ARM unaligned MMIO access with attribute((packed))

[arnd@arndb.de (Arnd Bergmann)]

On Wednesday 27 April 2011 18:25:40 Alan Stern wrote:
> 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.

The compiler does not complain, it just silently assumes that it needs
to do byte accesses. There is no way to tell the compiler to ignore
what it knows about the alignment, other than using inline assembly
for the actual pointer dereference. Most architectures today do that,
but on ARM it comes down to "*(u32 *)status_reg = temp".

	Arnd