i2c: ls2x: Fix frequency division register access

According to the chip manual, the I2C register access type of
Loongson-2K2000/LS7A is "B", so we can only access registers in byte
form (readb()/writeb()).

Although Loongson-2K0500/Loongson-2K1000 do not have similar
constraints, register accesses in byte form also behave correctly.

Also, in hardware, the frequency division registers are defined as two
separate registers (high 8-bit and low 8-bit), so we just access them
directly as bytes.

Fixes: 015e61f0bffd ("i2c: ls2x: Add driver for Loongson-2K/LS7A I2C controller")
Co-developed-by: Hongliang Wang <[e-mail]>
Signed-off-by: Hongliang Wang <[e-mail]>
Signed-off-by: Binbin Zhou <[e-mail]>
Cc: [e-mail] # v6.3+
Reviewed-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Andi Shyti <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/i2c/busses/i2c-ls2x.c

···
  * Rewritten for mainline by Binbin Zhou <[e-mail]>
  */
 
+#include <linux/bitfield.h>
 #include <linux/bits.h>
 #include <linux/completion.h>
 #include <linux/device.h>
···
 #include <linux/units.h>
 
 /* I2C Registers */
-#define I2C_LS2X_PRER		0x0 /* Freq Division Register(16 bits) */
+#define I2C_LS2X_PRER_LO	0x0 /* Freq Division Low Byte Register */
+#define I2C_LS2X_PRER_HI	0x1 /* Freq Division High Byte Register */
 #define I2C_LS2X_CTR		0x2 /* Control Register */
 #define I2C_LS2X_TXR		0x3 /* Transport Data Register */
 #define I2C_LS2X_RXR		0x3 /* Receive Data Register */
···
  */
 static void ls2x_i2c_adjust_bus_speed(struct ls2x_i2c_priv *priv)
 {
+	u16 val;
 	struct i2c_timings *t = &priv->i2c_t;
 	struct device *dev = priv->adapter.dev.parent;
 	u32 acpi_speed = i2c_acpi_find_bus_speed(dev);
···
 	else
 		t->bus_freq_hz = LS2X_I2C_FREQ_STD;
 
-	/* Calculate and set i2c frequency. */
-	writew(LS2X_I2C_PCLK_FREQ / (5 * t->bus_freq_hz) - 1,
-	       priv->base + I2C_LS2X_PRER);
+	/*
+	 * According to the chip manual, we can only access the registers as bytes,
+	 * otherwise the high bits will be truncated.
+	 * So set the I2C frequency with a sequential writeb() instead of writew().
+	 */
+	val = LS2X_I2C_PCLK_FREQ / (5 * t->bus_freq_hz) - 1;
+	writeb(FIELD_GET(GENMASK(7, 0), val), priv->base + I2C_LS2X_PRER_LO);
+	writeb(FIELD_GET(GENMASK(15, 8), val), priv->base + I2C_LS2X_PRER_HI);
 }
 
 static void ls2x_i2c_init(struct ls2x_i2c_priv *priv)