linux-kernel - Re: [PATCH v4] introduce macro spin_event

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Date:	Tue, 10 Mar 2009 10:50:15 -0500
From:	Timur Tabi <timur@...escale.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	linux-kernel@...r.kernel.org, rdreier@...co.com,
	jirislaby@...il.com, peterz@...radead.org, will.newton@...il.com,
	hancockrwd@...il.com, jeremy@...p.org
Subject: Re: [PATCH v4] introduce macro spin_event_timeout()

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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