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From: parker@finest.io
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jirislaby@kernel.org>,
	linux-kernel@vger.kernel.org,
	linux-serial@vger.kernel.org
Cc: Parker Newman <pnewman@connecttech.com>
Subject: [PATCH v2 2/7] serial: exar: add support for reading from Exar EEPROM
Date: Thu, 11 Apr 2024 16:25:40 -0400
Message-ID: <d16cb88f916914278e125023c856bbf85d0908c1.1712863999.git.pnewman@connecttech.com>
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From: Parker Newman <pnewman@connecttech.com>

- Adds support for reading a word from the Exar EEPROM.
- Adds exar_write_reg/exar_read_reg for reading and writing to the UART's
config registers.

Signed-off-by: Parker Newman <pnewman@connecttech.com>
=2D--
 drivers/tty/serial/8250/8250_exar.c | 110 ++++++++++++++++++++++++++++
 1 file changed, 110 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_exar.c b/drivers/tty/serial/8250=
/8250_exar.c
index 4d1e07343d0b..49d690344e65 100644
=2D-- a/drivers/tty/serial/8250/8250_exar.c
+++ b/drivers/tty/serial/8250/8250_exar.c
@@ -128,6 +128,16 @@
 #define UART_EXAR_DLD			0x02 /* Divisor Fractional */
 #define UART_EXAR_DLD_485_POLARITY	0x80 /* RS-485 Enable Signal Polarity =
*/

+/* EEPROM registers */
+#define UART_EXAR_REGB                  0x8e
+#define UART_EXAR_REGB_EECK             BIT(4)
+#define UART_EXAR_REGB_EECS             BIT(5)
+#define UART_EXAR_REGB_EEDI             BIT(6)
+#define UART_EXAR_REGB_EEDO             BIT(7)
+#define UART_EXAR_REGB_EE_ADDR_SIZE     6
+#define UART_EXAR_REGB_EE_DATA_SIZE     16
+
+
 /*
  * IOT2040 MPIO wiring semantics:
  *
@@ -195,6 +205,106 @@ struct exar8250 {
 	int			line[];
 };

+static inline void exar_write_reg(struct exar8250 *priv,
+				unsigned int reg, uint8_t value)
+{
+	if (!priv || !priv->virt)
+		return;
+
+	writeb(value, priv->virt + reg);
+}
+
+static inline uint8_t exar_read_reg(struct exar8250 *priv, unsigned int r=
eg)
+{
+	if (!priv || !priv->virt)
+		return 0;
+
+	return readb(priv->virt + reg);
+}
+
+static inline void exar_ee_select(struct exar8250 *priv, bool enable)
+{
+	uint8_t value =3D 0x00;
+
+	if (enable)
+		value |=3D UART_EXAR_REGB_EECS;
+
+	exar_write_reg(priv, UART_EXAR_REGB, value);
+	udelay(2);
+}
+
+static inline void exar_ee_write_bit(struct exar8250 *priv, int bit)
+{
+	uint8_t value =3D UART_EXAR_REGB_EECS;
+
+	if (bit)
+		value |=3D UART_EXAR_REGB_EEDI;
+
+	//Clock out the bit on the i2c interface
+	exar_write_reg(priv, UART_EXAR_REGB, value);
+	udelay(2);
+
+	value |=3D UART_EXAR_REGB_EECK;
+
+	exar_write_reg(priv, UART_EXAR_REGB, value);
+	udelay(2);
+}
+
+static inline uint8_t exar_ee_read_bit(struct exar8250 *priv)
+{
+	uint8_t regb;
+	uint8_t value =3D UART_EXAR_REGB_EECS;
+
+	//Clock in the bit on the i2c interface
+	exar_write_reg(priv, UART_EXAR_REGB, value);
+	udelay(2);
+
+	value |=3D UART_EXAR_REGB_EECK;
+
+	exar_write_reg(priv, UART_EXAR_REGB, value);
+	udelay(2);
+
+	regb =3D exar_read_reg(priv, UART_EXAR_REGB);
+
+	return (regb & UART_EXAR_REGB_EEDO ? 1 : 0);
+}
+
+/**
+ * exar_ee_read() - Read a word from the EEPROM
+ * @priv: Device's private structure
+ * @ee_addr: Offset of EEPROM to read word from
+ *
+ * Read a single 16bit word from an Exar UART's EEPROM
+ *
+ * Return: EEPROM word on success, negative error code on failure
+ */
+static int exar_ee_read(struct exar8250 *priv, uint8_t ee_addr)
+{
+	int i;
+	int data =3D 0;
+
+	exar_ee_select(priv, true);
+
+	//Send read command (opcode 110)
+	exar_ee_write_bit(priv, 1);
+	exar_ee_write_bit(priv, 1);
+	exar_ee_write_bit(priv, 0);
+
+	//Send address to read from
+	for (i =3D 1 << (UART_EXAR_REGB_EE_ADDR_SIZE - 1); i; i >>=3D 1)
+		exar_ee_write_bit(priv, (ee_addr & i));
+
+	//Read data 1 bit at a time
+	for (i =3D 0; i <=3D UART_EXAR_REGB_EE_DATA_SIZE; i++) {
+		data <<=3D 1;
+		data |=3D exar_ee_read_bit(priv);
+	}
+
+	exar_ee_select(priv, false);
+
+	return data;
+}
+
 static void exar_pm(struct uart_port *port, unsigned int state, unsigned =
int old)
 {
 	/*
=2D-
2.43.2