ARM unaligned MMIO access with attribute((packed))

Rabin Vincent rabin at
Tue Apr 26 11:00:51 EDT 2011

On Wed, Feb 2, 2011 at 21:30, Arnd Bergmann <arnd at> wrote:
> As noticed by Peter Maydell, the EHCI device driver in Linux gets
> miscompiled by some versions of arm-gcc (still need to find out which)
> due to a combination of problems:
> 1. In include/linux/usb/ehci_def.h, struct ehci_caps is defined
> with __attribute__((packed)), for no good reason. This is clearly
> a bug and needs to get fixed, but other drivers have the same bug

Was a patch submitted for this?  I couldn't find it in the archives.
U-Boot seems to be fixing this by adding an "aligned(4)" instead
of removing the packed:

> and it used to work. The attribute forces byte access on all members
> accessed through pointer dereference, which is not allowed on
> MMIO accesses in general. The specific code triggering the problem
> in Peter's case is in ehci-omap.c:
>        omap->ehci->regs = hcd->regs
>                + HC_LENGTH(readl(&omap->ehci->caps->hc_capbase));

In my case it's this writel() in ehci-hub.c that gets chopped into

	/* force reset to complete */
	ehci_writel(ehci, temp & ~(PORT_RWC_BITS | PORT_RESET),

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