linux-kernel - Re: [RFC PATCH] x86: Add safe_udelay() and safe

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Date:	Thu, 13 Jan 2011 17:12:37 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Greg KH <greg@...ah.com>
CC:	Jason Wessel <jason.wessel@...driver.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [RFC PATCH] x86: Add safe_udelay() and safe_msleep()

On 01/13/2011 04:44 PM, Greg KH wrote:
> On Thu, Jan 13, 2011 at 04:31:04PM -0800, Yinghai Lu wrote:
>> there are some udelay() in drivers/usb/early/ehci-dbgp.c
>>
>> that is for early USB debug port console support.
>>
>> We should replace them with early version udelay()
> 
> Again, I fail to see why.  They work fine today, right?

did not test that for a while.

from code review, we should not use udelay() that early.

in arch/x86/lib/delay.c we have
inline void __const_udelay(unsigned long xloops)
{
        int d0;

        xloops *= 4;
        asm("mull %%edx"
                :"=d" (xloops), "=&a" (d0)
                :"1" (xloops), "0"
                (this_cpu_read(cpu_info.loops_per_jiffy) * (HZ/4)));

        __delay(++xloops);
}
EXPORT_SYMBOL(__const_udelay);

that percpu cpu_info.loops_per_jiffy only have value after smp_prepare_cpus() is called.

> 
> You _really_ need to prove what you are trying to do here, with lots of
> details and descriptions of what the problem is, as it's getting a bit
> annoying...

sorry for that.

Yinghai
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