The current implementation of PCI support in the kernel was not available with version 2.0 of the kernel. With 2.0 the support API was much more raw, because it lacked the various objects that have been described in this chapter.
The six functions to access the configuration space received as
arguments the 16-bit low-level key to the PCI device instead of using
a pointer to struct pci_dev
. Also, you had to
include <asm/pcibios.h>
before being able to
read or write to the configuration space.
Fortunately, dealing with the difference is not a big problem, and if
you include sysdep.h
you’ll be able to use 2.4
semantics even when compiling under 2.0. PCI support for version 2.0
is available in the header pci-compat.h
,
automatically included by sysdep.h
when you
compile under 2.0. The header, as distributed, implements the most
important functions used to work with the PCI bus.
If you use pci-compat.h
to develop drivers that
work all the way from 2.0 through 2.4, you must call
pci_release_device
when you are done with a
pci_dev
item. This happens because the fake
pci_dev
structures created by the header are
allocated with kmalloc, whereas the real
structures of 2.2 and 2.4 are static resources in the kernel proper.
The extra function is defined to do nothing by
sysdep.h
whenever compiling for 2.2 or 2.4, so it
does no harm. Feel free to look at pciregions.c
or pcidata.c
to see portable code in action.
Another relevant difference in 2.0 is /proc
support for PCI. There was no /proc/bus/pci
file
hierarchy (and no /proc/bus
at all, actually), only
a single /proc/pci
file. It was meant more for
human reading than for machine reading, and it was not very readable
anyway. Under 2.2 it was possible to select a “backward-compatible
/proc/pci
” at compile time, but the obsolete
file was completely removed in version 2.4.
The concept of hot-pluggable PCI drivers (and struct pci_driver
) is new as of version 2.4. We do not offer
backward-compatible macros to use
the feature on older kernels.