linux-kernel - Re: [PATCH 3.17-rc3 v2] serial: asc: Adopt readl_/writel

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Date:	Tue, 09 Sep 2014 12:15:16 +0100
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Srinivas Kandagatla <srinivas.kandagatla@...il.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, patches@...aro.org,
	linaro-kernel@...ts.linaro.org,
	Maxime Coquelin <maxime.coquelin@...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 v2] serial: asc: Adopt readl_/writel_relaxed()

On 09/09/14 12:08, Srinivas Kandagatla wrote:
> Hi Dan,
> 
> After Greg's comment, Am ok with the patch and sorry for the noise.
> Good that you removed the barier() too.

Yes.

I got interested in Greg's question about which architectures don't have
readl_relaxed() and, in the process, discovered I was wrong about that
being needed anyway.


> You can have my Ack.
> 
> --srini
> 
> On Tue, Sep 9, 2014 at 11:03 AM, Daniel Thompson
> <daniel.thompson@...aro.org <mailto:daniel.thompson@...aro.org>> wrote:
> 
>     The architectures supported by this driver, 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 absence of spin locks during console I/O) if it uses the
>     relaxed variants.
> 
>     The driver supports COMPILE_TEST and therefore falls back to writel()
>     when writel_relaxed() does not exist.
> 
>     Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org
>     <mailto:daniel.thompson@...aro.org>>
>     Cc: Srinivas Kandagatla <srinivas.kandagatla@...il.com
>     <mailto:srinivas.kandagatla@...il.com>>
>     Cc: Maxime Coquelin <maxime.coquelin@...com
>     <mailto:maxime.coquelin@...com>>
>     Cc: Patrice Chotard <patrice.chotard@...com
>     <mailto:patrice.chotard@...com>>
>     Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org
>     <mailto:gregkh@...uxfoundation.org>>
>     Cc: Jiri Slaby <jslaby@...e.cz <mailto:jslaby@...e.cz>>
>     Cc: kernel@...inux.com <mailto:kernel@...inux.com>
>     Cc: linux-serial@...r.kernel.org <mailto:linux-serial@...r.kernel.org>
>     ---
> 
>     Notes:
>         Changes since v1:
> 
>         * Added fallback to writel() to permit COMPILE_TESTing (review
>           of Greg Kroah-Hartmam).
> 
>      drivers/tty/serial/st-asc.c | 6 +++++-
>      1 file changed, 5 insertions(+), 1 deletion(-)
> 
>     diff --git a/drivers/tty/serial/st-asc.c b/drivers/tty/serial/st-asc.c
>     index 8b2d735..a3fc167 100644
>     --- a/drivers/tty/serial/st-asc.c
>     +++ b/drivers/tty/serial/st-asc.c
>     @@ -151,12 +151,16 @@ 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);
>      }
> 
>      static inline void asc_out(struct uart_port *port, u32 offset, u32
>     value)
>      {
>     +#ifdef writel_relaxed
>     +       writel_relaxed(value, port->membase + offset);
>     +#else
>             writel(value, port->membase + offset);
>     +#endif
>      }
> 
>      /*
>     --
>     1.9.3
> 
> 

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