build_mem_type_table

start_kernel
    -->setup_arch
        -->paging_init
            -->build_mem_type_table

static unsigned int cachepolicy __initdata = CPOLICY_WRITEBACK;

#define CPOLICY_UNCACHED    0
#define CPOLICY_BUFFERED    1
#define CPOLICY_WRITETHROUGH    2
#define CPOLICY_WRITEBACK    3
#define CPOLICY_WRITEALLOC    4

static struct cachepolicy cache_policies[] __initdata = {
    {
        .policy        = "uncached",
        .cr_mask    = CR_W|CR_C,
        .pmd        = PMD_SECT_UNCACHED,
        .pte        = L_PTE_MT_UNCACHED,
    }, {
        .policy        = "buffered",
        .cr_mask    = CR_C,
        .pmd        = PMD_SECT_BUFFERED,
        .pte        = L_PTE_MT_BUFFERABLE,
    }, {
        .policy        = "writethrough",
        .cr_mask    = 0,
        .pmd        = PMD_SECT_WT,
        .pte        = L_PTE_MT_WRITETHROUGH,
    }, {
        .policy        = "writeback",
        .cr_mask    = 0,
        .pmd        = PMD_SECT_WB,
        .pte        = L_PTE_MT_WRITEBACK,
    }, {
        .policy        = "writealloc",
        .cr_mask    = 0,
        .pmd        = PMD_SECT_WBWA,
        .pte        = L_PTE_MT_WRITEALLOC,
    }
};