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

["Rafał Miłecki" <zajec5@gmail.com>]

From: Rafał Miłecki <rafal@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@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@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;
+
+	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) {
+	case 1:
+	case 2:
+	case 4:
+		return -EOPNOTSUPP;
+	default:
+		memcpy_toio(priv->base + offset, val, bytes);
+	}
+
+	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;
+
+	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
+	{},
+};
+
+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);
-- 
2.34.1