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>Accessing the device</H1
><P
> The part of the interface most used by drivers is reading and
writing memory-mapped registers on the device. Linux provides
interfaces to read and write 8-bit, 16-bit, 32-bit and 64-bit
quantities. Due to a historical accident, these are named byte,
word, long and quad accesses. Both read and write accesses are
supported; there is no prefetch support at this time.
</P
><P
> The functions are named <TT
CLASS="FUNCTION"
>readb</TT
>,
<TT
CLASS="FUNCTION"
>readw</TT
>, <TT
CLASS="FUNCTION"
>readl</TT
>,
<TT
CLASS="FUNCTION"
>readq</TT
>, <TT
CLASS="FUNCTION"
>writeb</TT
>,
<TT
CLASS="FUNCTION"
>writew</TT
>, <TT
CLASS="FUNCTION"
>writel</TT
> and
<TT
CLASS="FUNCTION"
>writeq</TT
>.
</P
><P
> Some devices (such as framebuffers) would like to use larger
transfers than 8 bytes at a time. For these devices, the
<TT
CLASS="FUNCTION"
>memcpy_toio</TT
>, <TT
CLASS="FUNCTION"
>memcpy_fromio</TT
>
and <TT
CLASS="FUNCTION"
>memset_io</TT
> functions are provided.
Do not use memset or memcpy on IO addresses; they
are not guaranteed to copy data in order.
</P
><P
> The read and write functions are defined to be ordered. That is the
compiler is not permitted to reorder the I/O sequence. When the
ordering can be compiler optimised, you can use <TT
CLASS="FUNCTION"
> __readb</TT
> and friends to indicate the relaxed ordering. Use
this with care. The <TT
CLASS="FUNCTION"
>rmb</TT
> provides a read memory
barrier. The <TT
CLASS="FUNCTION"
>wmb</TT
> provides a write memory barrier.
</P
><P
> While the basic functions are defined to be synchronous with respect
to each other and ordered with respect to each other the busses the
devices sit on may themselves have asynchronocity. In paticular many
authors are burned by the fact that PCI bus writes are posted
asynchronously. A driver author must issue a read from the same
device to ensure that writes have occurred in the specific cases the
author cares. This kind of property cannot be hidden from driver
writers in the API.
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