在我們使用ARM等嵌入式Linux系統的時候,一個頭疼的問題是GPU,Camera,HDMI等都需要預留大量連續內存,這部分內存平時不用,但是一般的做法又必須先預留著。目前,Marek Szyprowski和Michal Nazarewicz實現了一套全新的Contiguous Memory Allocator。通過這套機制,我們可以做到不預留內存,這些內存平時是可用的,只有當需要的時候才被分配給Camera,HDMI等設備。下面分析它的基本代碼流程。
聲明連續內存
內核啟動過程中arch/arm/mm/init.c中的arm_memblock_init()會調用dma_contiguous_reserve(min(arm_dma_limit, arm_lowmem_limit));
該函數位於:drivers/base/dma-contiguous.c
- /**
- * dma_contiguous_reserve() - reserve area for contiguous memory handling
- * @limit: End address of the reserved memory (optional, 0 for any).
- *
- * This function reserves memory from early allocator. It should be
- * called by arch specific code once the early allocator (memblock or bootmem)
- * has been activated and all other subsystems have already allocated/reserved
- * memory.
- */
- void __init dma_contiguous_reserve(phys_addr_t limit)
- {
- unsigned long selected_size = 0;
-
- pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit);
-
- if (size_cmdline != -1) {
- selected_size = size_cmdline;
- } else {
- #ifdef CONFIG_CMA_SIZE_SEL_MBYTES
- selected_size = size_bytes;
- #elif defined(CONFIG_CMA_SIZE_SEL_PERCENTAGE)
- selected_size = cma_early_percent_memory();
- #elif defined(CONFIG_CMA_SIZE_SEL_MIN)
- selected_size = min(size_bytes, cma_early_percent_memory());
- #elif defined(CONFIG_CMA_SIZE_SEL_MAX)
- selected_size = max(size_bytes, cma_early_percent_memory());
- #endif
- }
-
- if (selected_size) {
- pr_debug("%s: reserving %ld MiB for global area\n", __func__,
- selected_size / SZ_1M);
-
- dma_declare_contiguous(NULL, selected_size, 0, limit);
- }
- };
其中的size_bytes定義為:
static const unsigned long size_bytes = CMA_SIZE_MBYTES * SZ_1M; 默認情況下,CMA_SIZE_MBYTES會被定義為16MB,來源於CONFIG_CMA_SIZE_MBYTES=16
->
- int __init dma_declare_contiguous(struct device *dev, unsigned long size,
- phys_addr_t base, phys_addr_t limit)
- {
- ...
- /* Reserve memory */
- if (base) {
- if (memblock_is_region_reserved(base, size) ||
- memblock_reserve(base, size) < 0) {
- base = -EBUSY;
- goto err;
- }
- } else {
- /*
- * Use __memblock_alloc_base() since
- * memblock_alloc_base() panic()s.
- */
- phys_addr_t addr = __memblock_alloc_base(size, alignment, limit);
- if (!addr) {
- base = -ENOMEM;
- goto err;
- } else if (addr + size > ~(unsigned long)0) {
- memblock_free(addr, size);
- base = -EINVAL;
- base = -EINVAL;
- goto err;
- } else {
- base = addr;
- }
- }
-
- /*
- * Each reserved area must be initialised later, when more kernel
- * subsystems (like slab allocator) are available.
- */
- r->start = base;
- r->size = size;
- r->dev = dev;
- cma_reserved_count++;
- pr_info("CMA: reserved %ld MiB at %08lx\n", size / SZ_1M,
- (unsigned long)base);
-
- /* Architecture specific contiguous memory fixup. */
- dma_contiguous_early_fixup(base, size);
- return 0;
- err:
- pr_err("CMA: failed to reserve %ld MiB\n", size / SZ_1M);
- return base;
- }
由此可見,連續內存區域也是在內核啟動的早期,通過__memblock_alloc_base()拿到的。
另外:
drivers/base/dma-contiguous.c裡面的core_initcall()會導致cma_init_reserved_areas()被調用:
- static int __init cma_init_reserved_areas(void)
- {
- struct cma_reserved *r = cma_reserved;
- unsigned i = cma_reserved_count;
-
- pr_debug("%s()\n", __func__);
-
- for (; i; --i, ++r) {
- struct cma *cma;
- cma = cma_create_area(PFN_DOWN(r->start),
- r->size >> PAGE_SHIFT);
- if (!IS_ERR(cma))
- dev_set_cma_area(r->dev, cma);
- }
- return 0;
- }
- core_initcall(cma_init_reserved_areas);