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2.1、开发环境：Ubuntu12.04
2.2、软件版本：openwrt 官方15.05版本SDK开发包（CHAOS CALMER 15.05版本）
本文章所选择的目标芯片为MT7620，profile 选择的为“Xiaomi MiWiFi Mini ”。
3.1、在openwrt 系统上，移植mcp7940的rtc芯片驱动。
3.2、在openwrt系统上，增加对i2c总线的支持。
注意事项：openwrt系统比较奇怪，在menuconfig配置中，配置了i2c，仍然不能支持。需要另外修改“*.dts”文件，才能支持i2c总线。
对于系统增加i2c总线的支持，需要修改2个地方
1、openwrt增加对i2c支持。
2、目标芯片的kernel，增加i2c支持。
3、修改dts文件，增加对i2c支持。
在openwrt 目录下，执行“make menuconfig”命令。
进入菜单 Kernel modules --->I2C support --->，在菜单选项中，配置如图4-1所示。
图4-1 I2C support
在openwrt 目录下，执行"make kernel_menuconfig"命令，对目标芯片内核进行配置。
进入菜单， Device Drivers ---> I2C support --->，如图4-2所示。
图4-2 目标芯片的I2C总线配置
在图4-2中，选择“I2C Hardware Bus support ---> ”，配置I2C 硬件总线的支持，选择“Ralink I2C Controller”，如图4-3所示。
图4-3 I2C Hardware Bus support
默认的openwrt系统，没有对I2C总线的支持，需要自己修改DTS配置文件。由于本文章选择的芯片为MT7620A，目标profile 选择的为“Xiaomi MiWiFi Mini”，故需要修改的文件为“XIAOMI-MIWIFI-MINI.dts”
文件的路径为“openwrt/target/linux/ramips/dts/XIAOMI-MIWIFI-MINI.dts”
修改之前的DTS文件：
palmbus@10000000 {
gpio0: gpio@600 {
status = "okay";
};
gpio1: gpio@638 {
status = "okay";
};
gpio2: gpio@660 {
status = "okay";
};
spi@b00 {
status = "okay";
m25p80@0 {
palmbus@10000000 {
gpio0: gpio@600 {
status = "okay";
};
gpio1: gpio@638 {
status = "okay";
};
gpio2: gpio@660 {
status = "okay";
};
i2c@900 {
compatible ="ralink,mt7620a-i2c", "ralink,rt2880-i2c";
reg= <0x900 0x100>;
resets = <&rstctrl 16>;
reset-names= "i2c";
#address-cells = <1>;
#size-cells= <0>;
status= "okay";
rtc@6f {
compatible = "mcp,mcp7940";
reg = <0x6f>;
};
};
spi@b00 {
status = "okay";
m25p80@0 {
1、i2c@900为MT7620A的I2C节点；
2、对于MCP7940芯片，通过查找芯片手册，知道其通信地址为0x6f。如果需要换成其他的芯片，则将对应的地址0x6f都改成其对应的地址即可。
3、对于RTC的名称描述也需要注意，compatible = "mcp,mcp7940"，其中mcp7940对应本文5.1章节，驱动程序rtc_mcp7940.c中第93行，“struct i2c_device_id”中的名称。如果名字不匹配，则会导致驱动程序执行到probe函数不成功，导致rtc驱动加载失败。
static const struct i2c_device_id mcp7940_id[] = {
{ "mcp7940", mcp7940 },
{ }
};4、如果I2C的引脚，没有设置，则默认可能工作在GPIO模式，特别注意！在本文中的RTC驱动程序，将I2C的引脚设置为了I2C模式，所以在DTS文件中，就没有再设置I2C引脚的工作模式。如果自己的RTC驱动程序中，没有设置I2C引脚，请在DTS文件中设置其工作为I2C模式，以免它默认工作在GPIO模式中！
i2c@900 {
compatible ="ralink,mt7620a-i2c", "ralink,rt2880-i2c";
reg= <0x900 0x100>;
resets = <&rstctrl 16>;
reset-names= "i2c";
#address-cells = <1>;
#size-cells= <0>;
status= "okay";
rtc@6f {
compatible = "mcp,mcp7940";
reg = <0x6f>;
};
};
如果需要修改为ds1307芯片，则只需要将其中的rtc地址和comatible修改即可。ds1307的通信地址为0x68，则修改如下：
rtc@68 {
compatible = "dallas,ds1307";
reg = <0x68>;
}; 系统在./scripts/medatata.pl中判断并处理RTC_SUPPORT开关，分析之后，原来是在 target/linux/ramips/mt7620/target.mk中，将原始的内容:
FEATURES+=usb
FEATURES+=usb rtc
#
# Copyright (C) 2009 OpenWrt.org
#
SUBTARGET:=mt7620
BOARDNAME:=MT7620 based boards
ARCH_PACKAGES:=ramips_24kec
FEATURES+=usb rtc
CPU_TYPE:=24kec
CPU_SUBTYPE:=dsp
DEFAULT_PACKAGES += kmod-rt2800-pci kmod-rt2800-soc
define Target/Description
Build firmware images for Ralink MT7620 based boards.
endef
1、在openwrt的SDK开发包中，执行“make kernel_menuconfig”命令。
在弹出的菜单“Linux/mips 3.18.29 Kernel Configuration”中，选择“Device Drivers ---> Real Time Clock --->”，开启RTC功能选项，如图4-4所示。
图4-4 开启RTC功能
2、进入“ Real Time Clock--->”，对RTC进行配置
配置如图4-5所示，其中：
（1）去除选项“Set system time from RTC on startup and resume”，不再开机的时候就启动RTC驱动，因为本文中，打算开机完毕之后，再自行挂载RTC驱动。如果没有一开始就挂载了RTC驱动，开启这个选项，将出现错误提示“drivers/rtc/hctosys.c: unable to open rtc device (rtc0)”。
（2）对于“RTC debug support”，依据个人喜好而定，开启之后，可以查看RTC的调试信息。
图4-5 RTC 配置
驱动对应的文件名为：rtc_mcp7940.c，程序如下：
/*
* rtc-mcp7940.c - RTC driver for some mostly-compatible I2C chips.
*
* Copyright (C) 2005 James Chapman (ds1337 core)
* Copyright (C) 2006 David Brownell
* Copyright (C) 2009 Matthias Fuchs (rx8025 support)
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*/
#include <linux/module.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/i2c.h>
#include <linux/string.h>
#include <linux/rtc.h>
#include <linux/bcd.h>
#define RALINK_SYSCTL_BASE 0xB0000000
#define RALINK_TIMER_BASE 0xB0000100
#define RALINK_INTCL_BASE 0xB0000200
#define RALINK_SYSCTL_ADDR RALINK_SYSCTL_BASE // system control
#define RALINK_REG_GPIOMODE (RALINK_SYSCTL_ADDR + 0x60) // GPIO MODE
#define __devexit
#define __devinitdata
#define __devinit
#define __devexit_p
/* We can't determine type by probing, but if we expect pre-Linux code
* to have set the chip up as a clock (turning on the oscillator and
* setting the date and time), Linux can ignore the non-clock features.
* That's a natural job for a factory or repair bench.
*/
enum ds_type
{
mcp7940,
};
/* RTC registers don't differ much, except for the century flag */
#define MCP7940_REG_SECS 0x00 /* 00-59 */
#define MCP7940_BIT_CH 0x80
#define MCP7940_BIT_ST 0x80
#define MCP7940_REG_MIN 0x01 /* 00-59 */
#define MCP7940_REG_HOUR 0x02 /* 00-23, or 1-12{am,pm} */
#define MCP7940_BIT_12HR 0x40 /* in REG_HOUR */
#define MCP7940_BIT_PM 0x20 /* in REG_HOUR */
#define MCP7940_REG_WDAY 0x03 /* 01-07 */
#define MCP7940_REG_MDAY 0x04 /* 01-31 */
#define MCP7940_REG_MONTH 0x05 /* 01-12 */
#define MCP7940_REG_YEAR 0x06 /* 00-99 */
#define MCP7940_BIT_VBATEN 0x08
/* Other registers (control, status, alarms, trickle charge, NVRAM, etc)
* start at 7, and they differ a LOT. Only control and status matter for
* basic RTC date and time functionality; be careful using them.
*/
#define MCP7940_REG_CONTROL 0x07
#define MCP7940_BIT_OUT 0x80
#define MCP7940_BIT_SQWE 0x10
#define MCP7940_BIT_RS1 0x02
#define MCP7940_BIT_RS0 0x01
struct mcp7940 {
u8 offset; /* register's offset */
u8 regs[11];
enum ds_type type;
unsigned long flags;
#define HAS_NVRAM 0 /* bit 0 == sysfs file active */
#define HAS_ALARM 1 /* bit 1 == irq claimed */
struct i2c_client *client;
struct rtc_device *rtc;
struct work_struct work;
s32 (*read_block_data)(struct i2c_client *client, u8 command,
u8 length, u8 *values);
s32 (*write_block_data)(struct i2c_client *client, u8 command,
u8 length, const u8 *values);
};
struct chip_desc {
unsigned nvram56:1;
unsigned alarm:1;
};
static const struct i2c_device_id mcp7940_id[] = {
{ "mcp7940", mcp7940 },
{ }
};
MODULE_DEVICE_TABLE(i2c, mcp7940_id);
/*----------------------------------------------------------------------*/
#define BLOCK_DATA_MAX_TRIES 10
static s32 mcp7940_read_block_data_once(struct i2c_client *client, u8 command,
u8 length, u8 *values)
{
s32 i, data;
for (i = 0; i < length; i++) {
data = i2c_smbus_read_byte_data(client, command + i);
if (data < 0)
return data;
values[i] = data;
}
return i;
}
static s32 mcp7940_read_block_data(struct i2c_client *client, u8 command,
u8 length, u8 *values)
{
u8 oldvalues[I2C_SMBUS_BLOCK_MAX];
s32 ret;
int tries = 0;
dev_dbg(&client->dev, "mcp7940_read_block_data (length=%d)\n", length);
ret = mcp7940_read_block_data_once(client, command, length, values);
if (ret < 0)
return ret;
do {
if (++tries > BLOCK_DATA_MAX_TRIES) {
dev_err(&client->dev,
"mcp7940_read_block_data failed\n");
return -EIO;
}
memcpy(oldvalues, values, length);
ret = mcp7940_read_block_data_once(client, command, length,
values);
if (ret < 0)
return ret;
} while (memcmp(oldvalues, values, length));
return length;
}
static s32 mcp7940_write_block_data(struct i2c_client *client, u8 command,
u8 length, const u8 *values)
{
u8 currvalues[I2C_SMBUS_BLOCK_MAX];
int tries = 0;
dev_dbg(&client->dev, "mcp7940_write_block_data (length=%d)\n", length);
do
{
s32 i, ret;
if (++tries > BLOCK_DATA_MAX_TRIES) {
dev_err(&client->dev,
"mcp7940_write_block_data failed\n");
return -EIO;
}
for (i = 0; i < length; i++) {
ret = i2c_smbus_write_byte_data(client, command + i,
values[i]);
if (ret < 0)
return ret;
}
ret = mcp7940_read_block_data_once(client, command, length,
currvalues);
if (ret < 0)
return ret;
} while (memcmp(currvalues, values, length));
return length;
}
static int mcp7940_get_time(struct device *dev, struct rtc_time *t)
{
struct mcp7940 *mcp7940 = dev_get_drvdata(dev);
int tmp;
/* read the RTC date and time registers all at once */
tmp = mcp7940->read_block_data(mcp7940->client,
mcp7940->offset, 7, mcp7940->regs);
if (tmp != 7) {
dev_err(dev, "%s error %d\n", "read", tmp);
return -EIO;
}
dev_dbg(dev, "%s: %02x %02x %02x %02x %02x %02x %02x\n",
"read",
mcp7940->regs[0], mcp7940->regs[1],
mcp7940->regs[2], mcp7940->regs[3],
mcp7940->regs[4], mcp7940->regs[5],
mcp7940->regs[6]);
t->tm_sec = bcd2bin(mcp7940->regs[MCP7940_REG_SECS] & 0x7f);
t->tm_min = bcd2bin(mcp7940->regs[MCP7940_REG_MIN] & 0x7f);
tmp = mcp7940->regs[MCP7940_REG_HOUR] & 0x3f;
t->tm_hour = bcd2bin(tmp);
t->tm_wday = bcd2bin(mcp7940->regs[MCP7940_REG_WDAY] & 0x07) - 1;
t->tm_mday = bcd2bin(mcp7940->regs[MCP7940_REG_MDAY] & 0x3f);
tmp = mcp7940->regs[MCP7940_REG_MONTH] & 0x1f;
t->tm_mon = bcd2bin(tmp) - 1;
/* assume 20YY not 19YY, and ignore DS1337_BIT_CENTURY */
t->tm_year = bcd2bin(mcp7940->regs[MCP7940_REG_YEAR]) + 100;
dev_dbg(dev, "%s secs=%d, mins=%d, "
"hours=%d, mday=%d, mon=%d, year=%d, wday=%d\n",
"read", t->tm_sec, t->tm_min,
t->tm_hour, t->tm_mday,
t->tm_mon, t->tm_year, t->tm_wday);
/* initial clock setting can be undefined */
return rtc_valid_tm(t);
}
static int mcp7940_set_time(struct device *dev, struct rtc_time *t)
{
struct mcp7940 *mcp7940 = dev_get_drvdata(dev);
int result;
int tmp;
u8 *buf = mcp7940->regs;
dev_dbg(dev, "%s secs=%d, mins=%d, "
"hours=%d, mday=%d, mon=%d, year=%d, wday=%d\n",
"write", t->tm_sec, t->tm_min,
t->tm_hour, t->tm_mday,
t->tm_mon, t->tm_year, t->tm_wday);
buf[MCP7940_REG_SECS] = bin2bcd(t->tm_sec);
buf[MCP7940_REG_MIN] = bin2bcd(t->tm_min);
buf[MCP7940_REG_HOUR] = bin2bcd(t->tm_hour);
buf[MCP7940_REG_WDAY] = bin2bcd(t->tm_wday + 1);
buf[MCP7940_REG_MDAY] = bin2bcd(t->tm_mday);
buf[MCP7940_REG_MONTH] = bin2bcd(t->tm_mon + 1);
/* assume 20YY not 19YY */
tmp = t->tm_year - 100;
buf[MCP7940_REG_YEAR] = bin2bcd(tmp);
buf[MCP7940_REG_SECS] |= MCP7940_BIT_ST;
buf[MCP7940_REG_WDAY] |= MCP7940_BIT_VBATEN;
dev_dbg(dev, "%s: %02x %02x %02x %02x %02x %02x %02x\n",
"write", buf[0], buf[1], buf[2], buf[3],
buf[4], buf[5], buf[6]);
result = mcp7940->write_block_data(mcp7940->client,
mcp7940->offset, 7, buf);
if (result < 0)
{
dev_err(dev, "%s error %d\n", "write", result);
return result;
}
return 0;
}
static int mcp7940_ioctl(struct device *dev, unsigned int cmd, unsigned long arg)
{
struct rtc_time time;
void __user *uarg = (void __user *) arg;
switch (cmd)
{
case RTC_RD_TIME:
mcp7940_get_time(dev, &time);
if (copy_to_user(uarg, &time, sizeof(time)))
{
printk("RTC_RD_TIME error, can not copy to user\n");
return -EFAULT;
}
break;
case RTC_SET_TIME:
if (copy_from_user(&time, uarg, sizeof(time)))
{
printk("RTC_SET_TIME error, can not copy from user\n");
return -EFAULT;
}
mcp7940_set_time(dev, &time);
break;
default:
return -ENOIOCTLCMD;
}
return 0;
}
static const struct rtc_class_ops mcp7940_rtc_ops =
{
.read_time = mcp7940_get_time,
.set_time = mcp7940_set_time,
.ioctl = mcp7940_ioctl,
};
/*----------------------------------------------------------------------*/
static struct i2c_driver mcp7940_driver;
static int __devinit mcp7940_probe(struct i2c_client *client,
const struct i2c_device_id *id)
{
struct mcp7940 *mcp7940;
int err = -ENODEV;
int tmp;
struct i2c_adapter *adapter = to_i2c_adapter(client->dev.parent);
unsigned char *buf;
if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA)
&& !i2c_check_functionality(adapter, I2C_FUNC_SMBUS_I2C_BLOCK))
return -EIO;
if (!(mcp7940 = kzalloc(sizeof(struct mcp7940), GFP_KERNEL)))
return -ENOMEM;
i2c_set_clientdata(client, mcp7940);
mcp7940->client = client;
mcp7940->type = id->driver_data;
mcp7940->offset = 0;
buf = mcp7940->regs;
if (i2c_check_functionality(adapter, I2C_FUNC_SMBUS_I2C_BLOCK)) {
mcp7940->read_block_data = i2c_smbus_read_i2c_block_data;
mcp7940->write_block_data = i2c_smbus_write_i2c_block_data;
} else {
mcp7940->read_block_data = mcp7940_read_block_data;
mcp7940->write_block_data = mcp7940_write_block_data;
}
read_rtc:
/* read RTC registers */
tmp = mcp7940->read_block_data(mcp7940->client, 0, 8, buf);
if (tmp != 8)
{
pr_debug("read error %d\n", tmp);
err = -EIO;
goto exit_free;
}
/* minimal sanity checking; some chips (like DS1340) don't
* specify the extra bits as must-be-zero, but there are
* still a few values that are clearly out-of-range.
*/
tmp = mcp7940->regs[MCP7940_REG_SECS];
/* make sure that the backup battery is enabled */
if (!(mcp7940->regs[MCP7940_REG_WDAY] & MCP7940_BIT_VBATEN)) {
i2c_smbus_write_byte_data(client, MCP7940_REG_WDAY,
mcp7940->regs[MCP7940_REG_WDAY]
| MCP7940_BIT_VBATEN);
}
/* clock halted? turn it on, so clock can tick. */
if (!(tmp & MCP7940_BIT_ST)) {
i2c_smbus_write_byte_data(client, MCP7940_REG_SECS,
MCP7940_BIT_ST);
dev_warn(&client->dev, "SET TIME!\n");
goto read_rtc;
}
tmp = mcp7940->regs[MCP7940_REG_HOUR];
switch (mcp7940->type) {
default:
if (!(tmp & MCP7940_BIT_12HR))
break;
/* Be sure we're in 24 hour mode. Multi-master systems
* take note...
*/
tmp = bcd2bin(tmp & 0x1f);
if (tmp == 12)
tmp = 0;
if (mcp7940->regs[MCP7940_REG_HOUR] & MCP7940_BIT_PM)
tmp += 12;
i2c_smbus_write_byte_data(client,
MCP7940_REG_HOUR,
bin2bcd(tmp));
}
mcp7940->rtc = rtc_device_register(client->name, &client->dev,
&mcp7940_rtc_ops, THIS_MODULE);
if (IS_ERR(mcp7940->rtc)) {
err = PTR_ERR(mcp7940->rtc);
dev_err(&client->dev,
"unable to register the class device\n");
goto exit_free;
}
return 0;
exit_free:
kfree(mcp7940);
return err;
}
static int __devexit mcp7940_remove(struct i2c_client *client)
{
struct mcp7940 *mcp7940 = i2c_get_clientdata(client);
rtc_device_unregister(mcp7940->rtc);
kfree(mcp7940);
return 0;
}
static struct i2c_driver mcp7940_driver =
{
.driver =
{
.name = "rtc-mcp7940",
.owner = THIS_MODULE,
},
.probe = mcp7940_probe,
.remove = __devexit_p(mcp7940_remove),
.id_table = mcp7940_id,
};
static void rtc_pin_mux_init(void)
{
u32 mode = 0;
mode = le32_to_cpu(*(volatile u32 *)(RALINK_REG_GPIOMODE));
mode &= ~(0x1 << 0); // I2C_GPIO_MODE引脚，设置为I2C模式，即I2C_SD(GPIO#1)I2C_SCLK(GPIO#2)都设置为I2C模式
*(volatile u32 *)(RALINK_REG_GPIOMODE) = cpu_to_le32(mode);
}
static int __init mcp7940_init(void)
{
rtc_pin_mux_init();
return i2c_add_driver(&mcp7940_driver);
}
module_init(mcp7940_init);
static void __exit mcp7940_exit(void)
{
i2c_del_driver(&mcp7940_driver);
}
module_exit(mcp7940_exit);
MODULE_AUTHOR("sky.houfei");
MODULE_DESCRIPTION("RTC driver for MCP7940");
MODULE_LICENSE("GPL");
makefile 名称为makefile
############################## mcp7940 rtc driver makefile ######################### obj-m = rtc_mcp7940.o TARGET_NAME=rtc_mcp7940.ko PWD=$(shell pwd) KERNEL_DIR?=/home/sky/develop/openWrt/openwrt/build_dir/target-mipsel_24kec+dsp_glibc-2.21/linux-ramips_mt7620/linux-3.18.29/ TOOL_CAHIN="/home/sky/develop/openWrt/openwrt/staging_dir/toolchain-mipsel_24kec+dsp_gcc-4.8-linaro_glibc-2.21/bin/mipsel-openwrt-linux-gnu-" OUTPUT_DIR=$(PWD)/../../build_openwrt/mnt/sau2ag1/ ############################################################################### all: make -C $(KERNEL_DIR) ARCH=mips CROSS_COMPILE=$(TOOL_CAHIN) M=$(PWD) modules clean: rm -f $(obj-m) rm -f *.mod.c rm -f *.mod.o rm -f *.order rm -f *.sysvers #make command: #make #make clean
mcp7940应用程序为 rtc_app.c，应用中，存储的时间为UTC格式时间。
对于UTC时间，year = year -1900, month = month -1，应用程序如下。
/*
* Real Time Clock Driver Test/Example Program
*
* Compile with:
* gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
*
* Copyright (C) 1996, Paul Gortmaker.
*
* Released under the GNU General Public License, version 2,
* included herein by reference.
*
*/
#include <stdio.h>
#include <linux/rtc.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
/*
* This expects the new RTC class driver framework, working with
* clocks that will often not be clones of what the PC-AT had.
* Use the command line to specify another RTC if you need one.
*/
static const char default_rtc[] = "/dev/rtc0";
int main(int argc, char **argv)
{
int i, fd, retval, irqcount = 0;
unsigned int cmd = 0;
unsigned long tmp, data;
struct rtc_time rtc_tm;
const char *rtc = default_rtc;
fd = open(default_rtc, O_RDONLY);
if (fd == -1)
{
printf("Can not open %s, exit the app\n", default_rtc);
}
rtc_tm.tm_year = 2015 - 1900;
rtc_tm.tm_mon = 10;
rtc_tm.tm_mday = 28;
rtc_tm.tm_wday = 3;
rtc_tm.tm_hour = 15;
rtc_tm.tm_min = 22;
rtc_tm.tm_sec = 10;
i = atoi(argv[1]);
switch(i)
{
case 1:
cmd = RTC_RD_TIME;
break;
case 2:
cmd = RTC_SET_TIME;
printf( "app set time %d-%d-%d week%d, %02d:%02d:%02d.\n",
rtc_tm.tm_year + 1900, rtc_tm.tm_mon, rtc_tm.tm_mday, rtc_tm.tm_wday,
rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
break;
default:
cmd = RTC_RD_TIME;
break;
}
retval = ioctl(fd, cmd, &rtc_tm);
if (retval == -1)
{
printf("ioctl cmd = %d, error, exit the rtc app\n");
exit(errno);
}
printf( "rtc app date/time is %d-%d-%d week%d, %02d:%02d:%02d.\n",
rtc_tm.tm_year + 1900, rtc_tm.tm_mon, rtc_tm.tm_mday, rtc_tm.tm_wday,
rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
close(fd);
printf("rtc test end\n");
return 0;
}
在MT7620开发板上，输入命令 "ls /dev/i2c*"，如图6-1所示。在图中可以看到i2c-0，说明I2C总线已经开启。
在MT7620开发板上，输入命令“ls /sys/bus/i2c/devices/”，如图6-2所示。
在图6-2中，可以看到支持了0x006f的从机设备（mcp7940芯片的地址刚好为0x6f）。
图6-2查看I2C总线支持的从机device
说明：如果没有看到d0x006f或者除了i2c-0之外的设备，则说明i2c总线不支持device从机，此时任何i2c
芯片(从机device)挂载到MT7620上，都不可能通信，因为主机（MT620芯片）不支持从机的地址。此时需要查看下4.1.3章节，dts文件是否修改正确。
在MT7620开发板上，输入命令“ls /sys/bus/i2c/drivers/”，如图6-3所示。由图可知，设备已经成功将rtc-mcp7940驱动程序挂载至i2c总线。
图6-3 查看i2c总线挂载的驱动
说明：如果没有看到rtc-mcp7940，则说明rtc的驱动程序有问题，需要查看下5.1章节的驱动程序。
在MT7620开发板上，输入命令“ls
/dev/rtc*”，如图6-4所示。由图可知，rtc已经驱动成功，可以看到rtc设备。
图6-4 查看rtc设备
说明：如果没有看到rtc0或者其他的rtc设备，则说明rtc的配置有问题，系统不支持rtc，需要查看下4.2章节的rtc配置。
rtc编译后的驱动名称为rtc_mcp7940,执行挂载，提示信息如下，说明rtc成功挂载在0-006f节点下，并且注册为/dev/rtc0：
root@OpenWrt:/tmp# insmod ./rtc_mcp7940.ko [ 203.740000] rtc-mcp7940 0-006f: rtc core: registered mcp7940 as rtc0 root@OpenWrt:/tmp#
先设置时间，设置完毕之后，再次读取rtc时间，发现可以正常读写时间，且正确。而且将设备重启，重启之后，rtc时间也正常计时，重启过程中，时间没有丢失和停止。
root@OpenWrt:/tmp# ./rtc_app_openwrt 2 app set time 2015-10-28 week3, 15:22:10. rtc app date/time is 2015-10-28 week3, 15:22:10. rtc test end root@OpenWrt:/tmp# ./rtc_app_openwrt 1 rtc app date/time is 2015-10-28 week3, 15:22:26. rtc test end
openwrt 增加RTC（MCP7940 I2C总线）驱动详解
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原文地址：http://blog.csdn.net/xhoufei2010/article/details/51791322
