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: Mon, 20 Jun 2011 22:26:37 +0200 [thread overview]
Message-ID: <201106202226.37381.arnd@arndb.de> (raw)
In-Reply-To: <20110620184849.GI26089@n2100.arm.linux.org.uk>
On Monday 20 June 2011 20:48:49 Russell King - ARM Linux wrote:
> On Mon, Jun 20, 2011 at 01:35:35PM -0400, Alan Stern wrote:
> > According to Arnd, any remaining possible issues will be addressed by
> > changing the implementation of readl/writel on ARM. It doesn't look as
> > though the ehci files need anything else done.
>
> I'm not about to change their implementation because they've proven
> themselves over the last 10 years to be perfectly fine, and changing
> them has a habbit of causing GCC to play less optimally than it should
> do.
Well, we do know that gcc now makes different tradeoffs, and that it's
entirely within the C99 specification when it's generating byte accesses
from __raw_readl(). The case where the pointer is __packed is just the
obvious case where it would do that, and I fully agree that the __packed
in that case is a bug, but I'm much in favor of writing code so that
we instruct the compiler to create correct code rather than giving it
the choice between correct and incorrect.
> I've seen drivers where GCC reloads the base address from the driver
> private data structure each time a register access is performed, rather
> than caching the base address in a register. I've seen it issuing
> separate add instructions and using a zero pre-index load/store. The
> existing way is the only way I've found to get GCC to come anywhere
> close to producing "optimal" code for the IO accessors.
Two points here:
* What's the olders compiler that we really need to be able to build
efficient kernels? Would you consider it if we can show that gcc-4.2
and higher produce code that is as good as the existing macros?
How about making the code gcc version dependent?
* We already need a compiler barrier in the non-_relaxed() versions of
the I/O accessors, which will force a reload of the base address
in a lot of cases, so the code is already suboptimal. Yes, we don't
have the barrier today without CONFIG_ARM_DMA_MEM_BUFFERABLE, but that
is a bug, because it lets the compiler move accesses to DMA buffers
around readl/writel.
> 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?
Arnd
next prev parent reply other threads:[~2011-06-20 20:26 UTC|newest]
Thread overview: 64+ 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 15:15 ` Sergei Shtylyov
2011-04-27 15:37 ` [PATCHv2] " 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 17:09 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
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 19:46 ` Alan Stern
2011-06-16 20:10 ` Alexander Holler
2011-06-16 20:20 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-19 15:02 ` Nicolas Pitre
2011-06-19 19:00 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-19 20:02 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-19 20:11 ` Arnd Bergmann
2011-06-19 21:39 ` Nicolas Pitre
2011-06-19 21:27 ` Nicolas Pitre
2011-06-20 15:03 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
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 16:58 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-20 19:02 ` Russell King - ARM Linux
2011-06-20 19:20 ` Nicolas Pitre
2011-06-20 19:29 ` Nicolas Pitre
2011-06-20 17:10 ` Nicolas Pitre
2011-06-20 17:35 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-20 18:48 ` Russell King - ARM Linux
2011-06-20 20:26 ` Arnd Bergmann [this message]
2011-06-20 20:50 ` [PATCH] USB: ehci: use packed, aligned(4) " Nicolas Pitre
2011-06-20 20:55 ` [PATCH] USB: ehci: use packed,aligned(4) " Russell King - ARM Linux
2011-06-20 21:23 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-20 22:23 ` Nicolas Pitre
2011-06-21 11:25 ` 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 ` Nicolas Pitre
2011-06-20 19:32 ` Russell King - ARM Linux
2011-06-20 20:14 ` Arnd Bergmann
2011-06-20 20:42 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
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
2011-06-20 17:39 ` Alexander Holler
2011-06-20 18:39 ` Alan Stern
2011-06-20 18:46 ` Alexander Holler
2011-06-20 18:57 ` Alan Stern
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 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 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
2011-06-20 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 20:07 ` [PATCH] USB: ehci: use packed, aligned(4) " Arnd Bergmann
2011-06-20 20:28 ` Nicolas Pitre
2011-06-20 20:39 ` Arnd Bergmann
2011-06-20 21:03 ` Nicolas Pitre
2011-06-23 9:47 ` Alexander Holler
2011-06-23 14:25 ` Alan Stern
2011-06-24 11:40 ` Alexander Holler
2011-06-20 16:26 ` Arnd Bergmann
2011-06-16 20:30 ` [PATCH] USB: ehci: use packed,aligned(4) " Alan Stern
2011-06-16 18:16 ` Alexander Holler
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