Re: [PATCH 02/18] Blackfin Serial Driver: use barrier instead of cpu_relax for Blackfin SMP like patch

Andrew Morton
2009-03-06T22:39:00Z

On Fri, 6 Mar 2009 14:42:44 +0800
Bryan Wu <cooloney@kernel.org> wrote:

> From: Graf Yang <graf.yang@analog.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.