linux-kernel - Re: [PATCH v3] spi: dts: sun4i: Add support for hardware-based wait time between words

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Date:	Sun, 13 Dec 2015 19:29:37 -0600
From:	Rob Herring <robh@...nel.org>
To:	Marcus Weseloh <mweseloh42@...il.com>
Cc:	linux-sunxi@...glegroups.com, Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Mark Brown <broonie@...nel.org>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Chen-Yu Tsai <wens@...e.org>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3] spi: dts: sun4i: Add support for hardware-based wait
 time between words

On Mon, Dec 14, 2015 at 12:04:11AM +0100, Marcus Weseloh wrote:
> Adds a new property "spi-word-wait-ns" to the spi-bus binding that allows
> SPI slave devices to set a hardware based wait time between the
> transmission of words. Also modifies the sun4i SPI master driver to make
> use of the new 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>
> ---
> Changes from v1:
>  * renamed the property for more clarity
>  * wait time is set in nanoseconds instead of number of clock cycles
>  * transparently handle the 3 setup clock cycles
> 
> Changes from v2:
>  * fixed typo in comment
>  * moved parameter to spi-bus binding, dropping the vendor prefix
>  * changed commit summary and description to reflect the changes
> ---
>  Documentation/devicetree/bindings/spi/spi-bus.txt |  2 ++
>  drivers/spi/spi-sun4i.c                           | 23 +++++++++++++++++++++++
>  2 files changed, 25 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/spi/spi-bus.txt b/Documentation/devicetree/bindings/spi/spi-bus.txt
> index bbaa857..2d6034f 100644
> --- a/Documentation/devicetree/bindings/spi/spi-bus.txt
> +++ b/Documentation/devicetree/bindings/spi/spi-bus.txt
> @@ -61,6 +61,8 @@ contain the following properties.
>                        used for MOSI. Defaults to 1 if not present.
>  - spi-rx-bus-width - (optional) The bus width(number of data wires) that
>                        used for MISO. Defaults to 1 if not present.
> +- spi-word-wait-ns - (optional) Hardware based delay between transmission of
> +                      words in nanoseconds

Could be a software delay if the h/w doesn't support delays.

>  Some SPI controllers and devices support Dual and Quad SPI transfer mode.
>  It allows data in the SPI system to be transferred in 2 wires(DUAL) or 4 wires(QUAD).
> diff --git a/drivers/spi/spi-sun4i.c b/drivers/spi/spi-sun4i.c
> index f60a6d6..73995a1 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 +174,9 @@ static int sun4i_spi_transfer_one(struct spi_master *master,
>  	unsigned int tx_len = 0;
>  	int ret = 0;
>  	u32 reg;
> +	u32 wait_ns = 0;
> +	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 +265,25 @@ static int sun4i_spi_transfer_one(struct spi_master *master,
>  
>  	sun4i_spi_write(sspi, SUN4I_CLK_CTL_REG, reg);
>  
> +	/* Setup wait time between words */
> +	of_property_read_u32(spi->dev.of_node, "spi-word-wait-ns",
> +			     &wait_ns);

Read this in probe and save the value rather than fetching every 
transfer.

> +	if (wait_ns) {
> +		/* The 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.
> +		 */
> +		clk_ns = DIV_ROUND_UP(1000000000, tfr->speed_hz);
> +		wait_clk = DIV_ROUND_UP(wait_ns, clk_ns) - 3;
> +		if (wait_clk < 1) {
> +			wait_clk = 1;
> +			dev_info(&spi->dev,
> +				 "using minimum of 4 word wait cycles (%uns)",
> +				 4 * clk_ns);
> +		}
> +	}
> +	sun4i_spi_write(sspi, SUN4I_WAIT_REG, (u16)wait_clk);
> +
>  	/* Setup the transfer now... */
>  	if (sspi->tx_buf)
>  		tx_len = tfr->len;
> -- 
> 1.9.1
> 
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