ch340usb轉串口在linux上波特率誤差

ch340usb轉串口在linux上波特率誤差

前段時間在使用linux mint 19.01系統上使用ch340的usb轉串口工具，發現波特率在961200的情況下數據總是不正確，而在115200波特率下又是正常的，於是在谷歌上搜索了一下，發現我使用的系統採用的linux內核版本4.15.0上ch340的驅動存在波特率誤差的問題。

在一番搜索下在這裏找到了一個對內核驅動的patch：https://patchwork.kernel.org/patch/10983017/

這個作者針對內核裏的ch341驅動進行了詳細的測試並提供了一份新的驅動，目前該驅動已合併到內核main line上5.5版本了，可以在這裏查看該驅動源碼：https://elixir.bootlin.com/linux/v5.5-rc5/source/drivers/usb/serial/ch341.c

另外驅動的作者在github上創建了一個項目，專門針對ch340的波特率問題進行了相關研究，其中給出了非常詳細的波特率計算公式和誤差測試，地址如下：https://github.com/nospam2000/ch341-baudrate-calculation

由於目前大多數的linux發行版都還使用較老的內核版本。因此該問題一直沒被解決，那麼我們這裏就將5.5版本的驅動代碼在目前使用的發行版上重新編譯替換自帶的驅動以修復這個問題。

PC

• 首先創建ch341.c文件，內容如下：

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright 2007, Frank A Kingswood <[email protected]>
 * Copyright 2007, Werner Cornelius <[email protected]>
 * Copyright 2009, Boris Hajduk <[email protected]>
 *
 * 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");

• 創建Makefile，內容如下

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

• 在終端依次執行

make
sudo make install

• 重新載入驅動或者重啓系統，驅動更新完成，實測ch340波特率正常通信正常

樹莓派

最後，這個問題筆者在樹莓派上使用ch340也發現有相同問題，因此想更新樹莓派上的驅動，樹莓派上沒有相關驅動編譯工具，只能在pc上進行交叉編譯，所以編譯ko時還得先編譯內核

• 首先去樹莓派官方git倉庫找到你使用的系統版本，切記要找到和你使用內核版本一致的那一次提交，可以在這裏查找：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

• 安裝必要的工具，拉取交叉編譯工具鏈，並配置環境變量，請根據自己的路徑指定，注意這裏環境變量是一次性的，關閉終端之後就沒有了

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"

• 然後進入源碼的linux/drivers/usb/serial目錄內修改ch341.c文件
• 進入源碼根目錄，對於樹莓派4，其他樹莓派版本可查看：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

• 完成進入linux/drivers/usb/serial目錄將ch341.ko文件拷貝至樹莓派內，更新驅動模塊即可。