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SubjectRe: [PATCH 2/4] PCI legacy I/O port free driver (take 3) - Update Documentation/pci.txt
On Mon, Feb 27, 2006 at 01:52:59PM +0900, Kenji Kaneshige wrote:
> This patch adds the description about legacy I/O port free driver into
> Documentation/pci.txt.

Since you've done all the hard work so far, my small contribution
is to rewrite the pci.txt so it's easier to read.
Does this look better to you?

The new section on "Write Posting" is just a proposal (RFC).
Write Posting has been discussed on most linux driver mailing lists
but I didn't see it mentioned in Documentation/pci.txt
(which seems like a natural place to discuss it).


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, set the no_ioport flag in struct pci_dev before calling
pci_enable_device() and pci_request_regions(). If the no_ioport flag
is set, generic PCI support will ignore 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

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 */

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 */

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").

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