[PATCH 2/4] PCI legacy I/O port free driver (take 3) - Update Documentation/pci.txt

[Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>]

This patch adds the description about legacy I/O port free driver into
Documentation/pci.txt.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>

 Documentation/pci.txt |   28 ++++++++++++++++++++++++++++
 1 files changed, 28 insertions(+)

Index: linux-2.6.16-rc4/Documentation/pci.txt
===================================================================
--- linux-2.6.16-rc4.orig/Documentation/pci.txt	2006-02-27 13:29:34.000000000 +0900
+++ linux-2.6.16-rc4/Documentation/pci.txt	2006-02-27 13:29:42.000000000 +0900
@@ -269,3 +269,31 @@
 pci_find_device()		Superseded by pci_get_device()
 pci_find_subsys()		Superseded by pci_get_subsys()
 pci_find_slot()			Superseded by pci_get_slot()
+
+
+9. Legacy I/O port free driver
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+On the large servers, I/O port resources could not be assigned to all
+PCI devices because it is limited (64KB on Intel Architecture[1]) and
+it would be fragmented (I/O base register of PCI-to-PCI bridge will
+usually be aligned to a 4KB boundary[2]). On such systems,
+pci_enable_device() and pci_request_regions() for those devices will
+fail because those functions try to enable all the regions. However,
+it is a problem for some PCI devices which provide both I/O port and
+MMIO interface because some of them can be handled without using I/O
+port interface. The reason why such devices provide I/O port interface
+is for compatibility to legacy OSs. So this kind of devices should
+work even if enough I/O port resources are not assigned. The "PCI
+Local Bus Specification Revision 3.0" also mentions about this topic
+(Please see p.44, "IMPLEMENTATION NOTE").
+
+This problem is solved by telling the kernel if your driver needs to
+use I/O port to handle the device. If your driver doesn't need any I/O
+port regions to handle the device, you can tell it to the kernel by
+setting the no_ioport flag in struct pci_dev. If the no_ioport flag is
+set, kernel will never touch I/O port regions for the corresponding
+devices. Please note that you need to set the no_ioport flag before
+calling pci_enable_device() and pci_request_regions().
+
+[1] Some systems support 64KB I/O port space per PCI segment.
+[2] Some PCI-to-PCI bridges support optional 1KB aligned I/O base.