linux-kernel - Re: [PATCH 02/18] Blackfin Serial Driver: use barrier instead of cpu

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Date:	Tue, 10 Mar 2009 18:07:11 +0800
From:	gyang <graf.yang@...log.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Bryan Wu <cooloney@...nel.org>, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/18] Blackfin Serial Driver: use barrier instead of
 cpu_relax for Blackfin SMP like patch

On Fri, 2009-03-06 at 14:37 -0800, Andrew Morton wrote:
> On Fri,  6 Mar 2009 14:42:44 +0800
> Bryan Wu <cooloney@...nel.org> wrote:
> 
> > From: Graf Yang <graf.yang@...log.com>
> > 
> > We are making a SMP like patch to blackfin, cpu_relax() is replaced by a
> > data cache flush function which will count it to a per-cpu counter.
> > If this serial function is called too early, the per-cpu data area have
> > not been initialized, this call will cause crash.
> 
> That's a bug in blackfin architecture support.  The kernel should be
> able to call cpu_relax() at any time, surely.  It's a very low-level
> and simple thing.
> 
> > So we'd like to use barrier() instead of cpu_relax().
> > 
> 
> barrier() is purely a compiler concept.  We might as well just remove
> the cpu_relax() altogether.

Do you mean remove cpu_relax(), and either not add barrier() here?

> 
> 
> >  drivers/serial/bfin_5xx.c |    3 ++-
> >  1 files changed, 2 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/serial/bfin_5xx.c b/drivers/serial/bfin_5xx.c
> > index 52ccc27..350bfc4 100644
> > --- a/drivers/serial/bfin_5xx.c
> > +++ b/drivers/serial/bfin_5xx.c
> > @@ -1129,7 +1129,8 @@ static __init void early_serial_putc(struct uart_port *port, int ch)
> >  	struct bfin_serial_port *uart = (struct bfin_serial_port *)port;
> >  
> >  	while ((!(UART_GET_LSR(uart) & THRE)) && --timeout)
> > -		cpu_relax();
> > +		barrier();
> > +
> >  	UART_PUT_CHAR(uart, ch);
> >  }
> 
> I grumpily queued this, but it seems all wrong.
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