Re: ARM unaligned MMIO access with attribute((packed))

[Rabin Vincent]

On Wed, Feb 2, 2011 at 21:30, Arnd Bergmann  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:

http://www.mail-archive.com/u-boot@lists.denx.de/msg51418.html

> 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
strbs:

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

Re: ARM unaligned MMIO access with attribute((packed))

[Rabin Vincent]

On Wed, Apr 27, 2011 at 00:21, Alan Stern  wrote:
> On Tue, 26 Apr 2011, Rabin Vincent wrote:
>> In my case it's this writel() in ehci-hub.c that gets chopped into
>> strbs:
>>
>>       /* force reset to complete */
>>       ehci_writel(ehci, temp & ~(PORT_RWC_BITS | PORT_RESET),
>>                               status_reg);
>
> Why would that get messed up?  The status_reg variable doesn't have any
> __atribute__((packed)) associated with it.

The initialization of status_reg is:

u32 __iomem *status_reg
= &ehci->regs->port_status[(wIndex & 0xff) - 1];

where ehci->regs is a pointer to the packed struct ehci_regs.  So, this
is the same problem of casting pointers to stricter alignment.