linux内核空间申请超过2MB连续空间的实现函数。

 /*
kmalloc can apply 128KB memory only. This func support any continous memory allocate more than 2MB.
*/
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kallsyms.h>
#define KMEM_PAGES        //apply more than one page
#define KMEM_DEBUG        //debug message switch
/*
// Pure 2^n version of get_order
static __inline__ __attribute_const__ int get_order(unsigned long size)
{
    int order;
    size = (size - 1) >> (PAGE_SHIFT - 1);
    order = -1;
    do {
        size >>= 1;
        order++;
    } while (size);
    return order;
}
*/
/*
alloc memory for DMA using in kernel space.
*/
void *kmem_alloc(size_t size, dma_addr_t *dma_handle, unsigned long flags)
{
    struct page *page;        //start page address
    void *cpu_addr = NULL;        //cpu io address
    struct page *end;        //end of pages
    unsigned int  PageOrder=0;
    
    size = PAGE_ALIGN(size);    //must be times of 4KB
    PageOrder=get_order(size);
#ifdef CONFIG_ISOLATE_HIGHMEM        //allocate in high memory
    page = alloc_pages(GFP_HIGHUSER, PageOrder);
#else                                //allocate from low memory
    page = alloc_pages(GFP_KERNEL, PageOrder);        //get_order(size) means how many 4KB page do you want to apply. 2^n
#end