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

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Message-Id: <20090306143756.738db0ee.akpm@linux-foundation.org>
Date:	Fri, 6 Mar 2009 14:37:56 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Bryan Wu <cooloney@...nel.org>
Cc:	alan@...rguk.ukuu.org.uk, linux-kernel@...r.kernel.org,
	graf.yang@...log.com, cooloney@...nel.org
Subject: Re: [PATCH 02/18] Blackfin Serial Driver: use barrier instead of
 cpu_relax for Blackfin SMP like patch

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.


>  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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