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From: Hidetoshi Seto <>
To: Linux Kernel list <>,
Cc: Greg KH <>,
	Grant Grundler <>,
	Andrew Morton <>,,,
	Kenji Kaneshige <>
Subject: [PATCH 1/5] Update Documentation/pci.txt
Date: Wed, 22 Nov 2006 17:05:54 +0900	[thread overview]
Message-ID: <> (raw)

This patch adds the description about legacy I/O port free driver into

Signed-off-by: Kenji Kaneshige <>
Signed-off-by: Grant Grundler <>
Signed-off-by: Hidetoshi Seto <>

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

Index: linux-2.6.19-rc6/Documentation/pci.txt
--- linux-2.6.19-rc6.orig/Documentation/pci.txt
+++ linux-2.6.19-rc6/Documentation/pci.txt
@@ -287,3 +287,73 @@
 pci_find_device()		Superseded by pci_get_device()
 pci_find_subsys()		Superseded by pci_get_subsys()
 pci_find_slot()			Superseded by pci_get_slot()
+10. 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.
+11. 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").

             reply	other threads:[~2006-11-22  8:05 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2006-11-22  8:05 Hidetoshi Seto [this message]
2006-11-22  8:54 ` Arjan van de Ven
2006-11-22 18:28 ` Grant Grundler
2006-11-24  0:38   ` Hidetoshi Seto
2006-11-24  5:12     ` Grant Grundler
2006-11-24  6:05       ` Hidetoshi Seto
2006-12-06  7:26       ` Greg KH
2006-12-07  3:55         ` Grant Grundler
2006-12-10  7:25         ` Grant Grundler
2006-12-15 17:02           ` Greg KH
2006-12-18  7:11             ` Grant Grundler
2006-12-22 19:46               ` Randy Dunlap
2006-12-22 21:52                 ` Stefan Richter
2006-12-24  6:11                 ` Grant Grundler
2006-12-24  6:07               ` [PATCH] Update Documentation/pci.txt v7 Grant Grundler
2006-12-24 19:16                 ` Randy Dunlap
2006-12-25  7:59                   ` Grant Grundler
2006-12-25  8:06                 ` Grant Grundler
2006-12-25  8:08                   ` Grant Grundler
2007-01-02 21:45                     ` Greg KH
2007-01-03  7:15                       ` Grant Grundler
2006-12-25  9:04                   ` Kenji Kaneshige
2007-01-16 22:26                   ` patch pci-rework-documentation-pci.txt.patch added to gregkh-2.6 tree gregkh
2007-01-17  9:10                     ` Jiri Slaby
2007-01-17 19:21                       ` Greg KH

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