linux-kernel - Re: [PATCH 10/12] unicore32 machine related files: pci bus handling

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Date:	Thu, 17 Feb 2011 18:19:52 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	"Guan Xuetao" <gxt@...c.pku.edu.cn>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	"'Greg KH'" <greg@...ah.com>
Subject: Re: [PATCH 10/12] unicore32 machine related files: pci bus handling

On Wednesday 16 February 2011, Guan Xuetao wrote:
> +{
> +       PCICFG_ADDR = CONFIG_CMD(bus, devfn, where);
> +       switch (size) {
> +       case 1:
> +               *value = (PCICFG_DATA >> ((where & 3) * 8)) & 0xFF;
> +               break;
> +       case 2:

It took me a while to figure out what this actually does. PCICFG_ADDR
and PCICFG_DATA are pointers to MMIO registers, which you should not
simply dereference. A lot of things can go wrong there, especially
if future machines move to weakly ordered I/O subsystem or have
multiple CPUs, but even for the simple case, the compiler has
a lot of ways to mess this up.

As explained in my reply to the "hardware registers" patch, all these
pointers should be marked as __iomem, so that sparse warns about
dangerous accesses such as the one here.

The code above should be written as

{
       writel(CONFIG_CMD(bus, devfn, where), PCICFG_ADDR);
       switch (size) {
       case 1:
               *value = readl(PCICFG_DATA >> ((where & 3) * 8)) & 0xFF;
               break;
       case 2:

	Arnd
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