linux-kernel - Re: [PATCH v5 2/2] spi: sun4i: Add support for wait time between word transmissions

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Date:	Fri, 18 Dec 2015 12:16:27 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Marcus Weseloh <mweseloh42@...il.com>
Cc:	linux-sunxi@...glegroups.com, Chen-Yu Tsai <wens@...e.org>,
	devicetree@...r.kernel.org,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-spi@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v5 2/2] spi: sun4i: Add support for wait time between
 word transmissions

Hi,

On Thu, Dec 17, 2015 at 12:40:27PM +0100, Marcus Weseloh wrote:
> Modifies the sun4i SPI master driver to make use of the
> "spi-word-wait-ns" property. This specific SPI controller needs 3 clock
> cycles to set up the delay, which makes the minimum non-zero wait time
> on this hardware 4 clock cycles.
> 
> Signed-off-by: Marcus Weseloh <mweseloh42@...il.com>
> ---
>  drivers/spi/spi-sun4i.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/drivers/spi/spi-sun4i.c b/drivers/spi/spi-sun4i.c
> index f60a6d6..3b4f5f4 100644
> --- a/drivers/spi/spi-sun4i.c
> +++ b/drivers/spi/spi-sun4i.c
> @@ -19,6 +19,7 @@
>  #include <linux/module.h>
>  #include <linux/platform_device.h>
>  #include <linux/pm_runtime.h>
> +#include <linux/of.h>
>  
>  #include <linux/spi/spi.h>
>  
> @@ -173,6 +174,8 @@ static int sun4i_spi_transfer_one(struct spi_master *master,
>  	unsigned int tx_len = 0;
>  	int ret = 0;
>  	u32 reg;
> +	int wait_clk = 0;
> +	int clk_ns = 0;
>  
>  	/* We don't support transfer larger than the FIFO */
>  	if (tfr->len > SUN4I_FIFO_DEPTH)
> @@ -261,6 +264,25 @@ static int sun4i_spi_transfer_one(struct spi_master *master,
>  
>  	sun4i_spi_write(sspi, SUN4I_CLK_CTL_REG, reg);
>  
> +	/*
> +	 * Setup wait time between words.
> +	 *
> +	 * Wait time is set in SPI_CLK cycles. The SPI hardware needs 3
> +	 * additional cycles to setup the wait counter, so the minimum delay
> +	 * time is 4 cycles.
> +	 */
> +	if (spi->word_wait_ns) {
> +		clk_ns = DIV_ROUND_UP(1000000000, tfr->speed_hz);

You should use the actual rate of the clock returned by clk_get_rate
(or probably just use mclk_rate).

The clock driver might round the frequency to something else than what
was set in clk_set_rate, which would make your calculation here a bit
off.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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