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 22:26:37 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	Alan Stern <stern@...land.harvard.edu>,
	linux-usb@...r.kernel.org, Nicolas Pitre <nico@...xnic.net>,
	gregkh@...e.de, lkml <linux-kernel@...r.kernel.org>,
	Rabin Vincent <rabin@....in>,
	Alexander Holler <holler@...oftware.de>
Subject: Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removing the packed attribute

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
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