bios/sdram: add firmware for reading SPD EEPROM

litex/soc/software/liblitedram/Makefile

@@ -1,7 +1,7 @@
 include ../include/generated/variables.mak
 include $(SOC_DIRECTORY)/software/common.mak
 
-OBJECTS=sdram.o
+OBJECTS = sdram.o spd.o
 
 all: liblitedram.a
 

litex/soc/software/liblitedram/sdram.h

@@ -2,6 +2,7 @@
 #define __SDRAM_H
 
 #include <generated/csr.h>
+#include "spd.h"
 
 void sdrsw(void);
 void sdrhw(void);

litex/soc/software/liblitedram/spd.c

@@ -0,0 +1,180 @@
+// This file is Copyright (c) 2020 Antmicro <www.antmicro.com>
+
+#include <stdio.h>
+#include "spd.h"
+
+#ifdef CSR_I2C_BASE
+
+// SMBus uses frequency 10-100 kHz
+#define I2C_FREQ_HZ       50000
+#define I2C_PERIOD_CYCLES (CONFIG_CLOCK_FREQUENCY / I2C_FREQ_HZ)
+#define I2C_DELAY(n)      cdelay((n)*I2C_PERIOD_CYCLES/4)
+
+static void cdelay(int i)
+{
+	while(i > 0) {
+		__asm__ volatile(CONFIG_CPU_NOP);
+		i--;
+	}
+}
+
+static void i2c_oe_scl_sda(int oe, int scl, int sda)
+{
+	i2c_w_write(
+		((oe & 1)  << CSR_I2C_W_OE_OFFSET)  |
+		((scl & 1) << CSR_I2C_W_SCL_OFFSET) |
+		((sda & 1) << CSR_I2C_W_SDA_OFFSET)
+	);
+}
+
+
+// START condition: 1-to-0 transition of SDA when SCL is 1
+static void i2c_start(void)
+{
+	i2c_oe_scl_sda(1, 1, 1);
+	I2C_DELAY(1);
+	i2c_oe_scl_sda(1, 1, 0);
+	I2C_DELAY(1);
+	i2c_oe_scl_sda(1, 0, 0);
+	I2C_DELAY(1);
+}
+
+// STOP condition: 0-to-1 transition of SDA when SCL is 1
+static void i2c_stop(void)
+{
+	i2c_oe_scl_sda(1, 0, 0);
+	I2C_DELAY(1);
+	i2c_oe_scl_sda(1, 1, 0);
+	I2C_DELAY(1);
+	i2c_oe_scl_sda(1, 1, 1);
+	I2C_DELAY(1);
+	i2c_oe_scl_sda(0, 1, 1);
+}
+
+// Reset line state
+static void i2c_reset(void)
+{
+	int i;
+	i2c_oe_scl_sda(1, 1, 1);
+	I2C_DELAY(8);
+	for (i = 0; i < 9; ++i) {
+		i2c_oe_scl_sda(1, 0, 1);
+		I2C_DELAY(2);
+		i2c_oe_scl_sda(1, 1, 1);
+		I2C_DELAY(2);
+	}
+	i2c_oe_scl_sda(0, 0, 1);
+	I2C_DELAY(1);
+	i2c_stop();
+	i2c_oe_scl_sda(0, 1, 1);
+	I2C_DELAY(8);
+}
+
+// Call when in the middle of SCL low, advances one clk period
+static void i2c_transmit_bit(int value)
+{
+	i2c_oe_scl_sda(1, 0, value);
+	I2C_DELAY(1);
+	i2c_oe_scl_sda(1, 1, value);
+	I2C_DELAY(2);
+	i2c_oe_scl_sda(1, 0, value);
+	I2C_DELAY(1);
+	i2c_oe_scl_sda(0, 0, 0);  // release line
+}
+
+// Call when in the middle of SCL low, advances one clk period
+static int i2c_receive_bit(void)
+{
+	int value;
+	i2c_oe_scl_sda(0, 0, 0);
+	I2C_DELAY(1);
+	i2c_oe_scl_sda(0, 1, 0);
+	I2C_DELAY(1);
+	// read in the middle of SCL high
+	value = i2c_r_read() & 1;
+	I2C_DELAY(1);
+	i2c_oe_scl_sda(0, 0, 0);
+	I2C_DELAY(1);
+	return value;
+}
+
+// Send data byte and return 1 if slave sends ACK
+static int i2c_transmit(unsigned char data)
+{
+	int ack;
+	int i;
+
+	// SCL should have already been low for 1/4 cycle
+	i2c_oe_scl_sda(0, 0, 0);
+	for (i = 0; i < 8; ++i) {
+		// MSB first
+		i2c_transmit_bit((data & (1 << 7)) != 0);
+		data <<= 1;
+	}
+	ack = i2c_receive_bit();
+
+	// 0 from slave means ack
+	return ack == 0;
+}
+
+// Read data byte and send ACK if ack=1
+static unsigned char i2c_receive(int ack)
+{
+	unsigned char data = 0;
+	int i;
+
+	i2c_oe_scl_sda(0, 0, 0);
+	I2C_DELAY(1);
+	for (i = 0; i < 8; ++i) {
+		data <<= 1;
+		data |= i2c_receive_bit();
+	}
+	i2c_transmit_bit(!ack);
+
+	return data;
+}
+
+
+#define ADDR_PREAMBLE_RW  0b1010
+#define ADDR_7BIT(addr)   ((ADDR_PREAMBLE_RW << 3) | ((addr) & 0b111))
+#define ADDR_WRITE(addr)  ((ADDR_7BIT(addr) << 1) & (~1u))
+#define ADDR_READ(addr)   ((ADDR_7BIT(addr) << 1) | 1u)
+
+/*
+ * Read SPD memory content
+ *
+ * spdaddr: address of SPD EEPROM defined by pins A0, A1, A2
+ * addr: memory starting address
+ */
+int spdread(unsigned int spdaddr, unsigned int addr, unsigned char *buf, unsigned int len) {
+	int i;
+
+	i2c_reset();
+
+	// To read from random address, we have to first send a "data-less" WRITE,
+	// followed by START condition with a READ (no STOP condition)
+	i2c_start();
+
+	if(!i2c_transmit(ADDR_WRITE(spdaddr))) {
+		i2c_reset();
+		return 0;
+	}
+	if(!i2c_transmit(addr)) {
+		i2c_reset();
+		return 0;
+	}
+
+	I2C_DELAY(1);
+	i2c_start();
+	if(!i2c_transmit(ADDR_READ(spdaddr))) {
+		i2c_reset();
+		return 0;
+	}
+	for (i = 0; i < len; ++i) {
+		buf[i] = i2c_receive(i != len - 1);
+	}
+	i2c_stop();
+
+	return 1;
+}
+#endif /* CSR_I2C_BASE */

litex/soc/software/liblitedram/spd.h

@@ -0,0 +1,8 @@
+#ifndef __SPD_H
+#define __SPD_H
+
+#include <generated/csr.h>
+
+int spdread(unsigned int spdaddr, unsigned int addr, unsigned char *buf, unsigned int len);
+
+#endif /* __SPD_H */