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Re: ARM unaligned MMIO access with attribute((packed))
- From: Alan Stern <stern at rowland dot harvard dot edu>
- To: Rabin Vincent <rabin at rab dot in>
- Cc: Arnd Bergmann <arnd at arndb dot de>, <linux-arm-kernel at lists dot infradead dot org>, <linux-usb at vger dot kernel dot org>, Ulrich Weigand <Ulrich dot Weigand at de dot ibm dot com>, <linux-kernel at vger dot kernel dot org>, <gcc at gcc dot gnu dot org>, Peter Maydell <peter dot maydell at linaro dot org>
- Date: Wed, 27 Apr 2011 12:25:40 -0400 (EDT)
- Subject: Re: ARM unaligned MMIO access with attribute((packed))
On Wed, 27 Apr 2011, Rabin Vincent wrote:
> On Wed, Apr 27, 2011 at 00:21, Alan Stern <stern@rowland.harvard.edu> 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.
Right. I can understand the compiler complaining about the cast to
stricter alignment during the initialization. But I don't understand
why that would affect the code generated for the writel function.
Alan Stern