Re: [PATCH] Document Linux's memory barriers

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

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.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.15.4 on an i686 machine (5589.50 BogoMips).
Warning : 98.36% of all statistics are fiction, book release in April.
_


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