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SubjectRe: [PATCH 07/10] IOCHK interface for I/O error handling/detecting
>>>>> On Fri, 10 Jun 2005 19:29:58 +0900, Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com> said:

Hidetoshi> Hi David,
Hidetoshi> David Mosberger wrote:
>>>>>>> On Thu, 09 Jun 2005 21:58:26 +0900, Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com> said:
>>
Hidetoshi> +/*
Hidetoshi> + * Some I/O bridges may poison the data read, instead of
Hidetoshi> + * signaling a BERR. The consummation of poisoned data
Hidetoshi> + * triggers a local, imprecise MCA.
Hidetoshi> + * Note that the read operation by itself does not consume
Hidetoshi> + * the bad data, you have to do something with it, e.g.:
Hidetoshi> + *
Hidetoshi> + * ld.8 r9=[r10];; // r10 == I/O address
Hidetoshi> + * add.8 r8=r9,r9;; // fake operation
Hidetoshi> + */
Hidetoshi> +#define ia64_poison_check(val) \
Hidetoshi> +{ register unsigned long gr8 asm("r8"); \
Hidetoshi> + asm volatile ("add %0=%1,r0" : "=r"(gr8) : "r"(val)); }
Hidetoshi> +
Hidetoshi> #endif /* CONFIG_IOMAP_CHECK */

>> I have only looked that this briefly and I didn't see off hand where you get
>> the "r9=[r10]" sequence from --- I hope you're not relying on the compiler
>> happening to generate this sequence!

Hidetoshi> +static inline unsigned char
Hidetoshi> +___ia64_readb (const volatile void __iomem *addr)
Hidetoshi> +{
Hidetoshi> + unsigned char val;
Hidetoshi> +
Hidetoshi> + val = *(volatile unsigned char __force *)addr;
Hidetoshi> + ia64_poison_check(val);
Hidetoshi> +
Hidetoshi> + return val;
Hidetoshi> +}

Ah, I see now what you're trying to do. I think it's really a
machine-check barrier that you want there.

I'm doubtful whether this is the right approach, though: your
ia64_poison_check() will cause _every single_ readX() operation to
stall the CPU for 1,000+ cycles. Why not define an explicit
iochk_barrier() instead? Then you could do things like this:

a = readb(X);
b = readb(Y);
c = readb(Z);
iochk_barrier(a + b + c);

That is, if it's unimportant to know whether the read of X, Y, or Z
caused the MCA, you can amortize the cost of iochk_barrier() over 3
reads.

I'd probably make iochk_barrier() an out-of-line no-op assembly
routine. The cost of two branches compared to stalling for hundreds
of cycles is rather trivial.

--david
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