From: Arnd Bergmann <arnd@arndb.de>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Alexander Holler <holler@ahsoftware.de>,
gregkh@suse.de, Rabin Vincent <rabin@rab.in>,
linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removing the packed attribute
Date: Thu, 16 Jun 2011 19:55:50 +0200 [thread overview]
Message-ID: <201106161955.50972.arnd@arndb.de> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1106161308310.2204-100000@iolanthe.rowland.org>
On Thursday 16 June 2011, Alan Stern wrote:
>
> On Thu, 16 Jun 2011, Alexander Holler wrote:
>
> > In commit 139540170d9d9b7ead3caaf540f161756b356d56 the attribute
> > packed is removed from the structs which are used to access the EHCI-registers.
> >
> > This is done to circumvent a problem with gcc 4.6, which might access members of
> > packed structs on a byte by byte basis. But using packed, aligned(4) fixes that
> > too and is imho the better solution. Otherwise (without packed) the compiler would be free
> > to choose whatever alignment he thinks fits best, which might be e.g. 8-byte on 64-bit machines.
>
> Is that really true?
No.
> I thought the compiler was not allowed to insert
> padding if the natural alignment of the data types didn't require any.
It's architecture dependent. The alignment of the structure is the maximum alignment
of its members, so it gets to be 8 bytes if there is a 64 bit member in the struct
on most architectures, but 4 bytes on x86.
Arnd
WARNING: multiple messages have this Message-ID (diff)
From: arnd@arndb.de (Arnd Bergmann)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] USB: ehci: use packed, aligned(4) instead of removing the packed attribute
Date: Thu, 16 Jun 2011 19:55:50 +0200 [thread overview]
Message-ID: <201106161955.50972.arnd@arndb.de> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1106161308310.2204-100000@iolanthe.rowland.org>
On Thursday 16 June 2011, Alan Stern wrote:
>
> On Thu, 16 Jun 2011, Alexander Holler wrote:
>
> > In commit 139540170d9d9b7ead3caaf540f161756b356d56 the attribute
> > packed is removed from the structs which are used to access the EHCI-registers.
> >
> > This is done to circumvent a problem with gcc 4.6, which might access members of
> > packed structs on a byte by byte basis. But using packed, aligned(4) fixes that
> > too and is imho the better solution. Otherwise (without packed) the compiler would be free
> > to choose whatever alignment he thinks fits best, which might be e.g. 8-byte on 64-bit machines.
>
> Is that really true?
No.
> I thought the compiler was not allowed to insert
> padding if the natural alignment of the data types didn't require any.
It's architecture dependent. The alignment of the structure is the maximum alignment
of its members, so it gets to be 8 bytes if there is a 64 bit member in the struct
on most architectures, but 4 bytes on x86.
Arnd
next prev parent reply other threads:[~2011-06-16 17:56 UTC|newest]
Thread overview: 124+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-27 14:34 [PATCH] echi: remove structure packing from ehci_def Rabin Vincent
2011-04-27 14:34 ` Rabin Vincent
2011-04-27 15:15 ` Sergei Shtylyov
2011-04-27 15:15 ` Sergei Shtylyov
2011-04-27 15:37 ` [PATCHv2] " Rabin Vincent
2011-04-27 15:37 ` Rabin Vincent
2011-06-16 16:17 ` [PATCH] USB: ehci: use packed,aligned(4) instead of removing the packed attribute Alexander Holler
2011-06-16 16:17 ` [PATCH] USB: ehci: use packed, aligned(4) " Alexander Holler
2011-06-16 17:09 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-16 17:09 ` Alan Stern
2011-06-16 17:55 ` Arnd Bergmann [this message]
2011-06-16 17:55 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-16 19:25 ` [PATCH] USB: ehci: use packed,aligned(4) " Alexander Holler
2011-06-16 19:25 ` Alexander Holler
2011-06-16 19:46 ` Alan Stern
2011-06-16 19:46 ` Alan Stern
2011-06-16 20:10 ` Alexander Holler
2011-06-16 20:10 ` Alexander Holler
2011-06-16 20:20 ` Arnd Bergmann
2011-06-16 20:20 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-19 15:02 ` [PATCH] USB: ehci: use packed,aligned(4) " Nicolas Pitre
2011-06-19 15:02 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-19 19:00 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-19 19:00 ` Alan Stern
2011-06-19 20:02 ` Arnd Bergmann
2011-06-19 20:02 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-19 20:11 ` [PATCH] USB: ehci: use packed,aligned(4) " Arnd Bergmann
2011-06-19 20:11 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-19 21:39 ` [PATCH] USB: ehci: use packed,aligned(4) " Nicolas Pitre
2011-06-19 21:39 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-19 21:27 ` [PATCH] USB: ehci: use packed,aligned(4) " Nicolas Pitre
2011-06-19 21:27 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-20 15:03 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-20 15:03 ` Alan Stern
2011-06-20 16:16 ` Nicolas Pitre
2011-06-20 16:16 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-20 16:48 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-20 16:48 ` Alan Stern
2011-06-20 16:58 ` Arnd Bergmann
2011-06-20 16:58 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-20 19:02 ` Russell King - ARM Linux
2011-06-20 19:02 ` Russell King - ARM Linux
2011-06-20 19:20 ` Nicolas Pitre
2011-06-20 19:20 ` Nicolas Pitre
2011-06-20 19:29 ` Nicolas Pitre
2011-06-20 19:29 ` Nicolas Pitre
2011-06-20 17:10 ` [PATCH] USB: ehci: use packed,aligned(4) " Nicolas Pitre
2011-06-20 17:10 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-20 17:35 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-20 17:35 ` Alan Stern
2011-06-20 18:48 ` Russell King - ARM Linux
2011-06-20 18:48 ` Russell King - ARM Linux
2011-06-20 20:26 ` Arnd Bergmann
2011-06-20 20:26 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-20 20:50 ` Nicolas Pitre
2011-06-20 20:50 ` Nicolas Pitre
2011-06-20 20:55 ` [PATCH] USB: ehci: use packed,aligned(4) " Russell King - ARM Linux
2011-06-20 20:55 ` Russell King - ARM Linux
2011-06-20 21:23 ` Arnd Bergmann
2011-06-20 21:23 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-20 22:23 ` [PATCH] USB: ehci: use packed,aligned(4) " Nicolas Pitre
2011-06-20 22:23 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-21 11:25 ` [PATCH] USB: ehci: use packed,aligned(4) " Arnd Bergmann
2011-06-21 11:25 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-25 1:25 ` Nicolas Pitre
2011-06-25 8:09 ` Arnd Bergmann
2011-06-28 18:51 ` Nicolas Pitre
2011-06-29 10:56 ` Arnd Bergmann
2011-06-20 19:14 ` [PATCH] USB: ehci: use packed,aligned(4) " Nicolas Pitre
2011-06-20 19:14 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-20 19:32 ` Russell King - ARM Linux
2011-06-20 19:32 ` Russell King - ARM Linux
2011-06-20 20:14 ` Arnd Bergmann
2011-06-20 20:14 ` Arnd Bergmann
2011-06-20 20:42 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-20 20:42 ` Alan Stern
2011-06-20 22:36 ` Nicolas Pitre
2011-06-20 22:36 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-21 15:06 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-21 15:06 ` Alan Stern
2011-06-20 17:39 ` Alexander Holler
2011-06-20 17:39 ` Alexander Holler
2011-06-20 18:39 ` Alan Stern
2011-06-20 18:39 ` Alan Stern
2011-06-20 18:46 ` Alexander Holler
2011-06-20 18:46 ` Alexander Holler
2011-06-20 18:57 ` Alan Stern
2011-06-20 18:57 ` Alan Stern
2011-06-20 19:56 ` Nicolas Pitre
2011-06-20 19:56 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-20 21:04 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-20 21:04 ` Alan Stern
2011-06-20 22:31 ` Nicolas Pitre
2011-06-20 22:31 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-21 14:58 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-21 14:58 ` Alan Stern
2011-06-21 20:41 ` Nicolas Pitre
2011-06-21 20:41 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-22 6:23 ` [PATCH] USB: ehci: use packed,aligned(4) " Alexander Holler
2011-06-22 6:23 ` Alexander Holler
2011-06-20 20:09 ` Arnd Bergmann
2011-06-20 20:09 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-20 21:05 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-20 21:05 ` Alan Stern
2011-06-20 20:07 ` Arnd Bergmann
2011-06-20 20:07 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-20 20:28 ` [PATCH] USB: ehci: use packed,aligned(4) " Nicolas Pitre
2011-06-20 20:28 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-20 20:39 ` Arnd Bergmann
2011-06-20 20:39 ` Arnd Bergmann
2011-06-20 21:03 ` Nicolas Pitre
2011-06-20 21:03 ` Nicolas Pitre
2011-06-23 9:47 ` Alexander Holler
2011-06-23 9:47 ` Alexander Holler
2011-06-23 14:25 ` Alan Stern
2011-06-23 14:25 ` Alan Stern
2011-06-24 11:40 ` Alexander Holler
2011-06-24 11:40 ` Alexander Holler
2011-06-20 16:26 ` [PATCH] USB: ehci: use packed,aligned(4) " Arnd Bergmann
2011-06-20 16:26 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-16 20:30 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-16 20:30 ` Alan Stern
2011-06-16 18:16 ` Alexander Holler
2011-06-16 18:16 ` Alexander Holler
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