Message-ID: <49B68C37.8010803@freescale.com>
Date: Tue, 10 Mar 2009 10:50:15 -0500
From: Timur Tabi <timur@freescale.com>
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To: Alan Cox <alan@lxorguk.ukuu.org.uk>
CC: linux-kernel@vger.kernel.org, rdreier@cisco.com, jirislaby@gmail.com,
       peterz@infradead.org, will.newton@gmail.com, hancockrwd@gmail.com,
       jeremy@goop.org
Subject: Re: [PATCH v4] introduce macro spin_event_timeout()
References: <1236699004-1863-1-git-send-email-timur@freescale.com> <20090310153537.5fd5d84d@lxorguk.ukuu.org.uk>
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Alan Cox wrote:

> NAK this - on a lot of platforms 1uS is the wrong timescale. Also we
> shouldn't be encouraging this kind of polling by making it very easy to
> write.

Well, I can agree that the time scale might be wrong on some platforms.
  The original version of spin_event_timeout() used jiffies, but some
people said that a jiffy is too long of a timescale, so I changed it to
udelay.

However, I disagree about the encouragement part.  Polling a register
until a status bit changes is a common task that cannot be handled any
other way.  If the status bit change does not generate an interrupt, but
the wait for the change takes a few microseconds, what else are you
going to do?

The way I see it, I'm just extending the idea behind cpu_relax().  Just
doing a search for cpu_relax shows dozens, maybe hundreds, of drivers
doing stuff like this:

	while((inb(ioaddr+DAYNA_CARD_STATUS)&DAYNA_TX_READY)==0)
		cpu_relax();

This code doesn't even have a timeout!  In fact, I'd say that at least
90% of all uses of cpu_relax() are in a while loop reading some register
without a timeout.

Ironically, in the situations where there is a timeout, the drivers use
jiffies, not a delay.

Frankly, I just don't see how spin_event_timeout() is not an improvement
 over the current code that drivers use.

-- 
Timur Tabi
Linux kernel developer at Freescale
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