dev_dax
当我们生成了nd_dax设备后，nvdimm驱动的任务就基本结束了。或者应该说，nvdimm驱动最后的任务是创建一个dev_dax设备，由这个设备接过最后的枪。
构造dev_dax的过程
从nd_dax构造dev_dax的过程在dax_pmem_probe中完成。
dax_pmem_probe()
nvdimm_setup_pfn(nd_pfn, &pgmap);
nd_pfn_init(nd_pfn);
__nvdimm_setup_pfn(nd_pfn, pgmap);
dax_region = alloc_dax_region(dev, region_id, &res,
le32_to_cpu(pfn_sb->align), addr, PFN_DEV|PFN_MAP);
dev_dax = __devm_create_dev_dax(dax_region, id, &pgmap, subsys);
dax_dev = alloc_dax(dev_dax, NULL, NULL);
dax_dev = dax_dev_get(devt);
...
alloc_inode(super_block)
dev_dax->dax_dev = dax_dev;上面的流程就是从nd_dax/nd_pfn设备构造到dev_dax的过程。大致可以分成三个步骤：
nvdimm_setup_pfn() 对nd_pfn中的成员做了设置，比如对齐和计算page struct的空间
alloc_dax_region() 这是构造dev_dax所需的信息
函数__devm_create_dev_dax用来创建dax_dev，也就是dax文件系统。值得注意的是这个数据结构是通过super_block创建inode时候创建的。
dev_dax是个庞大的结构体，就让我们来看一看吧。
dev_dax, dax_dev, dax_region
dev_dax
+-------------------------------------+<----------------------------+
|id | |
| (int) | |
|dev | |
| (struct device) | |
| +---------------------------------+ |
| |devt | dax_dev.inode->i_rdev |
| |class | dax_class |
| |/ bus | dax_bus_type |
| |groups | dax_attribute_groups |
| |parent | dax_region->dev = nd_pfn |
| | | |
| |kobj->name | = "dax0.0" |
| +---------------------------------+ |
| | |
|dax_dev | |
| (struct dax_device*) | |
| +---------------------------------+ |
| |private ----| ----------------------------+
| | (void*) |
| |inode |
| | (struct inode) |
| | +-----------------------------+
| | |i_cdev ----|-----+
| | | (struct cdev*) | |
| | +-----------------------------+ |
| |cdev |<----+
| | (struct cdev) | /dev/dax0.0
| | +-----------------------------+
| | |ops | dax_fops
| | | |
| | +-----------------------------+
| | |
| |host |
| | (char*) |
| |ops |
| | (struct dax_operations*) |
| +---------------------------------+
| |
|region |
| (struct dax_region*) |
| +---------------------------------+
| |id |
| | (int) |
| |dev | = nd_pfn.dev
| | (struct device*) |
| |ida |
| | (struct ida) |
| |res |
| | (struct resource) |
| | +-----------------------------+
| | |start | nsio->res.start + start_pad + dataoff
| | |end | nsio->res.start + size - end_trunc
| | +-----------------------------+
| | |
+---+---------------------------------+
|ref <---|------------------------------------+
| (struct percpu_ref) | |
| +-------------------------------+ |
| |count | |
| | (atomic_long_t) | |
| |percpu_count_ptr | |
| | (unsigned long) | |
| |release | = dev_dax_percpu_release |
| |confirm_switch | |
| | (percpu_ref_func_t) | |
| |force_atomit | |
| | | |
| |rcu | |
| | (struct rcu_head) | |
+-----+-------------------------------+ |
|pgmap | setup in nvdimm_setup_pfn |
| (struct dev_pagemap) | |
| +-------------------------------+ |
| |dev | = nd_pfn.dev |
| | (struct device*) | |
| |ref | -----------------------------------+
| | (struct percpu_ref*) |
| |res |
| | (struct resource) |
| | +---------------------------+
| | |start | nsio->res.start + start_pad
| | |end | nsio->res.start + size - end_trunc
| | +---------------------------+
| |page_free |
| | (dev_page_free_t) |
| |kill | = dax_pmem_percpu_kill
| | () |
| | |
| |altmap |
| | (struct vmem_altmap) |
| | +---------------------------+
| | |base_pfn | = PFN_SECTION_ALIGN_DOWN(nsio->res.start + start_pad)
| | |reserve | = PHYS_PFN(SZ_8K)
| | | (const unsigned long) |
| | |free | = PHYS_PFN(offset - SZ_8K);
| | |align |
| | |alloc |
| | | (unsigned long) |
| | +---------------------------+
| | |
+-----+-------------------------------+着重讲几点：
最终的目标是生成dev_dax设备，该设备对应一个字符类型的文件/dev/dax0.0
/dev/dax0.0的fops是dax_fops，所以后面的mmap就靠它了
dev_dax生成的信息在dax_region中
dax_region中res就是对齐过后，去掉元数据的部分；
怎么样，这下是不是够爽？
dev_dax的驱动device_dax_driver
既然有了dev_dax，那么这个设备是如何开始它的工作的呢？再来回顾一下dev_dax结构体中新创建的设备。
dev
(struct device)
+---------------------------------+
|devt | dax_dev.inode->i_rdev
|class | dax_class
|/ bus | dax_bus_type
|groups | dax_attribute_groups
|parent | dax_region->dev = nd_pfn
| |
|kobj->name | = "dax0.0"
+---------------------------------+ 这部分已经超出了nvdimm驱动的范畴，但是为了把故事讲完，我们还是要看一下生成的这个设备是如何被使用到的。
因为这个设备是在dax_bus_type总线上的，所以它的驱动也应该是在同一个总线上的了。这个默认的驱动就是device_dax_driver.
static struct dax_device_driver device_dax_driver = {
.drv = {
.probe = dev_dax_probe,
.remove = dev_dax_remove,
},
.match_always = 1,
};探测
这个驱动的探测函数dev_dax_probe主要做了两件事情：
devm_memremap_pages(dev, &dev->pgmap)
cdev_init(cdev, &dax_fops);
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