linux-kernel - Re: [PATCH] USB: ehci: use packed, aligned(4) instead of removing the packed attribute

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Date:	Mon, 20 Jun 2011 16:50:34 -0400 (EDT)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org, gregkh@...e.de,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-usb@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
	Rabin Vincent <rabin@....in>,
	Alan Stern <stern@...land.harvard.edu>,
	Alexander Holler <holler@...oftware.de>
Subject: Re: [PATCH] USB: ehci: use packed,
 aligned(4) instead of removing the packed attribute

On Mon, 20 Jun 2011, Arnd Bergmann wrote:

> On Monday 20 June 2011 20:48:49 Russell King - ARM Linux wrote:
> > If it is the case that these structures do not require packing to get
> > their desired layout, then they don't require packing, and the packed
> > attribute should be dropped.
> 
> Yes. But are you going to audit every other use of __packed in the kernel
> to check if it is used on __iomem pointers?

The compiler might tell us about it:

diff --git a/arch/arm/include/asm/io.h b/arch/arm/include/asm/io.h
index d66605d..10c47e8 100644
--- a/arch/arm/include/asm/io.h
+++ b/arch/arm/include/asm/io.h
@@ -49,11 +49,11 @@ extern void __raw_readsl(const void __iomem *addr, void *data, int longlen);
 
 #define __raw_writeb(v,a)	(__chk_io_ptr(a), *(volatile unsigned char __force  *)(a) = (v))
 #define __raw_writew(v,a)	(__chk_io_ptr(a), *(volatile unsigned short __force *)(a) = (v))
-#define __raw_writel(v,a)	(__chk_io_ptr(a), *(volatile unsigned int __force   *)(a) = (v))
+#define __raw_writel(v,a)	(__chk_io_ptr(a), BUILD_BUG_ON_ZERO(__alignof(*(int *)a) != 4), *(volatile unsigned int __force   *)(a) = (v))
 
 #define __raw_readb(a)		(__chk_io_ptr(a), *(volatile unsigned char __force  *)(a))
 #define __raw_readw(a)		(__chk_io_ptr(a), *(volatile unsigned short __force *)(a))
-#define __raw_readl(a)		(__chk_io_ptr(a), *(volatile unsigned int __force   *)(a))
+#define __raw_readl(a)		(__chk_io_ptr(a), BUILD_BUG_ON_ZERO(__alignof(*(int *)a) != 4), *(volatile unsigned int __force   *)(a))
 
 /*
  * Architecture ioremap implementation.

And similar for readh/writeh, given that your GCC version is preserving 
the alignment attribute across the cast of course.

[...]

Scratch that.  The alignment of a void pointer dereference is 1.


Nicolas
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