[PATCH V3 2/2] nvmem: add generic driver for devices with MMIO access

[Srinivas Kandagatla]

Thanks Rafal for the patch,

On 20/02/2023 17:49, Rafał Miłecki wrote:
> From: Rafał Miłecki <rafal at milecki.pl>
> 
> Some NVMEM devices can be accessed by simply mapping memory and reading
> from / writing to it. This driver adds support for a generic
> "mmio-nvmem" DT binding used by such devices.
> 
> One of such devices is Broadcom's NVRAM. It's already supported (see
> NVMEM_BRCM_NVRAM) but existing driver covers both:
> 1. NVMEM device access
> 2. NVMEM content parsing

> 
> Once we get support for NVMEM layouts then existing NVRAM driver will
> get converted into a layout and generic driver will take over
> responsibility for data access. That's why it claims "brcm,nvram"
> (conditionally for now).
> 
> Signed-off-by: Rafał Miłecki <rafal at milecki.pl>
> ---
> V3: Support "reg-io-width", basic writing & "brcm,nvram" string
> ---
>   drivers/nvmem/Kconfig  |  10 ++++
>   drivers/nvmem/Makefile |   2 +
>   drivers/nvmem/mmio.c   | 125 +++++++++++++++++++++++++++++++++++++++++
>   3 files changed, 137 insertions(+)
>   create mode 100644 drivers/nvmem/mmio.c
> 
> diff --git a/drivers/nvmem/Kconfig b/drivers/nvmem/Kconfig
> index 6dec38805041..189ea85bd67d 100644
> --- a/drivers/nvmem/Kconfig
> +++ b/drivers/nvmem/Kconfig
> @@ -166,6 +166,16 @@ config NVMEM_MICROCHIP_OTPC
>   	  This driver enable the OTP controller available on Microchip SAMA7G5
>   	  SoCs. It controls the access to the OTP memory connected to it.
>   
> +config NVMEM_MMIO
> +	tristate "MMIO access based NVMEM support"
> +	depends on HAS_IOMEM
> +	help
> +	  This driver provides support for NVMEM devices that can be accessed
> +	  using MMIO.
> +
> +	  This driver can also be built as a module. If so, the module
> +	  will be called nvmem-mmio.
> +
>   config NVMEM_MTK_EFUSE
>   	tristate "Mediatek SoCs EFUSE support"
>   	depends on ARCH_MEDIATEK || COMPILE_TEST
> diff --git a/drivers/nvmem/Makefile b/drivers/nvmem/Makefile
> index 6a1efffa88f0..767a9db2bfc1 100644
> --- a/drivers/nvmem/Makefile
> +++ b/drivers/nvmem/Makefile
> @@ -35,6 +35,8 @@ obj-$(CONFIG_NVMEM_MESON_MX_EFUSE)	+= nvmem_meson_mx_efuse.o
>   nvmem_meson_mx_efuse-y			:= meson-mx-efuse.o
>   obj-$(CONFIG_NVMEM_MICROCHIP_OTPC)	+= nvmem-microchip-otpc.o
>   nvmem-microchip-otpc-y			:= microchip-otpc.o
> +obj-$(CONFIG_NVMEM_MMIO)		+= nvmem-mmio.o
> +nvmem-mmio-y				:= mmio.o
>   obj-$(CONFIG_NVMEM_MTK_EFUSE)		+= nvmem_mtk-efuse.o
>   nvmem_mtk-efuse-y			:= mtk-efuse.o
>   obj-$(CONFIG_NVMEM_MXS_OCOTP)		+= nvmem-mxs-ocotp.o
> diff --git a/drivers/nvmem/mmio.c b/drivers/nvmem/mmio.c
> new file mode 100644
> index 000000000000..e247c943eea2
> --- /dev/null
> +++ b/drivers/nvmem/mmio.c
> @@ -0,0 +1,125 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2023 Rafał Miłecki <rafal at milecki.pl>
> + */
> +
> +#include <linux/io.h>
> +#include <linux/mod_devicetable.h>
> +#include <linux/module.h>
> +#include <linux/nvmem-provider.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>
> +#include <linux/property.h>
> +#include <linux/slab.h>
> +
> +struct mmio_nvmem {
> +	void __iomem *base;
> +	u32 io_width;
> +};
> +
> +static int mmio_nvmem_read(void *context, unsigned int offset, void *val, size_t bytes)
> +{
> +	struct mmio_nvmem *priv = context;
> +	u32 *dst32;
> +	u16 *dst16;
> +	u8 *dst8;
> +
> +	if (priv->io_width && WARN_ON(bytes % priv->io_width))
> +		return -EINVAL;
> +
If there is a code path in core that would allow this to happen then it 
needs to be fixed at core level rather than handling it in provider.

> +	switch (priv->io_width) {
> +	case 1:
> +		for (dst8 = val; bytes; bytes -= 1, offset += 1)
> +			*dst8++ = readb(priv->base + offset);
> +		break;
> +	case 2:
> +		for (dst16 = val; bytes; bytes -= 2, offset += 2)
> +			*dst16++ = readw(priv->base + offset);
> +		break;
> +	case 4:
> +		for (dst32 = val; bytes; bytes -= 4, offset += 4)
> +			*dst32++ = readl(priv->base + offset);
> +		break;
> +	default:
> +		memcpy_fromio(val, priv->base + offset, bytes);
> +	}
> +
> +	return 0;
> +}
> +
> +static int mmio_nvmem_write(void *context, unsigned int offset, void *val, size_t bytes)
> +{
> +	struct mmio_nvmem *priv = context;
> +
> +	switch (priv->io_width) {
If io_width is 0 core considers defaults it to 1. But in this patch its 
possible to still have priv->io_width 0. This is the behavior that you want?

> +	case 1:
> +	case 2:
> +	case 4:
> +		return -EOPNOTSUPP;
> +	default:
> +		memcpy_toio(priv->base + offset, val, bytes);
so, are we saying that we only support writes to 8 bytes io_width? Any 
reason for doing this way?

> +	}
> +
> +	return 0;
> +}
> +
> +static int mmio_nvmem_probe(struct platform_device *pdev)
> +{
> +	struct nvmem_config config = {
> +		.name = "mmio-nvmem",
> +		.id = NVMEM_DEVID_AUTO,
> +		.read_only = true,
> +		.reg_read = mmio_nvmem_read,
> +	};
> +	struct device *dev = &pdev->dev;
> +	struct mmio_nvmem *priv;
> +	struct resource *res;
> +
> +	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
> +	if (!priv)
> +		return -ENOMEM;
> +
> +	priv->base = devm_platform_get_and_ioremap_resource(pdev, 0, &res);
> +	if (IS_ERR(priv->base))
> +		return PTR_ERR(priv->base);
> +
> +	of_property_read_u32(dev->of_node, "reg-io-width", &priv->io_width);
> +
> +	config.dev = dev;
> +	config.size = resource_size(res);
> +	config.word_size = priv->io_width;
> +	config.stride = priv->io_width;
> +	config.priv = priv;
> +	if (!device_property_present(dev, "read-only"))
> +		config.reg_write = mmio_nvmem_write;
core should have already marked it readonly based on this property.

> +
> +	return PTR_ERR_OR_ZERO(devm_nvmem_register(dev, &config));
> +}
> +
> +static const struct of_device_id mmio_nvmem_of_match_table[] = {
> +	{ .compatible = "mmio-nvmem", },
> +	/* Custom bindings */
> +#if !IS_ENABLED(CONFIG_NVMEM_BRCM_NVRAM)
> +	{ .compatible = "brcm,nvram", },
> +#endif

If CONFIG_NVMEM_BRCM_NVRAM is not enabled we want to add "brcm,nvram" 
to this driver compatibles, that is something confusing to be honest 
atleast at this transition stage.


How about making nvmem-mmio export functions for drivers like brcm to 
call directly.


> +	{},
> +};
> +
> +static struct platform_driver mmio_nvmem_driver = {
> +	.probe = mmio_nvmem_probe,
> +	.driver = {
> +		.name = "mmio_nvmem",
> +		.of_match_table = mmio_nvmem_of_match_table,
> +	},
> +};
> +
> +static int __init mmio_nvmem_init(void)
> +{
> +	return platform_driver_register(&mmio_nvmem_driver);
> +}
> +
> +subsys_initcall_sync(mmio_nvmem_init);
> +
> +MODULE_AUTHOR("Rafał Miłecki");
> +MODULE_LICENSE("GPL");
> +MODULE_DEVICE_TABLE(of, mmio_nvmem_of_match_table);