From: arnd@arndb.de (Arnd Bergmann)
To: linux-arm-kernel@lists.infradead.org
Subject: ARM unaligned MMIO access with attribute((packed))
Date: Thu, 28 Apr 2011 16:16:40 +0200 [thread overview]
Message-ID: <201104281616.40253.arnd@arndb.de> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1104280934080.2108-100000@iolanthe.rowland.org>
On Thursday 28 April 2011, Alan Stern wrote:
> > 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.
The behavior here is unspecified because the underlying typecase
is not valid. Gcc apparently uses some heuristics trying to do
the right thing, and in recent versions that heuristic seems to
have changed.
Arnd
WARNING: multiple messages have this Message-ID (diff)
From: Arnd Bergmann <arnd@arndb.de>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Rabin Vincent <rabin@rab.in>,
linux-arm-kernel@lists.infradead.org, linux-usb@vger.kernel.org,
Ulrich Weigand <Ulrich.Weigand@de.ibm.com>,
linux-kernel@vger.kernel.org, gcc@gcc.gnu.org,
Peter Maydell <peter.maydell@linaro.org>
Subject: Re: ARM unaligned MMIO access with attribute((packed))
Date: Thu, 28 Apr 2011 16:16:40 +0200 [thread overview]
Message-ID: <201104281616.40253.arnd@arndb.de> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1104280934080.2108-100000@iolanthe.rowland.org>
On Thursday 28 April 2011, Alan Stern wrote:
> > 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.
The behavior here is unspecified because the underlying typecase
is not valid. Gcc apparently uses some heuristics trying to do
the right thing, and in recent versions that heuristic seems to
have changed.
Arnd
next prev parent reply other threads:[~2011-04-28 14:16 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-02-02 16:00 ARM unaligned MMIO access with attribute((packed)) Arnd Bergmann
2011-02-02 16:00 ` Arnd Bergmann
2011-02-02 16:37 ` Russell King - ARM Linux
2011-02-02 16:37 ` Russell King - ARM Linux
2011-02-02 17:39 ` Arnd Bergmann
2011-02-02 17:39 ` Arnd Bergmann
2011-02-02 19:14 ` Ian Lance Taylor
2011-02-02 19:14 ` Ian Lance Taylor
2011-02-02 21:38 ` David Miller
2011-02-02 21:38 ` David Miller
2011-02-02 21:45 ` Russell King - ARM Linux
2011-02-02 21:45 ` Russell King - ARM Linux
2011-02-02 21:59 ` David Miller
2011-02-02 21:59 ` David Miller
2011-02-02 23:08 ` Måns Rullgård
2011-02-02 23:23 ` David Miller
2011-02-02 23:23 ` David Miller
2011-02-03 9:26 ` Arnd Bergmann
2011-02-03 9:26 ` Arnd Bergmann
2011-02-03 15:03 ` Arnd Bergmann
2011-02-03 15:03 ` Arnd Bergmann
2011-02-02 16:51 ` Richard Guenther
2011-02-02 16:51 ` Richard Guenther
2011-02-02 17:09 ` Russell King - ARM Linux
2011-02-02 17:09 ` Russell King - ARM Linux
2011-02-02 17:39 ` Joseph S. Myers
2011-02-02 17:39 ` Joseph S. Myers
2011-04-26 15:00 ` Rabin Vincent
2011-04-26 15:00 ` Rabin Vincent
2011-04-26 18:51 ` Alan Stern
2011-04-26 18:51 ` Alan Stern
2011-04-27 14:06 ` Rabin Vincent
2011-04-27 14:06 ` Rabin Vincent
2011-04-27 16:25 ` Alan Stern
2011-04-27 16:25 ` Alan Stern
2011-04-27 16:37 ` Arnd Bergmann
2011-04-27 16:37 ` Arnd Bergmann
2011-04-28 13:35 ` Alan Stern
2011-04-28 13:35 ` Alan Stern
2011-04-28 14:16 ` Arnd Bergmann [this message]
2011-04-28 14:16 ` Arnd Bergmann
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