void *ioremap(unsigned long phys_addr, unsigned long size)
入口： phys_addr：要映射的起始的IO地址；
size：要映射的空間的大小；
flags：要映射的IO空間的和許可權有關的標誌；
phys_addr：是要映射的物理地址，
size：是要映射的長度，
S3C2410的long是32位而非64位。
功能：將一個IO地址空間映射到核心的虛擬地址空間上去，便於訪問；
實現：對要映射的IO地址空間進行判斷，低PCI/ISA地址不需要重新映射，也不允許用戶將IO地址空間映射到正在使用的RAM中，最後申請一個 vm_area_struct結構，調用remap_area_pages填寫頁表，若填寫過程不成功則釋放申請的vm_area_struct空間；
ioremap 依靠 __ioremap實現,它只是在__ioremap中以第三個參數為0調用來實現.
ioremap是核心提供的用來映射外設暫存器到主存的函式，我們要映射的地址已經從pci_dev中讀了出來（上一步），這樣就水到渠成的成功映射了而不會和其他地址有衝突。映射完了有什麼效果呢，我舉個例子，比如某個網卡有100 個暫存器，他們都是連在一塊的，位置是固定的，假如每個暫存器占4個位元組，那么一共400個位元組的空間被映射到記憶體成功後，ioaddr就是這段地址的開頭（注意ioaddr是虛擬地址，而mmio_start是物理地址，它是BIOS得到的，肯定是物理地址，而保護模式下CPU不認物理地址，只認虛擬地址），ioaddr+0就是第一個暫存器的地址，ioaddr+4就是第二個暫存器地址（每個暫存器占4個位元組），以此類推，我們就能夠在記憶體中訪問到所有的暫存器進而操控他們了。
A new I/O memory access mechanism
--------------------------------------------------------------------------------
Most reasonably current cards for the PCI bus (and others) provide one or more I/O memory regions to the bus. By accessing those regions, the processor can communicate with the peripheral and make things happen. A look at /proc/iomem will show the I/O memory regions which have been registered on a given system.
Advertisement
To work with an I/O memory region, a driver is supposed to map that region with a call to ioremap(). The return value from ioremap() is a magic cookie which can be passed to a set of accessor functions (with names like readb() or writel()) to actually move data to or from the I/O memory. On some architectures ( notably x86), I/O memory is truly mapped into the kernel's memory space, so those accessor functions turn into a straightforward pointer dereference. Other architectures require more complicated operations.
There have been some longstanding problems with this scheme. Drivers written for the x86 architecture have often been known to simply dereference I/O memory addresses directly, rather than using the accessor functions. That approach works on the x86, but breaks on other architectures. Other drivers, knowing that I/O memory addresses are not real pointers, store them in integer variables; that works until they encounter a system with a physical address space which doesn't fit into 32 bits. And, in any case, readb() and friends perform no type checking, and thus fail to catch errors which could be found at compile time.
The 2.6.9 kernel will contain a series of changes designed to improve how the kernel works with I/O memory. The first of these is a new __iomem annotation used to mark pointers to I/O memory. These annotations work much like the __user markers, except that they reference a different address space. As with __user, the __iomem marker serves a documentation role in the kernel code; it is ignored by the compiler. When checking the code with sparse, however, developers will see a whole new set of warnings caused by code which mixes normal pointers with __iomem pointers, or which dereferences those pointers.
The Next Step is the addition of a new set of accessor functions which explicitly require a pointer argument. These functions are:
unsigned int ioread8(void __iomem *addr); unsigned int ioread16(void __iomem *addr); unsigned int ioread32(void __iomem *addr); void iowrite8(u8 value, void __iomem *addr); void iowrite16(u16 value, void __iomem *addr); void iowrite32(u32 value, void __iomem *addr);By default, these functions are simply wrappers around readb() and friends. The explicit pointer type for the argument will generate warnings, however, if a driver passes in an integer type.
There are "string" versions of these operations:
extern void ioread8_rep(void __iomem *port, void *buf, unsigned long count);All of the other variants are defined as well, of course.
There is actually one other twist to these functions. Some drivers have to be able to use either I/O memory or I/O ports, depending on the architecture and the device. Some such drivers have gone to considerable lengths to try to avoid duplicating code in those two cases. With the new accessors, a driver which finds it needs to work with x86-style ports can call:
void __iomem *ioport_map(unsigned long port, unsigned int count);The return value will be a cookie which allows the mapped ports to be treated as if they were I/O memory; functions like ioread8() will automatically do the right thing. For PCI devices, there is a new function:
void __iomem *pci_iomap(struct pci_dev *dev, int base, unsigned long maxlen);For this function, the base can be either a port number or an I/O memory address, and the right thing will be done.
As of 2.6.9-rc2, there are no in-tree users of the new interface. That can be expected to change soon as patches get merged and the kernel janitors get to work. For more information on the new I/O memory interface and the motivation behind it, see this explanation from Linus
本文來自 csdn部落格
