[PATCH v3] mfd: syscon: Add Spreadtrum physical regmap bus support

[Baolin Wang <baolin.wang7@gmail.com>]

Some platforms such as Spreadtrum platform, define a special method to
update bits of the registers instead of read-modify-write, which means
we should use a physical regmap bus to define the reg_update_bits()
operation instead of the MMIO regmap bus. Thus we can register a new
physical regmap bus into syscon core to support this.

Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
---
Changes from v2:
 - Fix building errors without enabling CONFIG_ARCH_SPRD.

Changes from v1:
 - Add WARN_ONCE() for seting bits and clearing bits at the same time.
 - Remove the Spreadtrum SoC syscon driver, instead moving the regmap_bus
 instance into syscon.c driver.

Changes from RFC v2:
 - Drop regmap change, which was applied by Mark.
 - Add more information about how to use set/clear.
 - Add checking to ensure the platform is compatible with
 using a new physical regmap bus.

Changes from RFC v1:
 - Add new helper to registers a physical regmap bus instead of
 using the MMIO bus.
---
 drivers/mfd/syscon.c | 83 ++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 81 insertions(+), 2 deletions(-)

diff --git a/drivers/mfd/syscon.c b/drivers/mfd/syscon.c
index 3a97816d0cba..ca91b7770e1a 100644
--- a/drivers/mfd/syscon.c
+++ b/drivers/mfd/syscon.c
@@ -40,6 +40,72 @@ static const struct regmap_config syscon_regmap_config = {
 	.reg_stride = 4,
 };
 
+#if IS_ENABLED(CONFIG_ARCH_SPRD)
+#define SPRD_REG_SET_OFFSET	0x1000
+#define SPRD_REG_CLR_OFFSET	0x2000
+
+/*
+ * The Spreadtrum platform defines a special set/clear method to update
+ * registers' bits, which means it can write values to the register's SET
+ * address (offset is 0x1000) to set bits, and write values to the register's
+ * CLEAR address (offset is 0x2000) to clear bits.
+ *
+ * This set/clear method can help to remove the race of accessing the global
+ * registers between the multiple subsystems instead of using hardware
+ * spinlocks.
+ *
+ * Note: there is a potential risk when users want to set and clear bits
+ * at the same time, since the set/clear method will always do bits setting
+ * before bits clearing, which may cause some unexpected results if the
+ * operation sequence is strict. Thus we recommend that do not set and
+ * clear bits at the same time if you are not sure about the results.
+ */
+static int sprd_syscon_update_bits(void *context, unsigned int reg,
+				   unsigned int mask, unsigned int val)
+{
+	void __iomem *base = context;
+	unsigned int set, clr;
+
+	set = val & mask;
+	clr = ~set & mask;
+
+	if (set)
+		writel(set, base + reg + SPRD_REG_SET_OFFSET);
+
+	if (clr)
+		writel(clr, base + reg + SPRD_REG_CLR_OFFSET);
+
+	WARN_ONCE(set && clr, "%s: non-atomic update", __func__);
+	return 0;
+}
+
+static int sprd_syscon_read(void *context, unsigned int reg, unsigned int *val)
+{
+	void __iomem *base = context;
+
+	*val = readl(base + reg);
+	return 0;
+}
+
+static int sprd_syscon_write(void *context, unsigned int reg, unsigned int val)
+{
+	void __iomem *base = context;
+
+	writel(val, base + reg);
+	return 0;
+}
+#endif
+
+static struct regmap_bus sprd_syscon_regmap_bus = {
+#if IS_ENABLED(CONFIG_ARCH_SPRD)
+	.fast_io = true,
+	.reg_write = sprd_syscon_write,
+	.reg_read = sprd_syscon_read,
+	.reg_update_bits = sprd_syscon_update_bits,
+	.val_format_endian_default = REGMAP_ENDIAN_LITTLE,
+#endif
+};
+
 static struct syscon *of_syscon_register(struct device_node *np, bool check_clk)
 {
 	struct clk *clk;
@@ -50,6 +116,7 @@ static struct syscon *of_syscon_register(struct device_node *np, bool check_clk)
 	int ret;
 	struct regmap_config syscon_config = syscon_regmap_config;
 	struct resource res;
+	bool use_phy_regmap_bus = false;
 
 	syscon = kzalloc(sizeof(*syscon), GFP_KERNEL);
 	if (!syscon)
@@ -106,14 +173,26 @@ static struct syscon *of_syscon_register(struct device_node *np, bool check_clk)
 	syscon_config.val_bits = reg_io_width * 8;
 	syscon_config.max_register = resource_size(&res) - reg_io_width;
 
-	regmap = regmap_init_mmio(NULL, base, &syscon_config);
+	/*
+	 * The Spreadtrum syscon need register a real physical regmap bus
+	 * with new atomic bits updating operation instead of using
+	 * read-modify-write.
+	 */
+	if (IS_ENABLED(CONFIG_ARCH_SPRD) &&
+	    of_device_is_compatible(np, "sprd,atomic-syscon")) {
+		use_phy_regmap_bus = true;
+		regmap = regmap_init(NULL, &sprd_syscon_regmap_bus, base,
+				     &syscon_config);
+	} else {
+		regmap = regmap_init_mmio(NULL, base, &syscon_config);
+	}
 	if (IS_ERR(regmap)) {
 		pr_err("regmap init failed\n");
 		ret = PTR_ERR(regmap);
 		goto err_regmap;
 	}
 
-	if (check_clk) {
+	if (!use_phy_regmap_bus && check_clk) {
 		clk = of_clk_get(np, 0);
 		if (IS_ERR(clk)) {
 			ret = PTR_ERR(clk);
-- 
2.17.1