[PATCH 2/4] 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>
Signed-off-by: Grant Grundler <grundler@parisc-linux.org>

---
 Documentation/pci.txt |   70 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 70 insertions(+)

Index: linux-2.6.17-rc6/Documentation/pci.txt
===================================================================
--- linux-2.6.17-rc6.orig/Documentation/pci.txt	2006-06-06 21:39:11.000000000 +0900
+++ linux-2.6.17-rc6/Documentation/pci.txt	2006-06-06 21:56:32.000000000 +0900
@@ -279,3 +279,73 @@
 pci_find_device()		Superseded by pci_get_device()
 pci_find_subsys()		Superseded by pci_get_subsys()
 pci_find_slot()			Superseded by pci_get_slot()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
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Large servers may not be able to provide I/O port resources to all PCI
+devices. I/O Port space is only 64KB on Intel Architecture[1] and is
+likely also fragmented since the 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() will fail when
+attempting to enable I/O Port regions that don't have I/O Port
+resources assigned.
+
+Fortunately, many PCI devices which request I/O Port resources also
+provide access to the same registers via MMIO BARs. These devices can
+be handled without using I/O port space and the drivers typically
+offer a CONFIG_ option to only use MMIO regions
+(e.g. CONFIG_TULIP_MMIO). PCI devices typically provide I/O port
+interface for legacy OSs and will work when I/O port resources are not
+assigned. The "PCI Local Bus Specification Revision 3.0" discusses
+this on p.44, "IMPLEMENTATION NOTE".
+
+If your PCI device driver doesn't need I/O port resources assigned to
+I/O Port BARs, you should use pci_enable_device_bars() instead of
+pci_enable_device() in order not to enable I/O port regions for the
+corresponding devices. In addition, you should use
+pci_request_selected_regions()/pci_release_selected_regions() instead
+of pci_request_regions()/pci_release_regions() in order not to
+request/release I/O port regions for the corresponding devices.
+
+[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.
+
+
+10. MMIO Space and "Write Posting"
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Converting a driver from using I/O Port space to using MMIO space
+often requires some additional changes. Specifically, "write posting"
+needs to be handled. Most drivers (e.g. tg3, acenic, sym53c8xx_2)
+already do. I/O Port space guarantees write transactions reach the PCI
+device before the CPU can continue. Writes to MMIO space allow to CPU
+continue before the transaction reaches the PCI device. HW weenies
+call this "Write Posting" because the write completion is "posted" to
+the CPU before the transaction has reached it's destination.
+
+Thus, timing sensitive code should add readl() where the CPU is
+expected to wait before doing other work.  The classic "bit banging"
+sequence works fine for I/O Port space:
+
+	for (i=8; --i; val >>= 1) {
+		outb(val & 1, ioport_reg);	/* write bit */
+		udelay(10);
+	}
+
+The same sequence for MMIO space should be:
+
+	for (i=8; --i; val >>= 1) {
+		writeb(val & 1, mmio_reg);	/* write bit */
+		readb(safe_mmio_reg);		/* flush posted write */
+		udelay(10);
+	}
+
+It is important that "safe_mmio_reg" not have any side effects that
+interferes with the correct operation of the device.
+
+Another case to watch out for is when resetting a PCI device. Use PCI
+Configuration space reads to flush the writel(). This will gracefully
+handle the PCI master abort on all platforms if the PCI device is
+expected to not respond to a readl().  Most x86 platforms will allow
+MMIO reads to master abort (aka "Soft Fail") and return garbage
+(e.g. ~0). But many RISC platforms will crash (aka "Hard Fail").