Linux内核：regmap机制

背景

在学习SPI框架的时候，看到了有一个rtc驱动用到了regmap，本想通过传统方式访问spi接口的我，突然有点不适应，翻了整个驱动，愣是没有找到读写spi的范式；因此了解了regmap以后，才发现regmap做了这个事情。

介绍

在Linu 3.1开始，Linux引入了regmap来统一管理内核的I2C, SPI等总线，将I2C, SPI驱动做了一次重构，把I/O读写的重复逻辑在regmap中实现。只需初始化时指定总线类型、寄存器位宽等关键参数，即可通过regmap模型接口来操作器件寄存器。

当然，regmap同样适用于操作cpu自身的寄存器。将i2c、spi、mmio、irq都抽象出统一的接口regmap_read、regmap_write、regmap_update_bits等接口 ，从而提高代码的可重用性，并且使得在使用如上内核基础组件时变得更为简单易用。

regmap是在 linux 内核为减少慢速 I/O 驱动上的重复逻辑，提供一种通用的接口来操作底层硬件寄存器的模型框架。

此外，如果在regmap在驱动和硬件寄存器之间使用cache，会减少底层低速 I/O 的操作次数，提高访问效率；但降低了实时性会有所降低。

传统的写法

用一个I2C设备为例，在3.1之前的I2C设备驱动，需要各自调用i2c_transfer来实现读写，比如：

static int raydium_i2c_pda2_write(struct i2c_client *client,
        unsigned char addr, unsigned char *w_data, unsigned short length)
{
    int retval = -1;
    unsigned char retry;
    unsigned char buf[MAX_WRITE_PACKET_SIZE + 1];
    struct raydium_ts_data *data = i2c_get_clientdata(client);

    struct i2c_msg msg[] = {
        {
            .addr = RAYDIUM_I2C_NID,
            .flags = RAYDIUM_I2C_WRITE,
            .len = length + 1,
            .buf = buf,
        },
    };


    if (length > MAX_WRITE_PACKET_SIZE)
    {
        return -EINVAL;
    }
    u8_i2c_mode = PDA2_MODE;
    buf[0] = addr;
    memcpy(&buf[1], w_data, length);

    for (retry = 0; retry < SYN_I2C_RETRY_TIMES; retry++)
    {
        if (i2c_transfer(client->adapter, msg, 1) == 1)
        {
            retval = length;
            break;
        }
        msleep(20);
    }

    if (retry == SYN_I2C_RETRY_TIMES)
    {
        dev_err(&data->client->dev,"[touch]%s: I2C write over retry limit.addr[0x%x]
",__func__, addr);
        retval = -EIO;
    }

    return retval;
}

需要自行构建i2c_msg，然后使用i2c_transfer传输数据。

使用regmap机制进行读写

但是使用regmap机制，就会变的更为简单。

使用regmap比较简单：

1、使用前，只需根据外设属性配置配置regmap信息（总线类型、寄存器位宽、缓存类型、读写属性等参数）；

2、接着注册一个regmap实例；

3、然后调用抽象访问接口访问寄存器；

4、不想使用的时候，释放regmap实例即可。

例如：

// 1.构建regmap_config结构
static const struct regmap_config mlx90632_regmap = {
        // 寄存器地址位宽
	.reg_bits = 16,
        // 寄存器值的位宽，必须填写
	.val_bits = 16,
        // 可选，判断寄存器是否可写，可读，是否可缓冲等回调
	.volatile_table = &mlx90632_volatile_regs_tbl,
	.rd_table = &mlx90632_readable_regs_tbl,
	.wr_table = &mlx90632_writeable_regs_tbl,

	.use_single_rw = true,
	.reg_format_endian = REGMAP_ENDIAN_BIG,
	.val_format_endian = REGMAP_ENDIAN_BIG,
	.cache_type = REGCACHE_RBTREE,
};

// 2. 初始化regmap（或者regmap_init_spi）
struct regmap *map= regmap_init_i2c(client, &mlx90632_regmap);

//3.读写操作
ret = regmap_read(mlx90632->regmap, MLX90632_EE_VERSION, &read);

//4. 释放regmap
regmap_exit(&mlx90632_regmap);

读写特性

regmap的write操作：

• 先尝试向缓冲cache中write.
• 如果bypass了,或者volatile,或者cache none情况,调用对应的底层协议进行写操作。

_regmap_read：

_regmap_read

• 先拿到context，在以后的map->reg_read会用到
• 如果不是bypass的，调用regcache_read从cache中读
• 如果不能从cache中read到，调用map->reg_read

regmap_config原型

在regmap_config，定义了寄存器的各种信息，比如寄存器地址长度，寄存器值的长度，读写寄存器的地址范围的信息，寄存器地址和值的大小端以及缓冲方式。

// include/linux/regmap.h
struct regmap_config {
    const char *name; // 可选，寄存器名字

    int reg_bits; // 寄存器地址位宽，必须填写
    int reg_stride; // 寄存器操作宽度，比如为1时，所有寄存器可操作，为2时，只有2^n可操作
    int pad_bits;
    int val_bits; // 寄存器值的位宽，必须填写
    // 可选，判断寄存器是否可写，可读，是否可缓冲等回调
    bool (*writeable_reg)(struct device *dev, unsigned int reg);
    bool (*readable_reg)(struct device *dev, unsigned int reg);
    bool (*volatile_reg)(struct device *dev, unsigned int reg);
    bool (*precious_reg)(struct device *dev, unsigned int reg);
    regmap_lock lock;
    regmap_unlock unlock;
    void *lock_arg;
    // 寄存器读写方法，可选
    int (*reg_read)(void *context, unsigned int reg, unsigned int *val);
    int (*reg_write)(void *context, unsigned int reg, unsigned int val);

    bool fast_io;

    unsigned int max_register;
    const struct regmap_access_table *wr_table; //可选，可写寄存器
    const struct regmap_access_table *rd_table;//可选，可读寄存器
    const struct regmap_access_table *volatile_table;
    const struct regmap_access_table *precious_table;
    const struct reg_default *reg_defaults;
    unsigned int num_reg_defaults;
    enum regcache_type cache_type; // 缓冲方式
    const void *reg_defaults_raw;
    unsigned int num_reg_defaults_raw;

    u8 read_flag_mask;
    u8 write_flag_mask;

    bool use_single_rw;
    bool can_multi_write;

    enum regmap_endian reg_format_endian;
    enum regmap_endian val_format_endian;

    const struct regmap_range_cfg *ranges;
    unsigned int num_ranges;
};

regmap_init_i2c

当设备驱动配置好config以后，调用对应的regmap_init_xx，例如，这里是regmap_init_i2c

// drivers/base/regmap/regmap-i2c.c

struct regmap *regmap_init_i2c(struct i2c_client *i2c,
                               const struct regmap_config *config)
{
    // regmap_bus 定位了对应总线的读写函数。
    const struct regmap_bus *bus = regmap_get_i2c_bus(i2c, config);

    if (IS_ERR(bus))
        return ERR_CAST(bus);

    return regmap_init(&i2c->dev, bus, &i2c->dev, config);
}

注意到2个新的数据结构：

• struct regmap_bus *bus
• struct regmap *regmap_init_i2c(..

regmap_get_i2c_bus

对于普通I2C设备，regmap_bus为：

// drivers/base/regmap/regmap-i2c.c
static const struct regmap_bus *regmap_get_i2c_bus(struct i2c_client *i2c,
                    const struct regmap_config *config)
{
    if (i2c_check_functionality(i2c->adapter, I2C_FUNC_I2C))
        return &regmap_i2c;
    // ...

    return ERR_PTR(-ENOTSUPP);
}

static struct regmap_bus regmap_i2c = {
    .write = regmap_i2c_write,
    .gather_write = regmap_i2c_gather_write,
    .read = regmap_i2c_read,
    .reg_format_endian_default = REGMAP_ENDIAN_BIG,
    .val_format_endian_default = REGMAP_ENDIAN_BIG,
};

regmap_bus结构定义了读写函数和默认的寄存器地址和寄存器值的大小端。

regmap_bus原型

/**
 * struct regmap_bus - Description of a hardware bus for the register map
 *                     infrastructure.
 *
 * @fast_io: Register IO is fast. Use a spinlock instead of a mutex
 *       to perform locking. This field is ignored if custom lock/unlock
 *       functions are used (see fields lock/unlock of
 *       struct regmap_config).
 * @write: Write operation.
 * @gather_write: Write operation with split register/value, return -ENOTSUPP
 *                if not implemented  on a given device.
 * @async_write: Write operation which completes asynchronously, optional and
 *               must serialise with respect to non-async I/O.
 * @reg_write: Write a single register value to the given register address. This
 *             write operation has to complete when returning from the function.
 * @reg_update_bits: Update bits operation to be used against volatile
 *                   registers, intended for devices supporting some mechanism
 *                   for setting clearing bits without having to
 *                   read/modify/write.
 * @read: Read operation.  Data is returned in the buffer used to transmit
 *         data.
 * @reg_read: Read a single register value from a given register address.
 * @free_context: Free context.
 * @async_alloc: Allocate a regmap_async() structure.
 * @read_flag_mask: Mask to be set in the top byte of the register when doing
 *                  a read.
 * @reg_format_endian_default: Default endianness for formatted register
 *     addresses. Used when the regmap_config specifies DEFAULT. If this is
 *     DEFAULT, BIG is assumed.
 * @val_format_endian_default: Default endianness for formatted register
 *     values. Used when the regmap_config specifies DEFAULT. If this is
 *     DEFAULT, BIG is assumed.
 * @max_raw_read: Max raw read size that can be used on the bus.
 * @max_raw_write: Max raw write size that can be used on the bus.
 */
struct regmap_bus {
    bool fast_io;
    regmap_hw_write write;
    regmap_hw_gather_write gather_write;
    regmap_hw_async_write async_write;
    regmap_hw_reg_write reg_write;
    regmap_hw_reg_update_bits reg_update_bits;
    regmap_hw_read read;
    regmap_hw_reg_read reg_read;
    regmap_hw_free_context free_context;
    regmap_hw_async_alloc async_alloc;
    u8 read_flag_mask;
    enum regmap_endian reg_format_endian_default;
    enum regmap_endian val_format_endian_default;
    size_t max_raw_read;
    size_t max_raw_write;
};

regmap_i2c_read

我们看下i2c对应的regmap_bus.read实现：

@/kernel/driver/base/rgmap/regmap-i2c.c
static int regmap_i2c_read(void *context,
               const void *reg, size_t reg_size,
               void *val, size_t val_size)
{
    struct device *dev = context;
    struct i2c_client *i2c = to_i2c_client(dev);
    struct i2c_msg xfer[2];
    int ret;

    xfer[0].addr = i2c->addr;
    xfer[0].flags = 0;
    xfer[0].len = reg_size;
    xfer[0].buf = (void *)reg;

    xfer[1].addr = i2c->addr;
    xfer[1].flags = I2C_M_RD;
    xfer[1].len = val_size;
    xfer[1].buf = val;

    ret = i2c_transfer(i2c->adapter, xfer, 2);
    if (ret == 2)
        return 0;
    else if (ret < 0)
        return ret;
    else
        return -EIO;
}

可以看到其实就是构造i2c_msg，选择操作寄存器地址然后读寄存器值。这正是regmap要做的事情，抽离出总线读写操作到regmap中，避免驱动去重复实现。

等到设备驱动调用regmap_read以后，最终会调用到regmap_i2c_read，这样子就会执行i2c驱动典型的驱动流程。

regmap_init

当在regmap_get_i2c_bus获取到对应的bus以后，就可以进行regmap的初始化了。

regmap_init主要是把config和描述i2c设备的bus设置到regmap中

struct regmap *regmap_init(struct device *dev,
               const struct regmap_bus *bus,
               void *bus_context,
               const struct regmap_config *config)
{
    struct regmap *map;
    int ret = -EINVAL;
    enum regmap_endian reg_endian, val_endian;

    map = kzalloc(sizeof(*map), GFP_KERNEL);
    // 将regmap_config定义的参数赋值到regmap中
    map->format.reg_bytes = DIV_ROUND_UP(config->reg_bits, 8);
    map->format.pad_bytes = config->pad_bits / 8;
    map->format.val_bytes = DIV_ROUND_UP(config->val_bits, 8);
    map->format.buf_size = DIV_ROUND_UP(config->reg_bits +
            config->val_bits + config->pad_bits, 8);
    map->reg_shift = config->pad_bits % 8;
    if (config->reg_stride)
        map->reg_stride = config->reg_stride;
    else
        map->reg_stride = 1;
    map->use_single_rw = config->use_single_rw;
    map->can_multi_write = config->can_multi_write;
    map->dev = dev;
    map->bus = bus;
    map->bus_context = bus_context;
    map->max_register = config->max_register;
    map->wr_table = config->wr_table;
    map->rd_table = config->rd_table;
    map->volatile_table = config->volatile_table;
    map->precious_table = config->precious_table;
    map->writeable_reg = config->writeable_reg;
    map->readable_reg = config->readable_reg;
    map->volatile_reg = config->volatile_reg;
    map->precious_reg = config->precious_reg;
    map->cache_type = config->cache_type;
    map->name = config->name;

    /* regmap中有reg_read操作方法，通过bus是否为空赋值 */
    if (!bus) {
        // 对于该驱动，此处只会走这里 
        map->reg_read  = config->reg_read;
        map->reg_write = config->reg_write;

        map->defer_caching = false;
        goto skip_format_initialization;
    } else if (!bus->read || !bus->write) {
        map->reg_read = _regmap_bus_reg_read;
        map->reg_write = _regmap_bus_reg_write;

        map->defer_caching = false;
        goto skip_format_initialization;
    } else {
        map->reg_read  = _regmap_bus_read;
    }
    // 设置地址和寄存器值的大小端
    reg_endian = regmap_get_reg_endian(bus, config);
    val_endian = regmap_get_val_endian(dev, bus, config);
    // 根据寄存器地址位宽和大小端解析寄存器地址
    switch (config->reg_bits + map->reg_shift) {
        case 32:
        switch (reg_endian) {
        case REGMAP_ENDIAN_BIG:
            map->format.format_reg = regmap_format_32_be;
            break;
        case REGMAP_ENDIAN_NATIVE:
            map->format.format_reg = regmap_format_32_native;
            break;
        default:
            goto err_map;
        }
        break;
    }
    // 根据寄存器值位宽和大小端解析寄存器值
    switch (config->val_bits) {
        case 16:
        switch (val_endian) {
        case REGMAP_ENDIAN_BIG:
            map->format.format_val = regmap_format_16_be;
            map->format.parse_val = regmap_parse_16_be;
            map->format.parse_inplace = regmap_parse_16_be_inplace;
            break;
        case REGMAP_ENDIAN_LITTLE:
            map->format.format_val = regmap_format_16_le;
            map->format.parse_val = regmap_parse_16_le;
            map->format.parse_inplace = regmap_parse_16_le_inplace;
            break;
        case REGMAP_ENDIAN_NATIVE:
            map->format.format_val = regmap_format_16_native;
            map->format.parse_val = regmap_parse_16_native;
            break;
        default:
            goto err_map;
        }
        break;
    }

    /* 对于val_bits = 16,reg_bits=16,regmap写函数选择_regmap_bus_raw_write */
    if (map->format.format_write) {
        map->defer_caching = false;
        map->reg_write = _regmap_bus_formatted_write;
    } else if (map->format.format_val) {
        map->defer_caching = true;
        map->reg_write = _regmap_bus_raw_write;
    }
    // 缓存初始化
    ret = regcache_init(map, config);
}

regmap_init()函数初始化了regmap，regmap中函数bus指针和config寄存器参数，已经足够用来操作I2C的寄存器了。

同时，也设置了regmap.reg_read为_regmap_bus_read

regmap原型

struct regmap {
    union {
        struct mutex mutex;
        struct {
            spinlock_t spinlock;
            unsigned long spinlock_flags;
        };
    };
    regmap_lock lock;
    regmap_unlock unlock;
    void *lock_arg; /* This is passed to lock/unlock functions */
    gfp_t alloc_flags;

    struct device *dev; /* Device we do I/O on */
    void *work_buf;     /* Scratch buffer used to format I/O */
    struct regmap_format format;  /* Buffer format */
    const struct regmap_bus *bus;
    void *bus_context;
    const char *name;

    bool async;
    spinlock_t async_lock;
    wait_queue_head_t async_waitq;
    struct list_head async_list;
    struct list_head async_free;
    int async_ret;

    unsigned int max_register;
    bool (*writeable_reg)(struct device *dev, unsigned int reg);
    bool (*readable_reg)(struct device *dev, unsigned int reg);
    bool (*volatile_reg)(struct device *dev, unsigned int reg);
    bool (*precious_reg)(struct device *dev, unsigned int reg);
    const struct regmap_access_table *wr_table;
    const struct regmap_access_table *rd_table;
    const struct regmap_access_table *volatile_table;
    const struct regmap_access_table *precious_table;

    int (*reg_read)(void *context, unsigned int reg, unsigned int *val);
    int (*reg_write)(void *context, unsigned int reg, unsigned int val);
    int (*reg_update_bits)(void *context, unsigned int reg,
                   unsigned int mask, unsigned int val);

    bool defer_caching;

    unsigned long read_flag_mask;
    unsigned long write_flag_mask;

    /* number of bits to (left) shift the reg value when formatting*/
    int reg_shift;
    int reg_stride;
    int reg_stride_order;

    /* regcache specific members */
    const struct regcache_ops *cache_ops;
    enum regcache_type cache_type;

    /* number of bytes in reg_defaults_raw */
    unsigned int cache_size_raw;
    /* number of bytes per word in reg_defaults_raw */
    unsigned int cache_word_size;
    /* number of entries in reg_defaults */
    unsigned int num_reg_defaults;
    /* number of entries in reg_defaults_raw */
    unsigned int num_reg_defaults_raw;

    /* if set, only the cache is modified not the HW */
    bool cache_only;
    /* if set, only the HW is modified not the cache */
    bool cache_bypass;
    /* if set, remember to free reg_defaults_raw */
    bool cache_free;

    struct reg_default *reg_defaults;
    const void *reg_defaults_raw;
    void *cache;
    /* if set, the cache contains newer data than the HW */
    bool cache_dirty;
    /* if set, the HW registers are known to match map->reg_defaults */
    bool no_sync_defaults;

    struct reg_sequence *patch;
    int patch_regs;

    /* if set, converts bulk read to single read */
    bool use_single_read;
    /* if set, converts bulk read to single read */
    bool use_single_write;
    /* if set, the device supports multi write mode */
    bool can_multi_write;

    /* if set, raw reads/writes are limited to this size */
    size_t max_raw_read;
    size_t max_raw_write;

    struct rb_root range_tree;
    void *selector_work_buf;    /* Scratch buffer used for selector */
};

_regmap_bus_read

reg_read回调函数是我们在读数据要使用的方法，

刚刚已经说了，map->reg_read = _regmap_bus_read;

我们需要看其实现：

// drivers/base/regmap/regmap.c

static int _regmap_bus_read(void *context, unsigned int reg,
                unsigned int *val)
{
    int ret;
    struct regmap *map = context;

    if (!map->format.parse_val)
        return -EINVAL;
    // 读出寄存器数据
    ret = _regmap_raw_read(map, reg, map->work_buf, map->format.val_bytes);
    if (ret == 0) // 根据寄存器位值宽和大小端得到寄存器值
        *val = map->format.parse_val(map->work_buf);

    return ret;
}

_regmap_raw_read

继续看：

static int _regmap_raw_read(struct regmap *map, unsigned int reg, void *val,
                unsigned int val_len)
{
    struct regmap_range_node *range;
    int ret;

    WARN_ON(!map->bus);

    if (!map->bus || !map->bus->read)
        return -EINVAL;
    // 从范围中找到某一页(但在这里没有用到)。
    range = _regmap_range_lookup(map, reg);
    if (range) {
        ret = _regmap_select_page(map, &reg, range,
                      val_len / map->format.val_bytes);
        if (ret != 0)
            return ret;
    }

    map->format.format_reg(map->work_buf, reg, map->reg_shift);
    regmap_set_work_buf_flag_mask(map, map->format.reg_bytes,
                      map->read_flag_mask);
    trace_regmap_hw_read_start(map, reg, val_len / map->format.val_bytes);
    // 使用bus定义的read函数指针读数据，这样子就会调用到`regmap_i2c_read`
    ret = map->bus->read(map->bus_context, map->work_buf,
                 map->format.reg_bytes + map->format.pad_bytes,
                 val, val_len);

    trace_regmap_hw_read_done(map, reg, val_len / map->format.val_bytes);

    return ret;
}

查找range（不理解）

这个部分的代码好像是不会执行的。

// drivers/base/regmap/internal.h
struct regmap_range_node {
    struct rb_node node;
    const char *name;
    struct regmap *map;

    unsigned int range_min;
    unsigned int range_max;

    unsigned int selector_reg;
    unsigned int selector_mask;
    int selector_shift;
    // range_min、range_max是当前reg范围的开始与结束地址，window_len是”窗”的长度.
    unsigned int window_start;
    unsigned int window_len;
};

// drivers/base/regmap/regmap.c
static struct regmap_range_node *_regmap_range_lookup(struct regmap *map,
                              unsigned int reg)
{
    struct rb_node *node = map->range_tree.rb_node;
    // reg是当前寄存器的index
    // range_min、range_max是当前reg范围的开始与结束地址
    while (node) {
        struct regmap_range_node *this =
            rb_entry(node, struct regmap_range_node, node);
        // 找到 落在 min 与 max 这个范围中的 reg
        if (reg < this->range_min)
            node = node->rb_left;
        else if (reg > this->range_max)
            node = node->rb_right;
        else
            return this;
    }

    return NULL;
}

static int _regmap_select_page(struct regmap *map, unsigned int *reg,
                   struct regmap_range_node *range,
                   unsigned int val_num)
{
    void *orig_work_buf;
    unsigned int win_offset;
    unsigned int win_page;
    bool page_chg;
    int ret;

    // 计算偏移
    win_offset = (*reg - range->range_min) % range->window_len;
    win_page = (*reg - range->range_min) / range->window_len;

    if (val_num > 1) {
        /* Bulk write shouldn't cross range boundary */
        if (*reg + val_num - 1 > range->range_max)
            return -EINVAL;

        /* ... or single page boundary */
        if (val_num > range->window_len - win_offset)
            return -EINVAL;
    }

    /* It is possible to have selector register inside data window.
       In that case, selector register is located on every page and
       it needs no page switching, when accessed alone. */
    /* 如果目标寄存器跨越1个寄存器(超过4个字节了)
        调用_regmap_update_bits来拿到新的数据
            _regmap_update_bits调用_regmap_read
            先从i2c中目标寄存器中读出值orig,
            然后把该值orig掩码掉mask,然后或上val,
            将得到的结果调用_regmap_write写入到目标寄存器
    */
    if (val_num > 1 ||
        range->window_start + win_offset != range->selector_reg) {
        /* Use separate work_buf during page switching */
        orig_work_buf = map->work_buf;
        map->work_buf = map->selector_work_buf;

        ret = _regmap_update_bits(map, range->selector_reg,
                      range->selector_mask,
                      win_page << range->selector_shift,
                      &page_chg, false);

        map->work_buf = orig_work_buf;

        if (ret != 0)
            return ret;
    }

    *reg = range->window_start + win_offset;

    return 0;
}

regmap_read

regmap_read函数用来读取寄存器值，实现如下：

int regmap_read(struct regmap *map, unsigned int reg, unsigned int *val)
{
    int ret;
    // 通过寄存器操作宽度判断寄存器操作是否合法
    if (reg % map->reg_stride)
        return -EINVAL;

    map->lock(map->lock_arg);

    ret = _regmap_read(map, reg, val);

    map->unlock(map->lock_arg);

    return ret;
}

static int _regmap_read(struct regmap *map, unsigned int reg,
                        unsigned int *val)
{
    // 1.直接从缓存取值
    if (!map->cache_bypass) {
        ret = regcache_read(map, reg, val);
        if (ret == 0)
            return 0;
    }
    // 2.最终调用bus定义的read读寄存器数据
    ret = map->reg_read(context, reg, val);
    // 3.将寄存器结果写入缓存
    if (!map->cache_bypass)
        regcache_write(map, reg, *val);

    return ret;
}

regmap_read函数最后调用bus的read函数，同理，regmap_write函数最终也会调用bus的write函数。

regmap机制中的缓存

关于缓冲，需要解释的是，在regmap中加入了一层缓存，减少IO操作次数，提供硬件操作效率；

/* An enum of all the supported cache types */
enum regcache_type {
	REGCACHE_NONE,
	REGCACHE_RBTREE,//红黑树类型
	REGCACHE_COMPRESSED,//压缩类型
	REGCACHE_FLAT,//普通数据类型
};

在Linux 4.0 版本中，已经有 3 种缓存类型，分别是数据（flat）、LZO 压缩和红黑树（rbtree）。

• 数据好理解，是最简单的缓存类型，当设备寄存器很少时，可以用这种类型来缓存寄存器值。
• LZO(Lempel–Ziv–Oberhumer) 是 Linux 中经常用到的一种压缩算法，Linux 编译后就会用这个算法来压缩。这个算法有 3 个特性：压缩快，解压不需要额外内存，压缩比可以自动调节。在这里，你可以理解为一个数组缓存，套了一层压缩，来节约内存。当设备寄存器数量中等时，可以考虑这种缓存类型。
• 而最后一类红黑树，它的特性就是索引快，所以当设备寄存器数量比较大，或者对寄存器操作延时要求低时，就可以用这种缓存类型。

缓存的类型是在 regmap 初始化时，由配置regmap_config的 cache_type 来指定的。

对于 regmap_read 来说，会先判断当前缓存是否有值，然后再检查是否需要 bypass，若没有，则可以直接从缓存里面取值，调用regcache_read来获取值，若需要从硬件上读取，则调用具体协议的读写函数，若是 I2C，调用i2c_transfer。

写的过程也是大同小异。

到此，在驱动程序中regmap的基本使用方法和调用简析就结束了。