[PATCH] Document Linux's memory barriers

linux-os (Dick Johnson) linux-os at analogic.com
Wed Mar 8 06:15:42 EST 2006

On Tue, 7 Mar 2006, Matthew Wilcox wrote:

> On Tue, Mar 07, 2006 at 01:54:33PM -0500, linux-os (Dick Johnson) wrote:
>> This might be a good place to document:
>>     dummy = readl(&foodev->ctrl);
>> Will flush all pending writes to the PCI bus and that:
>>     (void) readl(&foodev->ctrl);
>> ... won't because `gcc` may optimize it away. In fact, variable
>> "dummy" should be global or `gcc` may make it go away as well.
> static inline unsigned int readl(const volatile void __iomem *addr)
> {
> 	return *(volatile unsigned int __force *) addr;
> }
> The cast is volatile, so gcc knows not to optimise it away.

When the assignment is not made a.k.a., cast to void, or when the
assignment is made to an otherwise unused variable, `gcc` does,
indeed make it go away. These problems caused weeks of chagrin
after it was found that a PCI DMA operation took 20 or more times
than it should. The writel(START_DMA, &control), followed by
a dummy = readl(&control), ended up with the readl() missing.
That meant that the DMA didn't start until some timer code
read a status register, wondering why it hadn't completed yet.

Dick Johnson
Penguin : Linux version on an i686 machine (5589.50 BogoMips).
Warning : 98.36% of all statistics are fiction, book release in April.

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