Linux下platform_device中的dma_mask与coherent_dma_mask

一：dma_mask与coherent_dma_mask的定义在linux内核中,引入了platform_device与platform_driver，这样就很方便了平台的设备与驱动。在includelinuxplatform_device.h下：struct platform_device {
const char * name;
int  id;
struct device dev;
u32  num_resources;
struct resource * resource; const struct platform_device_id *id_entry; /* arch specific additions */
struct pdev_archdata archdata;
};而struct device dev，在includelinuxdevice.h中：struct device {
struct device  *parent; struct device_private *p; struct kobject kobj;
const char  *init_name; /* initial name of the device */
struct device_type *type; struct mutex  mutex; /* mutex to synchronize calls to
      * its driver.
      */ struct bus_type *bus;  /* type of bus device is on */
struct device_driver *driver; /* which driver has allocated this
        device */
void  *platform_data; /* Platform specific data, device
        core doesn"t touch it */
struct dev_pm_info power;#ifdef CONFIG_NUMA
int  numa_node; /* NUMA node this device is close to */
#endif
u64  *dma_mask; /* dma mask （if dma"able device） */
u64  coherent_dma_mask;/* Like dma_mask, but for
          alloc_coherent mappings as
          not all hardware supports
          64 bit addresses for consistent
          allocations such descriptors. */ struct device_dma_parameters *dma_parms; struct list_head dma_pools; /* dma pools （if dma"ble） */ struct dma_coherent_mem *dma_mem; /* internal for coherent mem
          override */
/* arch specific additions */
struct dev_archdata archdata;
#ifdef CONFIG_OF
struct device_node *of_node;
#endif dev_t   devt; /* dev_t, creates the sysfs "dev" */ spinlock_t  devres_lock;
struct list_head devres_head; struct klist_node knode_class;
struct class  *class;
const struct attribute_group **groups; /* optional groups */ void （*release）（struct device *dev）;
};
dma_mask与coherent_dma_mask这两个参数表示它能寻址的物理地址的范围，内核通过这两个参数分配合适的物理内存给 device。其中dma_coherent_mask则作用于申请一致性DMA缓冲区。因为不是所有的硬件都能够支持64bit的地址宽度。如果 addr_phy 是一个物理地址，且（u64）addr_phy <= *dev->dma_mask，那么该 device 就可以寻址该物理地址。如果 device 只能寻址32位地址，那么 mask 应为 0xffffffff。依此类推。例如，在linux2.6.38中：/* USB EHCI Host Controller */static u64 nuc900_device_usb_ehci_dmamask = 0xffffffffUL;static struct platform_device nuc900_device_ehci = {
        .name    = "nuc900-ehci",
        .id    = -1,
        .num_resources   = ARRAY_SIZE（nuc900_ehci_resource）,
        .resource   = nuc900_ehci_resource,
        .dev              = {
                .dma_mask = &nuc900_device_usb_ehci_dmamask,
                .coherent_dma_mask = 0xffffffffUL
        }
};二：sample code
这段代码摘录自 archarmmmdma-mapping.c    u64 limit;
    /*
     * Sanity check the allocation size.
     */
    size = PAGE_ALIGN（size）;
    //这个 limit 就是通过 mask 计算得到的设备最大寻址范围。
    limit = （mask + 1） & ~mask;
    //当 size 超出 limit 时，说明分配的地址超出了设备的最大寻址能力，这时返回错误。
    if （（limit && size >= limit） ||
        size >= （CONSISTENT_END - CONSISTENT_BASE）） {
        printk（KERN_WARNING "coherent allocation too big "
               "（requested %#x mask %#llx） ", size, mask）;
        goto no_page;
    }