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Date: Fri, 01 May 2009 16:44:19 -0500
From: Timur Tabi <timur@freescale.com>
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To: Sean MacLennan <smaclennan@pikatech.com>
Subject: Re: [PATCH v6] introduce macro spin_event_timeout()
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Sean MacLennan wrote:

> Would cpu_relax be a good thing to put here?
> 
> Something like:
> 
> while (!(rc = (condition)) && (tb_ticks_since(__start) <= __loops)) \
> 	if (delay) \
> 		udelay(delay); \
> 	else \
> 		cpu_relax(); \
> 

I had that at one point, but then I looked at the code for udelay(), and
it appears that if I do udelay(0), it does something similar to cpu_relax:

	start = get_tbl();
	while (get_tbl() - start < loops)
		HMT_low();
	HMT_medium();

cpu_relax does this:

do { HMT_low(); HMT_medium(); barrier(); } while (0)

Well, now that I look at it, cpu_relax() changes the thread priority to
low and then back to medium, whereas udelay() never sets it to low if
'loops' is 0.

I'm okay with changing my code, but I wonder if udelay() should look
like this:

	start = get_tbl();
	HMT_low();
	while (get_tbl() - start < loops)
		HMT_low();
	HMT_medium();

-- 
Timur Tabi
Linux kernel developer at Freescale