | TIOCM_RTS);
> > +}
> > +
> > +static void xr_set_termios(struct tty_struct *tty,
> > +			   struct usb_serial_port *port,
> > +			   struct ktermios *old_termios)
> > +{
> > +	struct ktermios *termios = &tty->termios;
> > +	int ret;
> > +	u8 bits = 0;
> > +
> > +	if ((old_termios && tty->termios.c_ospeed != old_termios->c_ospeed) ||
> > +	    !old_termios)
> 
> This can be simplified.
> 
> > +		xr_set_baudrate(tty, port);
> > +
> > +	switch (C_CSIZE(tty)) {
> > +	case CS5:
> > +	fallthrough;
> 
> No need for fallthrough for empty cases.
> 
> > +	case CS6:
> > +		/* CS5 and CS6 are not supported, so just restore old setting */
> > +		termios->c_cflag &= ~CSIZE;
> > +		if (old_termios)
> > +			termios->c_cflag |= old_termios->c_cflag & CSIZE;
> > +		else
> > +			bits |= XR21V141X_UART_DATA_8;
> > +		break;
> > +	case CS7:
> > +		bits |= XR21V141X_UART_DATA_7;
> > +		break;
> > +	case CS8:
> > +	fallthrough;
> > +	default:
> > +		bits |= XR21V141X_UART_DATA_8;
> > +		break;
> > +	}
> > +
> > +	if (C_PARENB(tty)) {
> > +		if (C_CMSPAR(tty)) {
> > +			if (C_PARODD(tty))
> > +				bits |= XR21V141X_UART_PARITY_MARK <<
> > +					XR21V141X_UART_PARITY_SHIFT;
> 
> I'd prefer just shifting the values when defining them, which is more
> consistent with how CSIZE is handled too.
> 
> > +			else
> > +				bits |= XR21V141X_UART_PARITY_SPACE <<
> > +					XR21V141X_UART_PARITY_SHIFT;
> > +		} else {
> > +			if (C_PARODD(tty))
> > +				bits |= XR21V141X_UART_PARITY_ODD <<
> > +					XR21V141X_UART_PARITY_SHIFT;
> > +			else
> > +				bits |= XR21V141X_UART_PARITY_EVEN <<
> > +					XR21V141X_UART_PARITY_SHIFT;
> > +		}
> > +	}
> > +
> > +	if (C_CSTOPB(tty))
> > +		bits |= XR21V141X_UART_STOP_2 << XR21V141X_UART_STOP_SHIFT;
> > +	else
> > +		bits |= XR21V141X_UART_STOP_1 << XR21V141X_UART_STOP_SHIFT;
> > +
> > +	ret = xr_set_reg_uart(port, XR21V141X_REG_FORMAT, bits);
> > +	if (ret)
> > +		return;
> > +
> > +	/* If baud rate is B0, clear DTR and RTS */
> > +	if (C_BAUD(tty) == B0)
> > +		xr_dtr_rts(port, 0);
> 
> This isn't sufficient; RTS will not be dropped when CRTSCTS is enabled,
> and we should reassert the lines when moving from B0 too.
> 
> > +
> > +	xr_set_flow_mode(tty, port);
> > +}
> > +
> > +static int xr_open(struct tty_struct *tty, struct usb_serial_port *port)
> > +{
> > +	int ret;
> > +
> > +	ret = xr_uart_enable(port);
> > +	if (ret) {
> > +		dev_err(&port->dev, "Failed to enable UART\n");
> > +		return ret;
> > +	}
> > +
> > +	/* Setup termios */
> > +	if (tty)
> > +		xr_set_termios(tty, port, NULL);
> > +
> > +	ret = usb_serial_generic_open(tty, port);
> > +	if (ret) {
> > +		xr_uart_disable(port);
> > +		return ret;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static void xr_close(struct usb_serial_port *port)
> > +{
> > +	usb_serial_generic_close(port);
> > +
> > +	xr_uart_disable(port);
> > +}
> > +
> > +static int xr_probe(struct usb_serial *serial, const struct usb_device_id *id)
> > +{
> > +	struct usb_device *usb_dev = interface_to_usbdev(serial->interface);
> 
> You can just use serial->dev directly.
> 
> > +	struct usb_driver *driver = serial->type->usb_driver;
> > +	struct usb_interface *control_interface;
> > +	int ret;
> > +
> > +	/* Don't bind to control interface */
> > +	if (serial->interface->cur_altsetting->desc.bInterfaceNumber == 0)
> > +		return -ENODEV;
> > +
> > +	/* But claim the control interface during data interface probe */
> > +	control_interface = usb_ifnum_to_if(usb_dev, 0);
> 
> You must check for NULL here since a device may not have an interface 0
> in case you'll oops here. This can be triggered by a malicious device or
> when fuzzing USB descriptors.
> 
> > +	ret = usb_driver_claim_interface(driver, control_interface, NULL);
> > +	if (ret) {
> > +		dev_err(&serial->interface->dev, "Can't claim control interface\n");
> > +		return ret;
> > +	}
> 
> And you must release the control interface again when unbinding the
> driver to avoid leaking resources and to allow the driver to be rebound.
> 
> > +
> > +	return 0;
> > +}
> > +
> > +static int xr_tiocmget(struct tty_struct *tty)
> > +{
> > +	struct usb_serial_port *port = tty->driver_data;
> > +	u8 status;
> > +	int ret;
> > +
> > +	ret = xr_get_reg_uart(port, XR21V141X_REG_GPIO_STATUS, &status);
> > +	if (ret)
> > +		return ret;
> > +
> > +	/*
> > +	 * Modem control pins are active low, so reading '0' means it is active
> > +	 * and '1' means not active.
> > +	 */
> > +	ret = ((status & XR21V141X_UART_MODE_DTR) ? 0 : TIOCM_DTR) |
> > +	      ((status & XR21V141X_UART_MODE_RTS) ? 0 : TIOCM_RTS) |
> > +	      ((status & XR21V141X_UART_MODE_CTS) ? 0 : TIOCM_CTS) |
> > +	      ((status & XR21V141X_UART_MODE_DSR) ? 0 : TIOCM_DSR) |
> > +	      ((status & XR21V141X_UART_MODE_RI) ? 0 : TIOCM_RI) |
> > +	      ((status & XR21V141X_UART_MODE_CD) ? 0 : TIOCM_CD);
> > +
> > +	return ret;
> > +}
> > +
> > +static void xr_break_ctl(struct tty_struct *tty, int break_state)
> > +{
> > +	struct usb_serial_port *port = tty->driver_data;
> > +	u8 state;
> > +
> > +	if (break_state == 0)
> > +		state = XR21V141X_UART_BREAK_OFF;
> > +	else
> > +		state = XR21V141X_UART_BREAK_ON;
> > +
> > +	dev_dbg(&port->dev, "Turning break %s\n",
> > +		state == XR21V141X_UART_BREAK_OFF ? "off" : "on");
> > +	xr_set_reg_uart(port, XR21V141X_REG_TX_BREAK, state);
> > +}
> > +
> > +static int xr_port_probe(struct usb_serial_port *port)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int xr_port_remove(struct usb_serial_port *port)
> > +{
> > +	return 0;
> > +}
> 
> Again, don't add these until you need them.
> 
> > +
> > +static const struct usb_device_id id_table[] = {
> > +	{ USB_DEVICE(0x04e2, 0x1410) }, /* XR21V141X */
> > +	{ }
> > +};
> > +MODULE_DEVICE_TABLE(usb, id_table);
> > +
> > +static struct usb_serial_driver xr_device = {
> > +	.driver = {
> > +		.owner = THIS_MODULE,
> > +		.name =	"xr_serial",
> > +	},
> > +	.id_table		= id_table,
> > +	.num_ports		= 1,
> > +	.open			= xr_open,
> > +	.close			= xr_close,
> > +	.break_ctl		= xr_break_ctl,
> > +	.set_termios		= xr_set_termios,
> > +	.tiocmget		= xr_tiocmget,
> > +	.tiocmset		= xr_tiocmset,
> > +	.probe			= xr_probe,
> > +	.port_probe		= xr_port_probe,
> > +	.port_remove		= xr_port_remove,
> > +	.dtr_rts		= xr_dtr_rts
> > +};
> > +
> > +static struct usb_serial_driver * const serial_drivers[] = {
> > +	&xr_device, NULL
> > +};
> > +
> > +module_usb_serial_driver(serial_drivers, id_table);
> > +
> > +MODULE_AUTHOR("Manivannan Sadhasivam <mani@kernel.org>");
> > +MODULE_DESCRIPTION("MaxLinear/Exar USB to Serial driver");
> > +MODULE_LICENSE("GPL");
> 
> Looks good overall otherwise.
> 
> I've applied this one now, and will follow up with a series addressing
> the non-trivial bits pointed out below.
> 
> Johan