Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removing the packed attribute
From: Arnd Bergmann
Date: Mon Jun 20 2011 - 16:27:08 EST
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
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
> 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?
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