lkml.org 
[lkml]   [2011]   [Apr]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: ARM unaligned MMIO access with attribute((packed))
On Wed, 27 Apr 2011, Arnd Bergmann wrote:

> 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".

Ah -- so the compiler associates the alignment attribute with the data
value and not with the variable's type? I didn't know that.

Alan Stern

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2011-04-28 15:37    [W:0.066 / U:0.184 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site