关于__iomem的解释

lunziyu

原文链接：http://lwn.net/Articles/102232/
A new I/O memory access mechanismMost 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.
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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.
               
               
               

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