ch340usb转串口在linux上波特率误差

前段时间在使用linux mint 19.01系统上使用ch340的usb转串口工具，发现波特率在961200的情况下数据总是不正确，而在115200波特率下又是正常的，于是在谷歌上搜索了一下，发现我使用的系统采用的linux内核版本4.15.0上ch340的驱动存在波特率误差的问题。

在一番搜索下在这里找到了一个对内核驱动的patch：https://patchwork.kernel.org/patch/10983017/

这个作者针对内核里的ch341驱动进行了详细的测试并提供了一份新的驱动，目前该驱动已合并到内核main line上5.5版本了，可以在这里查看该驱动源码：https://elixir.bootlin.com/linux/v5.5-rc5/source/drivers/usb/serial/ch341.c

另外驱动的作者在github上创建了一个项目，专门针对ch340的波特率问题进行了相关研究，其中给出了非常详细的波特率计算公式和误差测试，地址如下：https://github.com/nospam2000/ch341-baudrate-calculation

由于目前大多数的linux发行版都还使用较老的内核版本。因此该问题一直没被解决，那么我们这里就将5.5版本的驱动代码在目前使用的发行版上重新编译替换自带的驱动以修复这个问题。

PC

• 首先创建ch341.c文件，内容如下：

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright 2007, Frank A Kingswood <frank@kingswood-consulting.co.uk>
 * Copyright 2007, Werner Cornelius <werner@cornelius-consult.de>
 * Copyright 2009, Boris Hajduk <boris@hajduk.org>
 *
 * ch341.c implements a serial port driver for the Winchiphead CH341.
 *
 * The CH341 device can be used to implement an RS232 asynchronous
 * serial port, an IEEE-1284 parallel printer port or a memory-like
 * interface. In all cases the CH341 supports an I2C interface as well.
 * This driver only supports the asynchronous serial interface.
 */

#include <linux/kernel.h>
#include <linux/tty.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/usb.h>
#include <linux/usb/serial.h>
#include <linux/serial.h>
#include <asm/unaligned.h>

#define DEFAULT_BAUD_RATE 9600
#define DEFAULT_TIMEOUT   1000

/* flags for IO-Bits */
#define CH341_BIT_RTS (1 << 6)
#define CH341_BIT_DTR (1 << 5)

/******************************/
/* interrupt pipe definitions */
/******************************/
/* always 4 interrupt bytes */
/* first irq byte normally 0x08 */
/* second irq byte base 0x7d + below */
/* third irq byte base 0x94 + below */
/* fourth irq byte normally 0xee */

/* second interrupt byte */
#define CH341_MULT_STAT 0x04 /* multiple status since last interrupt event */

/* status returned in third interrupt answer byte, inverted in data
   from irq */
#define CH341_BIT_CTS 0x01
#define CH341_BIT_DSR 0x02
#define CH341_BIT_RI  0x04
#define CH341_BIT_DCD 0x08
#define CH341_BITS_MODEM_STAT 0x0f /* all bits */

/* Break support - the information used to implement this was gleaned from
 * the Net/FreeBSD uchcom.c driver by Takanori Watanabe.  Domo arigato.
 */

#define CH341_REQ_READ_VERSION 0x5F
#define CH341_REQ_WRITE_REG    0x9A
#define CH341_REQ_READ_REG     0x95
#define CH341_REQ_SERIAL_INIT  0xA1
#define CH341_REQ_MODEM_CTRL   0xA4

#define CH341_REG_BREAK        0x05
#define CH341_REG_LCR          0x18
#define CH341_NBREAK_BITS      0x01

#define CH341_LCR_ENABLE_RX    0x80
#define CH341_LCR_ENABLE_TX    0x40
#define CH341_LCR_MARK_SPACE   0x20
#define CH341_LCR_PAR_EVEN     0x10
#define CH341_LCR_ENABLE_PAR   0x08
#define CH341_LCR_STOP_BITS_2  0x04
#define CH341_LCR_CS8          0x03
#define CH341_LCR_CS7          0x02
#define CH341_LCR_CS6          0x01
#define CH341_LCR_CS5          0x00

static const struct usb_device_id id_table[] = {
	{ USB_DEVICE(0x4348, 0x5523) },
	{ USB_DEVICE(0x1a86, 0x7523) },
	{ USB_DEVICE(0x1a86, 0x5523) },
	{ },
};
MODULE_DEVICE_TABLE(usb, id_table);

struct ch341_private {
	spinlock_t lock; /* access lock */
	unsigned baud_rate; /* set baud rate */
	u8 mcr;
	u8 msr;
	u8 lcr;
};

static void ch341_set_termios(struct tty_struct *tty,
			      struct usb_serial_port *port,
			      struct ktermios *old_termios);

static int ch341_control_out(struct usb_device *dev, u8 request,
			     u16 value, u16 index)
{
	int r;

	dev_dbg(&dev->dev, "%s - (%02x,%04x,%04x)\n", __func__,
		request, value, index);

	r = usb_control_msg(dev, usb_sndctrlpipe(dev, 0), request,
			    USB_TYPE_VENDOR | USB_RECIP_DEVICE | USB_DIR_OUT,
			    value, index, NULL, 0, DEFAULT_TIMEOUT);
	if (r < 0)
		dev_err(&dev->dev, "failed to send control message: %d\n", r);

	return r;
}

static int ch341_control_in(struct usb_device *dev,
			    u8 request, u16 value, u16 index,
			    char *buf, unsigned bufsize)
{
	int r;

	dev_dbg(&dev->dev, "%s - (%02x,%04x,%04x,%u)\n", __func__,
		request, value, index, bufsize);

	r = usb_control_msg(dev, usb_rcvctrlpipe(dev, 0), request,
			    USB_TYPE_VENDOR | USB_RECIP_DEVICE | USB_DIR_IN,
			    value, index, buf, bufsize, DEFAULT_TIMEOUT);
	if (r < (int)bufsize) {
		if (r >= 0) {
			dev_err(&dev->dev,
				"short control message received (%d < %u)\n",
				r, bufsize);
			r = -EIO;
		}

		dev_err(&dev->dev, "failed to receive control message: %d\n",
			r);
		return r;
	}

	return 0;
}

#define CH341_CLKRATE		48000000
#define CH341_CLK_DIV(ps, fact)	(1 << (12 - 3 * (ps) - (fact)))
#define CH341_MIN_RATE(ps)	(CH341_CLKRATE / (CH341_CLK_DIV((ps), 1) * 512))

static const speed_t ch341_min_rates[] = {
	CH341_MIN_RATE(0),
	CH341_MIN_RATE(1),
	CH341_MIN_RATE(2),
	CH341_MIN_RATE(3),
};

/*
 * The device line speed is given by the following equation:
 *
 *	baudrate = 48000000 / (2^(12 - 3 * ps - fact) * div), where
 *
 *		0 <= ps <= 3,
 *		0 <= fact <= 1,
 *		2 <= div <= 256 if fact = 0, or
 *		9 <= div <= 256 if fact = 1
 */
static int ch341_get_divisor(speed_t speed)
{
	unsigned int fact, div, clk_div;
	int ps;

	/*
	 * Clamp to supported range, this makes the (ps < 0) and (div < 2)
	 * sanity checks below redundant.
	 */
	speed = clamp(speed, 46U, 3000000U);

	/*
	 * Start with highest possible base clock (fact = 1) that will give a
	 * divisor strictly less than 512.
	 */
	fact = 1;
	for (ps = 3; ps >= 0; ps--) {
		if (speed > ch341_min_rates[ps])
			break;
	}

	if (ps < 0)
		return -EINVAL;

	/* Determine corresponding divisor, rounding down. */
	clk_div = CH341_CLK_DIV(ps, fact);
	div = CH341_CLKRATE / (clk_div * speed);

	/* Halve base clock (fact = 0) if required. */
	if (div < 9 || div > 255) {
		div /= 2;
		clk_div *= 2;
		fact = 0;
	}

	if (div < 2)
		return -EINVAL;

	/*
	 * Pick next divisor if resulting rate is closer to the requested one,
	 * scale up to avoid rounding errors on low rates.
	 */
	if (16 * CH341_CLKRATE / (clk_div * div) - 16 * speed >=
			16 * speed - 16 * CH341_CLKRATE / (clk_div * (div + 1)))
		div++;

	return (0x100 - div) << 8 | fact << 2 | ps;
}

static int ch341_set_baudrate_lcr(struct usb_device *dev,
				  struct ch341_private *priv, u8 lcr)
{
	int val;
	int r;

	if (!priv->baud_rate)
		return -EINVAL;

	val = ch341_get_divisor(priv->baud_rate);
	if (val < 0)
		return -EINVAL;

	/*
	 * CH341A buffers data until a full endpoint-size packet (32 bytes)
	 * has been received unless bit 7 is set.
	 */
	val |= BIT(7);

	r = ch341_control_out(dev, CH341_REQ_WRITE_REG, 0x1312, val);
	if (r)
		return r;

	r = ch341_control_out(dev, CH341_REQ_WRITE_REG, 0x2518, lcr);
	if (r)
		return r;

	return r;
}

static int ch341_set_handshake(struct usb_device *dev, u8 control)
{
	return ch341_control_out(dev, CH341_REQ_MODEM_CTRL, ~control, 0);
}

static int ch341_get_status(struct usb_device *dev, struct ch341_private *priv)
{
	const unsigned int size = 2;
	char *buffer;
	int r;
	unsigned long flags;

	buffer = kmalloc(size, GFP_KERNEL);
	if (!buffer)
		return -ENOMEM;

	r = ch341_control_in(dev, CH341_REQ_READ_REG, 0x0706, 0, buffer, size);
	if (r < 0)
		goto out;

	spin_lock_irqsave(&priv->lock, flags);
	priv->msr = (~(*buffer)) & CH341_BITS_MODEM_STAT;
	spin_unlock_irqrestore(&priv->lock, flags);

out:	kfree(buffer);
	return r;
}

/* -------------------------------------------------------------------------- */

static int ch341_configure(struct usb_device *dev, struct ch341_private *priv)
{
	const unsigned int size = 2;
	char *buffer;
	int r;

	buffer = kmalloc(size, GFP_KERNEL);
	if (!buffer)
		return -ENOMEM;

	/* expect two bytes 0x27 0x00 */
	r = ch341_control_in(dev, CH341_REQ_READ_VERSION, 0, 0, buffer, size);
	if (r < 0)
		goto out;
	dev_dbg(&dev->dev, "Chip version: 0x%02x\n", buffer[0]);

	r = ch341_control_out(dev, CH341_REQ_SERIAL_INIT, 0, 0);
	if (r < 0)
		goto out;

	r = ch341_set_baudrate_lcr(dev, priv, priv->lcr);
	if (r < 0)
		goto out;

	r = ch341_set_handshake(dev, priv->mcr);

out:	kfree(buffer);
	return r;
}

static int ch341_port_probe(struct usb_serial_port *port)
{
	struct ch341_private *priv;
	int r;

	priv = kzalloc(sizeof(struct ch341_private), GFP_KERNEL);
	if (!priv)
		return -ENOMEM;

	spin_lock_init(&priv->lock);
	priv->baud_rate = DEFAULT_BAUD_RATE;
	/*
	 * Some CH340 devices appear unable to change the initial LCR
	 * settings, so set a sane 8N1 default.
	 */
	priv->lcr = CH341_LCR_ENABLE_RX | CH341_LCR_ENABLE_TX | CH341_LCR_CS8;

	r = ch341_configure(port->serial->dev, priv);
	if (r < 0)
		goto error;

	usb_set_serial_port_data(port, priv);
	return 0;

error:	kfree(priv);
	return r;
}

static int ch341_port_remove(struct usb_serial_port *port)
{
	struct ch341_private *priv;

	priv = usb_get_serial_port_data(port);
	kfree(priv);

	return 0;
}

static int ch341_carrier_raised(struct usb_serial_port *port)
{
	struct ch341_private *priv = usb_get_serial_port_data(port);
	if (priv->msr & CH341_BIT_DCD)
		return 1;
	return 0;
}

static void ch341_dtr_rts(struct usb_serial_port *port, int on)
{
	struct ch341_private *priv = usb_get_serial_port_data(port);
	unsigned long flags;

	/* drop DTR and RTS */
	spin_lock_irqsave(&priv->lock, flags);
	if (on)
		priv->mcr |= CH341_BIT_RTS | CH341_BIT_DTR;
	else
		priv->mcr &= ~(CH341_BIT_RTS | CH341_BIT_DTR);
	spin_unlock_irqrestore(&priv->lock, flags);
	ch341_set_handshake(port->serial->dev, priv->mcr);
}

static void ch341_close(struct usb_serial_port *port)
{
	usb_serial_generic_close(port);
	usb_kill_urb(port->interrupt_in_urb);
}


/* open this device, set default parameters */
static int ch341_open(struct tty_struct *tty, struct usb_serial_port *port)
{
	struct ch341_private *priv = usb_get_serial_port_data(port);
	int r;

	if (tty)
		ch341_set_termios(tty, port, NULL);

	dev_dbg(&port->dev, "%s - submitting interrupt urb\n", __func__);
	r = usb_submit_urb(port->interrupt_in_urb, GFP_KERNEL);
	if (r) {
		dev_err(&port->dev, "%s - failed to submit interrupt urb: %d\n",
			__func__, r);
		return r;
	}

	r = ch341_get_status(port->serial->dev, priv);
	if (r < 0) {
		dev_err(&port->dev, "failed to read modem status: %d\n", r);
		goto err_kill_interrupt_urb;
	}

	r = usb_serial_generic_open(tty, port);
	if (r)
		goto err_kill_interrupt_urb;

	return 0;

err_kill_interrupt_urb:
	usb_kill_urb(port->interrupt_in_urb);

	return r;
}

/* Old_termios contains the original termios settings and
 * tty->termios contains the new setting to be used.
 */
static void ch341_set_termios(struct tty_struct *tty,
		struct usb_serial_port *port, struct ktermios *old_termios)
{
	struct ch341_private *priv = usb_get_serial_port_data(port);
	unsigned baud_rate;
	unsigned long flags;
	u8 lcr;
	int r;

	/* redundant changes may cause the chip to lose bytes */
	if (old_termios && !tty_termios_hw_change(&tty->termios, old_termios))
		return;

	baud_rate = tty_get_baud_rate(tty);

	lcr = CH341_LCR_ENABLE_RX | CH341_LCR_ENABLE_TX;

	switch (C_CSIZE(tty)) {
	case CS5:
		lcr |= CH341_LCR_CS5;
		break;
	case CS6:
		lcr |= CH341_LCR_CS6;
		break;
	case CS7:
		lcr |= CH341_LCR_CS7;
		break;
	case CS8:
		lcr |= CH341_LCR_CS8;
		break;
	}

	if (C_PARENB(tty)) {
		lcr |= CH341_LCR_ENABLE_PAR;
		if (C_PARODD(tty) == 0)
			lcr |= CH341_LCR_PAR_EVEN;
		if (C_CMSPAR(tty))
			lcr |= CH341_LCR_MARK_SPACE;
	}

	if (C_CSTOPB(tty))
		lcr |= CH341_LCR_STOP_BITS_2;

	if (baud_rate) {
		priv->baud_rate = baud_rate;

		r = ch341_set_baudrate_lcr(port->serial->dev, priv, lcr);
		if (r < 0 && old_termios) {
			priv->baud_rate = tty_termios_baud_rate(old_termios);
			tty_termios_copy_hw(&tty->termios, old_termios);
		} else if (r == 0) {
			priv->lcr = lcr;
		}
	}

	spin_lock_irqsave(&priv->lock, flags);
	if (C_BAUD(tty) == B0)
		priv->mcr &= ~(CH341_BIT_DTR | CH341_BIT_RTS);
	else if (old_termios && (old_termios->c_cflag & CBAUD) == B0)
		priv->mcr |= (CH341_BIT_DTR | CH341_BIT_RTS);
	spin_unlock_irqrestore(&priv->lock, flags);

	ch341_set_handshake(port->serial->dev, priv->mcr);
}

static void ch341_break_ctl(struct tty_struct *tty, int break_state)
{
	const uint16_t ch341_break_reg =
			((uint16_t) CH341_REG_LCR << 8) | CH341_REG_BREAK;
	struct usb_serial_port *port = tty->driver_data;
	int r;
	uint16_t reg_contents;
	uint8_t *break_reg;

	break_reg = kmalloc(2, GFP_KERNEL);
	if (!break_reg)
		return;

	r = ch341_control_in(port->serial->dev, CH341_REQ_READ_REG,
			ch341_break_reg, 0, break_reg, 2);
	if (r < 0) {
		dev_err(&port->dev, "%s - USB control read error (%d)\n",
				__func__, r);
		goto out;
	}
	dev_dbg(&port->dev, "%s - initial ch341 break register contents - reg1: %x, reg2: %x\n",
		__func__, break_reg[0], break_reg[1]);
	if (break_state != 0) {
		dev_dbg(&port->dev, "%s - Enter break state requested\n", __func__);
		break_reg[0] &= ~CH341_NBREAK_BITS;
		break_reg[1] &= ~CH341_LCR_ENABLE_TX;
	} else {
		dev_dbg(&port->dev, "%s - Leave break state requested\n", __func__);
		break_reg[0] |= CH341_NBREAK_BITS;
		break_reg[1] |= CH341_LCR_ENABLE_TX;
	}
	dev_dbg(&port->dev, "%s - New ch341 break register contents - reg1: %x, reg2: %x\n",
		__func__, break_reg[0], break_reg[1]);
	reg_contents = get_unaligned_le16(break_reg);
	r = ch341_control_out(port->serial->dev, CH341_REQ_WRITE_REG,
			ch341_break_reg, reg_contents);
	if (r < 0)
		dev_err(&port->dev, "%s - USB control write error (%d)\n",
				__func__, r);
out:
	kfree(break_reg);
}

static int ch341_tiocmset(struct tty_struct *tty,
			  unsigned int set, unsigned int clear)
{
	struct usb_serial_port *port = tty->driver_data;
	struct ch341_private *priv = usb_get_serial_port_data(port);
	unsigned long flags;
	u8 control;

	spin_lock_irqsave(&priv->lock, flags);
	if (set & TIOCM_RTS)
		priv->mcr |= CH341_BIT_RTS;
	if (set & TIOCM_DTR)
		priv->mcr |= CH341_BIT_DTR;
	if (clear & TIOCM_RTS)
		priv->mcr &= ~CH341_BIT_RTS;
	if (clear & TIOCM_DTR)
		priv->mcr &= ~CH341_BIT_DTR;
	control = priv->mcr;
	spin_unlock_irqrestore(&priv->lock, flags);

	return ch341_set_handshake(port->serial->dev, control);
}

static void ch341_update_status(struct usb_serial_port *port,
					unsigned char *data, size_t len)
{
	struct ch341_private *priv = usb_get_serial_port_data(port);
	struct tty_struct *tty;
	unsigned long flags;
	u8 status;
	u8 delta;

	if (len < 4)
		return;

	status = ~data[2] & CH341_BITS_MODEM_STAT;

	spin_lock_irqsave(&priv->lock, flags);
	delta = status ^ priv->msr;
	priv->msr = status;
	spin_unlock_irqrestore(&priv->lock, flags);

	if (data[1] & CH341_MULT_STAT)
		dev_dbg(&port->dev, "%s - multiple status change\n", __func__);

	if (!delta)
		return;

	if (delta & CH341_BIT_CTS)
		port->icount.cts++;
	if (delta & CH341_BIT_DSR)
		port->icount.dsr++;
	if (delta & CH341_BIT_RI)
		port->icount.rng++;
	if (delta & CH341_BIT_DCD) {
		port->icount.dcd++;
		tty = tty_port_tty_get(&port->port);
		if (tty) {
			usb_serial_handle_dcd_change(port, tty,
						status & CH341_BIT_DCD);
			tty_kref_put(tty);
		}
	}

	wake_up_interruptible(&port->port.delta_msr_wait);
}

static void ch341_read_int_callback(struct urb *urb)
{
	struct usb_serial_port *port = urb->context;
	unsigned char *data = urb->transfer_buffer;
	unsigned int len = urb->actual_length;
	int status;

	switch (urb->status) {
	case 0:
		/* success */
		break;
	case -ECONNRESET:
	case -ENOENT:
	case -ESHUTDOWN:
		/* this urb is terminated, clean up */
		dev_dbg(&urb->dev->dev, "%s - urb shutting down: %d\n",
			__func__, urb->status);
		return;
	default:
		dev_dbg(&urb->dev->dev, "%s - nonzero urb status: %d\n",
			__func__, urb->status);
		goto exit;
	}

	usb_serial_debug_data(&port->dev, __func__, len, data);
	ch341_update_status(port, data, len);
exit:
	status = usb_submit_urb(urb, GFP_ATOMIC);
	if (status) {
		dev_err(&urb->dev->dev, "%s - usb_submit_urb failed: %d\n",
			__func__, status);
	}
}

static int ch341_tiocmget(struct tty_struct *tty)
{
	struct usb_serial_port *port = tty->driver_data;
	struct ch341_private *priv = usb_get_serial_port_data(port);
	unsigned long flags;
	u8 mcr;
	u8 status;
	unsigned int result;

	spin_lock_irqsave(&priv->lock, flags);
	mcr = priv->mcr;
	status = priv->msr;
	spin_unlock_irqrestore(&priv->lock, flags);

	result = ((mcr & CH341_BIT_DTR)		? TIOCM_DTR : 0)
		  | ((mcr & CH341_BIT_RTS)	? TIOCM_RTS : 0)
		  | ((status & CH341_BIT_CTS)	? TIOCM_CTS : 0)
		  | ((status & CH341_BIT_DSR)	? TIOCM_DSR : 0)
		  | ((status & CH341_BIT_RI)	? TIOCM_RI  : 0)
		  | ((status & CH341_BIT_DCD)	? TIOCM_CD  : 0);

	dev_dbg(&port->dev, "%s - result = %x\n", __func__, result);

	return result;
}

static int ch341_reset_resume(struct usb_serial *serial)
{
	struct usb_serial_port *port = serial->port[0];
	struct ch341_private *priv = usb_get_serial_port_data(port);
	int ret;

	/* reconfigure ch341 serial port after bus-reset */
	ch341_configure(serial->dev, priv);

	if (tty_port_initialized(&port->port)) {
		ret = usb_submit_urb(port->interrupt_in_urb, GFP_NOIO);
		if (ret) {
			dev_err(&port->dev, "failed to submit interrupt urb: %d\n",
				ret);
			return ret;
		}

		ret = ch341_get_status(port->serial->dev, priv);
		if (ret < 0) {
			dev_err(&port->dev, "failed to read modem status: %d\n",
				ret);
		}
	}

	return usb_serial_generic_resume(serial);
}

static struct usb_serial_driver ch341_device = {
	.driver = {
		.owner	= THIS_MODULE,
		.name	= "ch341-uart",
	},
	.id_table          = id_table,
	.num_ports         = 1,
	.open              = ch341_open,
	.dtr_rts	   = ch341_dtr_rts,
	.carrier_raised	   = ch341_carrier_raised,
	.close             = ch341_close,
	.set_termios       = ch341_set_termios,
	.break_ctl         = ch341_break_ctl,
	.tiocmget          = ch341_tiocmget,
	.tiocmset          = ch341_tiocmset,
	.tiocmiwait        = usb_serial_generic_tiocmiwait,
	.read_int_callback = ch341_read_int_callback,
	.port_probe        = ch341_port_probe,
	.port_remove       = ch341_port_remove,
	.reset_resume      = ch341_reset_resume,
};

static struct usb_serial_driver * const serial_drivers[] = {
	&ch341_device, NULL
};

module_usb_serial_driver(serial_drivers, id_table);

MODULE_LICENSE("GPL v2");

• 创建Makefile，内容如下

ifeq ($(KERNELRELEASE), )
KERNELDIR := /lib/modules/$(shell uname -r)/build
PWD :=$(shell pwd)
default:
	$(MAKE) -C $(KERNELDIR)  M=$(PWD)
clean:
	rm -rf .tmp_versions Module.symvers *.mod.c *.o *.ko .*.cmd Module.markers modules.order .cache.mk
load:
	modprobe usbserial
	insmod ch341.ko
unload:
	rmmod ch341
install:
	cp ch341.ko /lib/modules/$(shell uname -r)/kernel/drivers/usb/serial/ch341.ko
else
	obj-m := ch341.o
endif

• 在终端依次执行

make
sudo make install

• 重新载入驱动或者重启系统，驱动更新完成，实测ch340波特率正常通信正常

树莓派

最后，这个问题笔者在树莓派上使用ch340也发现有相同问题，因此想更新树莓派上的驱动，树莓派上没有相关驱动编译工具，只能在pc上进行交叉编译，所以编译ko时还得先编译内核

• 首先去树莓派官方git仓库找到你使用的系统版本，切记要找到和你使用内核版本一致的那一次提交，可以在这里查找：https://github.com/raspberrypi/linux/releases ，使用git命令拉取，示例如下：

git clone --depth=1 --branch raspberrypi-kernel_1.20190925-1 https://github.com/raspberrypi/linux.git

• 安装必要的工具，拉取交叉编译工具链，并配置环境变量，请根据自己的路径指定，注意这里环境变量是一次性的，关闭终端之后就没有了

sudo apt install bc bison flex libssl-dev make libc6-dev libncurses5-dev
git clone https://github.com/raspberrypi/tools
export PATH="$PATH:$HOME/Desktop/tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin"

• 然后进入源码的linux/drivers/usb/serial目录内修改ch341.c文件
• 进入源码根目录，对于树莓派4，其他树莓派版本可查看：https://www.raspberrypi.org/documentation/linux/kernel/building.md

cd linux
KERNEL=kernel7l
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- bcm2711_defconfig
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- zImage modules dtbs

• 完成进入linux/drivers/usb/serial目录将ch341.ko文件拷贝至树莓派内，更新驱动模块即可。