ARM Linux靜態(tài)映射分析

作者：易松華,華清遠(yuǎn)見(jiàn)嵌入式學(xué)院講師。

在華清遠(yuǎn)見(jiàn)上課過(guò)程中，發(fā)現(xiàn)靜態(tài)映射方面初學(xué)者比較難于掌握和理解，下面分析一下靜態(tài)映射機(jī)制的原理并通過(guò)GPIO和USB、LCD等的靜態(tài)映射作為例子來(lái)說(shuō)明如何通過(guò)這種靜態(tài)映射的方式訪問(wèn)外設(shè)資源。

內(nèi)核提供了一個(gè)重要的結(jié)構(gòu)體struct machine_desc ,這個(gè)結(jié)構(gòu)體在內(nèi)核移植中起到相當(dāng)重要的作用,內(nèi)核通過(guò)machine_desc結(jié)構(gòu)體來(lái)控制系統(tǒng)體系架構(gòu)相關(guān)部分的初始化。machine_desc結(jié)構(gòu)體的成員包含了體系架構(gòu)相關(guān)部分的幾個(gè)最重要的初始化函數(shù)，包括map_io,init_irq, init_machine以及phys_io , timer成員等。

machine_desc結(jié)構(gòu)體定義如下：

struct machine_desc {
            /*
                * Note! The first four elements are used
                * by assembler code in head-armv.S
                */
                unsigned int                  nr;         /* architecture number         */
                unsigned int                  phys_io;          /* start of physical io         */
                unsigned int                  io_pg_offst; /* byte offset for io
                                                * page tabe entry         */
                const char                 *name;          /* architecture name          */
                unsigned long                     ;boot_params;          /* tagged list          */
                unsigned int                  video_start;         /* start of video RAM          */
                unsigned int                  video_end;         /* end of video RAM          */
                unsigned int                 reserve_lp0 :1;          /* never has lp0          */
                unsigned int                 reserve_lp1 :1;          /* never has lp1          */
                unsigned int                  reserve_lp2 :1;          /* never has lp2          */
                unsigned int                  soft_reboot :1;          /* soft reboot          */
                void                          (*fixup)(struct machine_desc *,
                                                        struct tag *, char **,
                                                        struct meminfo *);
                void                         (*map_io)(void);/* IO mapping function          */
                void                         (*init_irq)(void);
                struct sys_timer          *timer;          /* system tick timer          */
                void                         (*init_machine)(void);
        };

machine_desc結(jié)構(gòu)體通過(guò)MACHINE_START宏來(lái)初始化,這里以s3c2410平臺(tái)為例:

s3c2410 machine_desc結(jié)構(gòu)體定義如下：
        /* arch/arm/mach-s3c2410/mach-smdk2410.c */
        MACHINE_START(SMDK2410, "SMDK2410") /* @TODO: request a new identifier and switch
                                        * to SMDK2410 */
                /* Maintainer: Jonas Dietsche */
                .phys_io = S3C2410_PA_UART,
                .io_pg_offst = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
                .boot_params = S3C2410_SDRAM_PA + 0x100,
                .map_io = smdk2410_map_io,
                .init_irq = s3c24xx_init_irq,
                .init_machine = smdk2410_init,
                .timer = &s3c24xx_timer,
        MACHINE_END

其中的宏MACHINE_START和MACHINE_END定義如下：
        /*
          * Set of macros to define architecture features. This is built into
          * a table by the linker.
          */
        #define MACHINE_START(_type,_name) \
        const struct machine_desc __mach_desc_##_type \
          __attribute__((__section__(".arch.info.init"))) = { \
                .nr = MACH_TYPE_##_type, \
                .name = _name,

#define MACHINE_END \
        };

其中MACH_TYPE_##_type 為GCC擴(kuò)展語(yǔ)法中的字符拼接標(biāo)識(shí)，在預(yù)編譯的時(shí)候會(huì)用真正的字符代替，比如我們這里就是MACH_TYPE_SMDK2410

MACHINE_START的使用及各個(gè)成員函數(shù)的的放置位置以及調(diào)用過(guò)程如下：
        MACH_TYPE_SMDK2410這個(gè)值是目標(biāo)板的類型值，定義在arch/include/asm-arm/mach-types.h內(nèi)，值為193.

/* arch/include/asm-arm/mach-types.h */
        #define MACH_TYPE_SMDK2410 193

由上發(fā)現(xiàn)，MACHINE_START主要是定義了"struct machine_desc"的類型，放在 section(".arch.info.init")，是初始化數(shù)據(jù)，其所占用的內(nèi)存在內(nèi)核起來(lái)之后將會(huì)被釋放。

這里的map_io成員即內(nèi)核提供給用戶的創(chuàng)建外設(shè)I/O資源到內(nèi)核虛擬地址靜態(tài)映射表的接口函數(shù)。map_io成員函數(shù)會(huì)在系統(tǒng)初始化過(guò)程中被調(diào)用,流程如下：
        start_kernel -> setup_arch() --> paging_init()中被調(diào)用
        struct machine_desc 結(jié)構(gòu)體的各個(gè)成員函數(shù)在不同時(shí)期被調(diào)用：
        1. .init_machine 在 arch/arm/kernel/setup.c 中被 customize_machine 調(diào)用，放在 arch_initcall( ) 段里面，會(huì)自動(dòng)按順序被調(diào)用（另外博客分析，敬請(qǐng)關(guān)注）。
        2. init_irq在start_kernel( ) --> init_IRQ( ) --> init_arch_irq( ) 被調(diào)用
        3. map_io 在 setup_arch( ) --> paging_init( )被調(diào)用

其他主要都在 setup_arch() 中用到。

用戶可以在定義machine_desc結(jié)構(gòu)體時(shí)指定map_io的接口函數(shù)，我們也正是這樣做的。

接下來(lái)我們繼續(xù)分析smdk2410_map_io的執(zhí)行過(guò)程，流程如下：

smdk2410_map_io-> s3c24xx_init_io(smdk2410_iodesc, ARRAY_SIZE(smdk2410_iodesc))

下面來(lái)看一下s3c24xx_init_io函數(shù)：

void __init s3c24xx_init_io(struct map_desc *mach_desc, int mach_size)
        {
                /* register our io-tables */
                iotable_init(s3c_iodesc, ARRAY_SIZE(s3c_iodesc));
                ……
        }

iotable_init內(nèi)核提供，定義如下：

/*
        * Create the architecture specific mappings
        */
        void __init iotable_init(struct map_desc *io_desc, int nr)
        {
                int i;
                for (i = 0; i nr; i++)
                create_mapping(io_desc + i);
        }

由上知道，smdk2410_map_io最終調(diào)用iotable_init建立映射表。

iotable_init函數(shù)的參數(shù)有兩個(gè)：一個(gè)是map_desc類型的結(jié)構(gòu)體，另一個(gè)是該結(jié)構(gòu)體的數(shù)量nr。這里最關(guān)鍵的就是struct map_desc。map_desc結(jié)構(gòu)體定義如下：

/* include/asm-arm/mach/map.h */
                struct map_desc {
                unsigned long virtual;
                unsigned long physical;
                unsigned long length;
                unsigned int type;
        };

create_mapping( )函數(shù)就是通過(guò)map_desc提供的信息創(chuàng)建線性映射表的。

這樣的話我們就知道了創(chuàng)建I/O映射表的大致流程為：只要定義相應(yīng)I/O資源的map_desc結(jié)構(gòu)體，并將該結(jié)構(gòu)體傳給iotable_init函數(shù)執(zhí)行，就可以創(chuàng)建相應(yīng)的I/O資源到內(nèi)核虛擬地址空間的映射表了。

我們來(lái)看看s3c2410是怎么定義map_desc結(jié)構(gòu)體的(即上面iotable_init()函數(shù)內(nèi)的s3c_iodesc)。

[arch/arm/mach-s3c2410/cpu.c]
        /* minimal IO mapping */
        static struct map_desc s3c_iodesc[] __initdata = {
                IODESC_ENT(GPIO),
                IODESC_ENT(IRQ),
                IODESC_ENT(MEMCTRL),
                IODESC_ENT(UART)
        };

IODESC_ENT宏如下：

#define IODESC_ENT(x) { (unsigned long)S3C24XX_VA_##x, S3C2410_PA_##x, S3C24XX_SZ_##x, MT_DEVICE }

展開(kāi)后等價(jià)于：

static struct map_desc s3c_iodesc[] __initdata = {
                {
                        .virtual = S3C24XX_VA_GPIO,
                        .physical = S3C24XX_PA_GPIO,
                        .length = S3C24XX_SZ_GPIO,
                        .type = MT_DEVICE
                },
                ……
        };

至此，我們可以比較清晰看到GPIO被靜態(tài)映射的過(guò)程，由于我們?cè)谇懊娴撵o態(tài)映射中已經(jīng)做好了GPIO的映射，也就是我們寫(xiě)GPIO相關(guān)驅(qū)動(dòng)的時(shí)候可以如下配置引腳的原因：
        s3c2410_gpio_cfgpin（xxx,xxx);

其實(shí)，s3c2410_gpio_cfgpin定義如下：

void s3c2410_gpio_cfgpin(unsigned int pin, unsigned int function)
        {
                void __iomem *base = S3C2410_GPIO_BASE(pin);
                unsigned long mask;
                unsigned long con;
                unsigned long flags;

        if (pin < S3C2410_GPIO_BANKB) {
                        mask = 1 << S3C2410_GPIO_OFFSET(pin);
                } else {
                        mask = 3 << S3C2410_GPIO_OFFSET(pin)*2;
                }

        local_irq_save(flags);

        con = __raw_readl(base + 0x00);
                con &= ~mask;
                con |= function;

        __raw_writel(con, base + 0x00);

        local_irq_restore(flags);
        }

其中，比較關(guān)鍵的一個(gè)地方：
        void __iomem *base = S3C2410_GPIO_BASE(pin);
        這一行中，S3C2410_GPIO_BASE定義如下：

#define S3C2410_GPIO_BASE(pin) ((((pin) & ~31) >> 1) + S3C24XX_VA_GPIO)

至此，GPIO的靜態(tài)映射就看得很明白了。

下面來(lái)看其他外設(shè)的靜態(tài)映射：

在s3c24xx_init_io（）函數(shù)中，除了iotable_init（）以為，還會(huì)在最后調(diào)用，
        (cpu->map_io)(mach_desc, size);

而CPU的這個(gè)map_io在arch/arm/mach-s3c2410/cpu.c里面定義如下：

static struct cpu_table cpu_ids[] __initdata = {
                {
                        .idcode = 0x32410000,
                        .idmask = 0xffffffff,
                        .map_io = s3c2410_map_io,
                        .init_clocks = s3c2410_init_clocks,
                        .init_uarts = s3c2410_init_uarts,
                        .init = s3c2410_init,
                        .name = name_s3c2410
                },
                        ...
         }

再查看s3c2410_map_io(),函數(shù)代碼如下：

void __init s3c2410_map_io(struct map_desc *mach_desc, int mach_size)
        {
                /* register our io-tables */

        iotable_init(s3c2410_iodesc, ARRAY_SIZE(s3c2410_iodesc));
                iotable_init(mach_desc, mach_size);
        }

接下來(lái)看結(jié)構(gòu)s3c2410_iodesc [arch/arm/mach-s3c2410/s3c2410.c]，代碼如下，

/* Initial IO mappings */
        static struct map_desc s3c2410_iodesc[] __initdata = {
                IODESC_ENT(USBHOST),
                IODESC_ENT(USBDEV),
                IODESC_ENT(CLKPWR),
                IODESC_ENT(LCD),
                IODESC_ENT(TIMER),
                IODESC_ENT(ADC),
                IODESC_ENT(WATCHDOG),
        };

赫然發(fā)現(xiàn)IODESC_ENT(TIMER)這一行，結(jié)合之前GPIO的類似分析，IODESC_ENT宏如下：
        #define IODESC_ENT(x) { (unsigned long)S3C24XX_VA_##x, S3C2410_PA_##x, S3C24XX_SZ_##x, MT_DEVICE }

至此，TIMER, USBHOST,USBDEV,lcd,adc,watchdog等的靜態(tài)映射都看得很明白了。