linux-kernel - Re: [PATCH] Update Documentation/pci.txt v7

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Date:	Sun, 24 Dec 2006 11:16:22 -0800
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Grant Grundler <grundler@...isc-linux.org>
Cc:	Andrew Morton <akpm@...l.org>, Greg KH <greg@...ah.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-pci@...ey.karlin.mff.cuni.cz,
	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>
Subject: Re: [PATCH] Update Documentation/pci.txt v7

On Sat, 23 Dec 2006 23:07:26 -0700 Grant Grundler wrote:

> +10. Legacy I/O port free driver
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

That subject (and patches with similar subject) confuses me.
It's difficult to associate the adjectives correctly.
I suppose it just needs some hyphens/dashes, like:

10. Legacy-I/O-port-free driver

but that's ETOOMUCH.  Maybe ?

10.  Stop using legacy I/O space

> +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_region() will fail when
> +attempting to enable I/O Port regions that don't have I/O Port
> +resources assigned.

> +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. Many drivers (e.g. tg3, acenic, sym53c8xx_2)
> +already do this. I/O Port space guarantees write transactions reach the PCI
> +device before the CPU can continue. Writes to MMIO space allow the CPU
> +to 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 its 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) {

Please use:	i = 8;
to match CodingStyle.  (and below)

> +               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);
> +       }

Rest looks good to me.

---
~Randy
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