linux-kernel - Re: ARM unaligned MMIO access with attribute((packed))

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Date:	Wed, 27 Apr 2011 12:25:40 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Rabin Vincent <rabin@....in>
cc:	Arnd Bergmann <arnd@...db.de>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-usb@...r.kernel.org>,
	Ulrich Weigand <Ulrich.Weigand@...ibm.com>,
	<linux-kernel@...r.kernel.org>, <gcc@....gnu.org>,
	Peter Maydell <peter.maydell@...aro.org>
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@...land.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

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