generic: RTC7301 byte-addressed IO

This is a backport of the patch for byte addressed IO to the
Epson RTC7301 driver. This is used by the IXP4xx-based
USRobotics USR8200.

Signed-off-by: Linus Walleij <[email protected]>

target/linux/generic/backport-6.1/831-v6.7-rtc-rtc7301-Support-byte-addressed-IO.patch

@@ -0,0 +1,93 @@
+From edd25a77e69b7c546c28077e5dffe72c54c0afe8 Mon Sep 17 00:00:00 2001
+From: Linus Walleij <[email protected]>
+Date: Thu, 21 Sep 2023 22:18:12 +0200
+Subject: [PATCH 2/4] rtc: rtc7301: Support byte-addressed IO
+
+The old RTC7301 driver in OpenWrt used byte access, but the
+current mainline Linux driver uses 32bit word access.
+
+Make this configurable using device properties using the
+standard property "reg-io-width" in e.g. device tree.
+
+This is needed for the USRobotics USR8200 which has the
+chip connected using byte accesses.
+
+Debugging and testing by Howard Harte.
+
+Signed-off-by: Linus Walleij <[email protected]>
+---
+ drivers/rtc/rtc-r7301.c | 35 +++++++++++++++++++++++++++++++++--
+ 1 file changed, 33 insertions(+), 2 deletions(-)
+
+--- a/drivers/rtc/rtc-r7301.c
++++ b/drivers/rtc/rtc-r7301.c
+@@ -14,6 +14,7 @@
+ #include <linux/module.h>
+ #include <linux/mod_devicetable.h>
+ #include <linux/delay.h>
++#include <linux/property.h>
+ #include <linux/regmap.h>
+ #include <linux/platform_device.h>
+ #include <linux/rtc.h>
+@@ -55,12 +56,23 @@ struct rtc7301_priv {
+ 	u8 bank;
+ };
+ 
+-static const struct regmap_config rtc7301_regmap_config = {
++/*
++ * When the device is memory-mapped, some platforms pack the registers into
++ * 32-bit access using the lower 8 bits at each 4-byte stride, while others
++ * expose them as simply consecutive bytes.
++ */
++static const struct regmap_config rtc7301_regmap_32_config = {
+ 	.reg_bits = 32,
+ 	.val_bits = 8,
+ 	.reg_stride = 4,
+ };
+ 
++static const struct regmap_config rtc7301_regmap_8_config = {
++	.reg_bits = 8,
++	.val_bits = 8,
++	.reg_stride = 1,
++};
++
+ static u8 rtc7301_read(struct rtc7301_priv *priv, unsigned int reg)
+ {
+ 	int reg_stride = regmap_get_reg_stride(priv->regmap);
+@@ -356,7 +368,9 @@ static int __init rtc7301_rtc_probe(stru
+ 	void __iomem *regs;
+ 	struct rtc7301_priv *priv;
+ 	struct rtc_device *rtc;
++	static const struct regmap_config *mapconf;
+ 	int ret;
++	u32 val;
+ 
+ 	priv = devm_kzalloc(&dev->dev, sizeof(*priv), GFP_KERNEL);
+ 	if (!priv)
+@@ -366,8 +380,25 @@ static int __init rtc7301_rtc_probe(stru
+ 	if (IS_ERR(regs))
+ 		return PTR_ERR(regs);
+ 
++	ret = device_property_read_u32(&dev->dev, "reg-io-width", &val);
++	if (ret)
++		/* Default to 32bit accesses */
++		val = 4;
++
++	switch (val) {
++	case 1:
++		mapconf = &rtc7301_regmap_8_config;
++		break;
++	case 4:
++		mapconf = &rtc7301_regmap_32_config;
++		break;
++	default:
++		dev_err(&dev->dev, "invalid reg-io-width %d\n", val);
++		return -EINVAL;
++	}
++
+ 	priv->regmap = devm_regmap_init_mmio(&dev->dev, regs,
+-					     &rtc7301_regmap_config);
++					     mapconf);
+ 	if (IS_ERR(priv->regmap))
+ 		return PTR_ERR(priv->regmap);
+