linux-kernel - Re: [PATCH v3 2/5] mfd: syscon: add a DT property to set value width

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Date:	Tue, 17 Nov 2015 09:19:46 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Damien Riegel <damien.riegel@...oirfairelinux.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-watchdog@...r.kernel.org, shawnguo@...nel.org,
	kernel@...gutronix.de, wim@...ana.be, robh+dt@...nel.org,
	sameo@...ux.intel.com, dinh.linux@...il.com, linux@...ck-us.net,
	kernel@...oirfairelinux.com
Subject: Re: [PATCH v3 2/5] mfd: syscon: add a DT property to set value width

On Mon, 16 Nov 2015, Damien Riegel wrote:

> Currently syscon has a fixed configuration of 32 bits for register and
> values widths. In some cases, it would be desirable to be able to
> customize the value width.
> 
> For example, certain boards (like the ones manufactured by Technologic
> Systems) have a FPGA that is memory-mapped, but its registers are only
> 16-bit wide.
> 
> This patch adds an optional "bus-width" DT binding for syscon that
> allows to change the width for the data bus (i.e. val_bits). If this
> property is provided, it will also adjust the register stride to
> bus-width / 8. If not provided, the default configuration is used.
> 
> Signed-off-by: Damien Riegel <damien.riegel@...oirfairelinux.com>
> ---
>  Documentation/devicetree/bindings/mfd/syscon.txt |  3 +++
>  drivers/mfd/syscon.c                             | 12 ++++++++++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/mfd/syscon.txt b/Documentation/devicetree/bindings/mfd/syscon.txt
> index fe8150b..4c9d187 100644
> --- a/Documentation/devicetree/bindings/mfd/syscon.txt
> +++ b/Documentation/devicetree/bindings/mfd/syscon.txt
> @@ -13,6 +13,9 @@ Required properties:
>  - compatible: Should contain "syscon".
>  - reg: the register region can be accessed from syscon
>  
> +Optional property:
> +- bus-width: width of the data bus. Can be <8>, <16>, <32>, or <64>.
> +
>  Examples:
>  gpr: iomuxc-gpr@...e0000 {
>  	compatible = "fsl,imx6q-iomuxc-gpr", "syscon";
> diff --git a/drivers/mfd/syscon.c b/drivers/mfd/syscon.c
> index 176bf0f..00b0995 100644
> --- a/drivers/mfd/syscon.c
> +++ b/drivers/mfd/syscon.c
> @@ -47,6 +47,7 @@ static struct syscon *of_syscon_register(struct device_node *np)
>  	struct syscon *syscon;
>  	struct regmap *regmap;
>  	void __iomem *base;
> +	u32 bus_width;
>  	int ret;
>  	struct regmap_config syscon_config = syscon_regmap_config;
>  
> @@ -69,6 +70,17 @@ static struct syscon *of_syscon_register(struct device_node *np)
>  	 else if (of_property_read_bool(np, "little-endian"))
>  		syscon_config.val_format_endian = REGMAP_ENDIAN_LITTLE;
>  
> +	 ret = of_property_read_u32(np, "bus-width", &bus_width);
> +	 if (!ret) {

This syntax is confusing, as we normally associate it with an error
condition.  Instead, I'd use:

  if (of_property_read_u32(np, "bus-width", &bus_width) == 0)

Or, for more clarity:

  of_property_read_u32(np, "bus-width", &bus_width);
  if (bus_width)

If you choose this version (which I think is my preferred method, don't
forget to initialise 'bus_width' to zero.

> +		 /*
> +		  * the property has been provided, regmap_init_mmio
> +		  * will return an error if these values are invalid
> +		  * so there is no need to check them here.
> +		  */
> +		 syscon_config.val_bits = bus_width;
> +		 syscon_config.reg_stride = syscon_config.val_bits / 8;
> +	 }
> +
>  	regmap = regmap_init_mmio(NULL, base, &syscon_config);
>  	if (IS_ERR(regmap)) {
>  		pr_err("regmap init failed\n");

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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