netdev - Re: New driver "sfc" for Solarstorm SFC4000 controller.

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Message-Id: <20080506072442.82a834c8.akpm@linux-foundation.org>
Date:	Tue, 6 May 2008 07:24:42 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	netdev@...r.kernel.org, linux-net-drivers@...arflare.com
Subject: Re: New driver "sfc" for Solarstorm SFC4000 controller.

On Tue, 6 May 2008 13:50:09 +0100 Ben Hutchings <bhutchings@...arflare.com> wrote:

> Andrew Morton wrote:
> > On Fri, 2 May 2008 17:05:35 +0100 Ben Hutchings <bhutchings@...arflare.com> wrote:
> > 
> > > > Oh dear, it found
> > > > 
> > > > #5617: FILE: drivers/net/sfc/falcon.c:1877:
> > > > +               if (*(volatile u32 *)dma_done == FALCON_STATS_DONE)
> > > > 
> > > > which was naughty of you.  Perhaps this was already discussed in review
> > > > with the people who actually know what they're talking about.
> > > 
> > > There wasn't any specific discussion of this.  Is it wrong?  We want to
> > > prevent the compiler from caching *dma_done, which is itself written by DMA.
> > 
> > Documentation/volatile-considered-harmful.txt has some dicussion.
> 
> Looks like we should be using something like:
> 
> 	while (*dma_done != FALCON_STATS_DONE)
> 		cpu_relax();
> 
> But then how do we time-out?

Dunno, something like

 	while (*dma_done != FALCON_STATS_DONE) {
		if (time_after(jiffies, when_to_timeout))
			goto timeout;
 		cpu_relax();
	}

or

 	while (*dma_done != FALCON_STATS_DONE) {
		udelay(1);
		if (++timeout > TIMEOUT)
			goto timeout;
	}

?
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