linux-kernel - Re: [PATCH v2] Add poll_get_char and poll_put_char uart_ops to atmel

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Date:	Thu, 12 Nov 2009 14:24:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Albin Tonnerre <albin.tonnerre@...e-electrons.com>
Cc:	avictor.za@...il.com, linux-kernel@...r.kernel.org,
	haavard.skinnemoen@...el.com, nicolas.ferre@...el.com
Subject: Re: [PATCH v2] Add poll_get_char and poll_put_char uart_ops to
 atmel_serial.

On Mon,  7 Sep 2009 11:52:50 +0200
Albin Tonnerre <albin.tonnerre@...e-electrons.com> wrote:

> +#ifdef CONFIG_CONSOLE_POLL
> +static int atmel_poll_get_char(struct uart_port *port)
> +{
> +	while (!(UART_GET_CSR(port) & ATMEL_US_RXRDY))
> +		barrier();
> +
> +	return UART_GET_CHAR(port);
> +}
> +
> +static void atmel_poll_put_char(struct uart_port *port, unsigned char ch)
> +{
> +	while (!(UART_GET_CSR(port) & ATMEL_US_TXRDY))
> +		barrier();
> +
> +	UART_PUT_CHAR(port, ch);
> +}
> +#endif

The use of barrier() is not appropriate here - it's purely a compiler
concept.

I'll switch it to cpu_relax().  Even that's probably unneeded, as the
IO operation is usually slow and should be an opportunity for the CPU
to take a little nap.

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