From: Nicolas Pitre <nico@fluxnic.net> To: Alan Stern <stern@rowland.harvard.edu> Cc: Alexander Holler <holler@ahsoftware.de>, Arnd Bergmann <arnd@arndb.de>, linux-arm-kernel@lists.infradead.org, linux-usb@vger.kernel.org, gregkh@suse.de, lkml <linux-kernel@vger.kernel.org>, Rabin Vincent <rabin@rab.in> Subject: Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removing the packed attribute Date: Mon, 20 Jun 2011 18:31:13 -0400 (EDT) [thread overview] Message-ID: <alpine.LFD.2.00.1106201824320.2142@xanadu.home> (raw) In-Reply-To: <Pine.LNX.4.44L0.1106201700500.2113-100000@iolanthe.rowland.org> On Mon, 20 Jun 2011, Alan Stern wrote: > On Mon, 20 Jun 2011, Nicolas Pitre wrote: > > > On Mon, 20 Jun 2011, Alan Stern wrote: > > > > > On Mon, 20 Jun 2011, Alexander Holler wrote: > > > > > > > I see it that way: packed is needed to be sure that at least for struct > > > > ehci_regs there are no padding bytes inbetween the members. > > > > > > But is it _really_ needed? > > > > > > > It might > > > > work without, but that depends on the compiler (-version, architecture, > > > > whatever). > > > > > > Have there _ever_ been _any_ combinations of compiler, version, > > > architecture, whatever, that had unwanted padding bytes in this > > > structure? > > > > This can be determined by simple code inspection. > > > > If you must have struct members which are not aligned to their natural > > size then you need __packed. Example: > > > > struct foo { > > u8 a; > > u16 b; > > u32 c; > > u64 d; > > }; > > > > Without __packed, there will be padding between a and b, and between c > > and d. > > One byte of padding between a and b is enough. No more is needed, and > the compiler would have to be pretty stupid to add anything else. Obviously, my mistake. I meant to make c a u16 too but failed to correct the example before posting. > > If the order of the members in this struct were reversed, then > > everything would be naturally aligned and no padding between members > > would be inserted. > > > > The size of structures is normally rounded up with padding to the size > > of the largest basic element it contains. Example: > > > > struct foo { > > u64 a; > > u8 b; > > }; > > > > Here sizeof(struct foo) would return 16, even if the actual content > > occupies 9 bytes only. That's because the largest basic element is u64 > > i.e. 8 bytes. Normally this trailing padding is not an issue, unless > > you have an array of such a struct or if it is a member of another > > struct. If you want to get rid of that padding, you need to use > > __packed again (which of course would make all subsequent instances of > > that structure in your array completely misaligned too). > > > > Two odd exceptions with the old ABI on ARM: > > > > - The alignment of a 64-bit value is always 4 bytes not 8. > > > > - The size of all structures are always rounded up to a 4-byte boundary, > > irrespective of their content. > > > > If you fall into none of the above issues, then you don't need any > > __packed, period. > > We don't fall into any of these cases, and therefore as you say, we > don't need packed. Arnd and I have both explained this. So why do you > keep arguing that we do need it? Please show me where I keep arguing that you need it? Nicolas
WARNING: multiple messages have this Message-ID (diff)
From: nico@fluxnic.net (Nicolas Pitre) To: linux-arm-kernel@lists.infradead.org Subject: [PATCH] USB: ehci: use packed, aligned(4) instead of removing the packed attribute Date: Mon, 20 Jun 2011 18:31:13 -0400 (EDT) [thread overview] Message-ID: <alpine.LFD.2.00.1106201824320.2142@xanadu.home> (raw) In-Reply-To: <Pine.LNX.4.44L0.1106201700500.2113-100000@iolanthe.rowland.org> On Mon, 20 Jun 2011, Alan Stern wrote: > On Mon, 20 Jun 2011, Nicolas Pitre wrote: > > > On Mon, 20 Jun 2011, Alan Stern wrote: > > > > > On Mon, 20 Jun 2011, Alexander Holler wrote: > > > > > > > I see it that way: packed is needed to be sure that at least for struct > > > > ehci_regs there are no padding bytes inbetween the members. > > > > > > But is it _really_ needed? > > > > > > > It might > > > > work without, but that depends on the compiler (-version, architecture, > > > > whatever). > > > > > > Have there _ever_ been _any_ combinations of compiler, version, > > > architecture, whatever, that had unwanted padding bytes in this > > > structure? > > > > This can be determined by simple code inspection. > > > > If you must have struct members which are not aligned to their natural > > size then you need __packed. Example: > > > > struct foo { > > u8 a; > > u16 b; > > u32 c; > > u64 d; > > }; > > > > Without __packed, there will be padding between a and b, and between c > > and d. > > One byte of padding between a and b is enough. No more is needed, and > the compiler would have to be pretty stupid to add anything else. Obviously, my mistake. I meant to make c a u16 too but failed to correct the example before posting. > > If the order of the members in this struct were reversed, then > > everything would be naturally aligned and no padding between members > > would be inserted. > > > > The size of structures is normally rounded up with padding to the size > > of the largest basic element it contains. Example: > > > > struct foo { > > u64 a; > > u8 b; > > }; > > > > Here sizeof(struct foo) would return 16, even if the actual content > > occupies 9 bytes only. That's because the largest basic element is u64 > > i.e. 8 bytes. Normally this trailing padding is not an issue, unless > > you have an array of such a struct or if it is a member of another > > struct. If you want to get rid of that padding, you need to use > > __packed again (which of course would make all subsequent instances of > > that structure in your array completely misaligned too). > > > > Two odd exceptions with the old ABI on ARM: > > > > - The alignment of a 64-bit value is always 4 bytes not 8. > > > > - The size of all structures are always rounded up to a 4-byte boundary, > > irrespective of their content. > > > > If you fall into none of the above issues, then you don't need any > > __packed, period. > > We don't fall into any of these cases, and therefore as you say, we > don't need packed. Arnd and I have both explained this. So why do you > keep arguing that we do need it? Please show me where I keep arguing that you need it? Nicolas
next prev parent reply other threads:[~2011-06-20 22:31 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 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 [this message] 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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