linux-kernel - Re: [PATCH 3.17-rc3] serial: asc: Adopt readl_/writel

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Date:	Mon, 8 Sep 2014 16:27:27 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	linux-kernel@...r.kernel.org, patches@...aro.org,
	linaro-kernel@...ts.linaro.org,
	Srinivas Kandagatla <srinivas.kandagatla@...il.com>,
	Patrice Chotard <patrice.chotard@...com>,
	Jiri Slaby <jslaby@...e.cz>, kernel@...inux.com,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH 3.17-rc3] serial: asc: Adopt readl_/writel_relaxed()

On Wed, Sep 03, 2014 at 01:00:51PM +0100, Daniel Thompson wrote:
> The architectures where this peripheral exists (ARM and SH) have expensive
> implementations of writel(), reliant on spin locks and explicit L2 cache
> management. These architectures provide a cheaper writel_relaxed() which
> is much better suited to peripherals that do not perform DMA. The
> situation with readl()/readl_relaxed()is similar although less acute.
> 
> This driver does not use DMA and will be more power efficient and more
> robust (due to absense of spin locks during console I/O) if it uses the
> relaxed variants.
> 
> This change means the driver is no longer portable and therefore no
> longer suitable for compile testing.
> 
> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> Cc: Srinivas Kandagatla <srinivas.kandagatla@...il.com>
> Cc: Patrice Chotard <patrice.chotard@...com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jiri Slaby <jslaby@...e.cz>
> Cc: kernel@...inux.com
> Cc: linux-serial@...r.kernel.org
> Acked-by: Maxime Coquelin <maxime.coquelin@...com>
> Acked-by: Peter Griffin <peter.griffin@...aro.org>
> ---
>  drivers/tty/serial/Kconfig  | 2 +-
>  drivers/tty/serial/st-asc.c | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
> index 26cec64d..e9b1735 100644
> --- a/drivers/tty/serial/Kconfig
> +++ b/drivers/tty/serial/Kconfig
> @@ -1527,7 +1527,7 @@ config SERIAL_FSL_LPUART_CONSOLE
>  config SERIAL_ST_ASC
>  	tristate "ST ASC serial port support"
>  	select SERIAL_CORE
> -	depends on ARM || COMPILE_TEST
> +	depends on ARM

I really don't like stuff that does this, sorry.  I want to test build
as many drivers as I can.  COMPILE_TEST does not mean that the driver is
"portable", only that it builds properly on all platforms.

>  	help
>  	  This driver is for the on-chip Asychronous Serial Controller on
>  	  STMicroelectronics STi SoCs.
> diff --git a/drivers/tty/serial/st-asc.c b/drivers/tty/serial/st-asc.c
> index 8b2d735..adadbc1 100644
> --- a/drivers/tty/serial/st-asc.c
> +++ b/drivers/tty/serial/st-asc.c
> @@ -151,12 +151,12 @@ static inline struct asc_port *to_asc_port(struct uart_port *port)
> 
>  static inline u32 asc_in(struct uart_port *port, u32 offset)
>  {
> -	return readl(port->membase + offset);
> +	return readl_relaxed(port->membase + offset);

What plaforms do not provide readl_relaxed()?

thanks,

greg k-h
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