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Date: Fri, 12 May 2006 08:23:40 -0700
From: Andrew Morton <akpm@osdl.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: mingo@elte.hu, markh@compro.net, linux-kernel@vger.kernel.org,
       dwalker@mvista.com, tglx@linutronix.de
Subject: Re: 3c59x vortex_timer rt hack (was: rt20 patch question)
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Steven Rostedt <rostedt@goodmis.org> wrote:
>
> 
> On Fri, 12 May 2006, Andrew Morton wrote:
> 
> >
> > So yes, doing spin_lock_irq() (irqrestore isn't needed in a timer handler)
> > instead of disable_irq() in vortex_timer() looks OK.
> >
> > One does wonder how long we'll hold off interrupts though.
> 
> Any longer than this!
> 
> in boomerang_start_xmit()
> 
> 	spin_lock_irqsave(&vp->lock, flags);
> 
> 	/* Wait for the stall to complete. */
> 	issue_and_wait(dev, DownStall);
> 
>   Pretty big wait!
> 
>     [...]
> 
> 	spin_unlock_irqrestore(&vp->lock, flags);
> 
> 
> Where we have in issue_and_wait
> 
> static void
> issue_and_wait(struct net_device *dev, int cmd)
> {
> 
>  [...]
> 
> 	/* OK, that didn't work.  Do it the slow way.  One second */
> 	for (i = 0; i < 100000; i++) {
> 
>  [...]
> }
> 
> So this can have interrupts off for over a second!
> 

Well, only if the hardware's fratzed.  Normally this is quick.

otoh vortex_timer() will play with the MII interface, which is slooooow.