i2c: Add software_i2c driver for I2C debugging and emulation

This patch adds I2C emulation in software through raw toggling of the
SDA/SCL lines. Platforms need to provide bindings to toggle their
respective I2C busses for this to work (e.g. by pinmuxing them as GPIOs,
currently only enabled for Tegra).

This is mostly useful as a debugging feature, to drive unusual states on
a bus and closely monitor the device output without the need of a bus
analyzer. It provides a few functions to "wedge" an I2C bus by aborting
a transaction at certain points, which can be used to test if a system
can correctly recover from an ill-timed reboot. However, it can also
dynamically replace the existing I2C transfer functions and drive
some/all I2C transfers on the system, which might be useful if a driver
for the actual I2C controller hardware is not (yet) available.

Based on original code by Doug Anderson <dianders@chromium.org> and
Hung-ying Tyan <tyanh@chromium.org> for the ChromeOS embedded
controller project.

BRANCH=None
BUG=chrome-os-partner:28323
TEST=Spread tegra_software_i2c_init()/tegra_software_i2c_disable()
through the code and see that everything still works.

Original-Change-Id: I9ee7ccbd1efb38206669a35d0c3318af16f8be63
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/198791
Original-Reviewed-by: Doug Anderson <dianders@chromium.org>
Original-Reviewed-by: Tom Warren <twarren@nvidia.com>
Original-Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
(cherry picked from commit 8f71503dbbd74c5298e90e2163b67d4efe3e89db)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>

Change-Id: Id6c5f75bb5baaabd62b6b1fc26c2c71d9f1ce682
Reviewed-on: http://review.coreboot.org/7947
Tested-by: build bot (Jenkins)
Reviewed-by: David Hendricks <dhendrix@chromium.org>

3 files changed, 359 insertions, 0 deletions

diff --git a/src/device/Kconfig b/src/device/Kconfig
index bd2f34b618..67d01e077a 100644
--- a/src/device/Kconfig
+++ b/src/device/Kconfig
@@ -376,6 +376,16 @@ config PXE_ROM_ID
 
 	  Under GNU/Linux you can run `lspci -nn` to list the IDs of your PCI devices.
 
+config SOFTWARE_I2C
+	bool "Enable I2C controller emulation in software"
+	default n
+	help
+	  This config option will enable code to override the i2c_transfer
+	  routine with a (simple) software emulation of the protocol. This may
+	  be useful for debugging or on platforms where a driver for the real
+	  I2C controller is not (yet) available. The platform code needs to
+	  provide bindings to manually toggle I2C lines.
+	  
 endmenu
 
 menu "Display"
diff --git a/src/device/Makefile.inc b/src/device/Makefile.inc
index a7f336489d..7fdd9b587d 100644
--- a/src/device/Makefile.inc
+++ b/src/device/Makefile.inc
@@ -24,3 +24,7 @@ romstage-$(CONFIG_PCI) += pci_early.c
 subdirs-y += oprom dram
 
 ramstage-$(CONFIG_VGA_ROM_RUN) += pci_rom.c
+
+bootblock-$(CONFIG_SOFTWARE_I2C) += software_i2c.c
+romstage-$(CONFIG_SOFTWARE_I2C) += software_i2c.c
+ramstage-$(CONFIG_SOFTWARE_I2C) += software_i2c.c
diff --git a/src/device/software_i2c.c b/src/device/software_i2c.c
new file mode 100644
index 0000000000..6c9c200dc7
--- /dev/null
+++ b/src/device/software_i2c.c
@@ -0,0 +1,345 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * Copyright (C) 2014 Google, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA, 02110-1301 USA
+ */
+
+#include <assert.h>
+#include <timer.h>
+#include <console/console.h>
+#include <device/i2c.h>
+
+/*
+ * The implementation is based on Wikipedia.
+ */
+
+#define DEBUG 0		/* Set to 1 for per-byte output */
+#define SPEW 0		/* Set to 1 for verbose bitwise/line-state output */
+#define DELAY_US 5	/* Default setup delay: 4us (+1 for timer inaccuracy) */
+#define TIMEOUT_US 50000 /* Maximum clock stretching time we want to allow */
+
+#define spew(...) do { if (SPEW) printk(BIOS_SPEW, ##__VA_ARGS__); } while (0)
+
+struct software_i2c_ops *software_i2c[SOFTWARE_I2C_MAX_BUS];
+
+/*
+ * Waits until either timeout_us have passed or (iff for_scl is set) until SCL
+ * goes high. Will report random line changes during the wait and return SCL.
+ */
+static int __wait(unsigned bus, int timeout_us, int for_scl)
+{
+	int us;
+	int sda = software_i2c[bus]->get_sda(bus);
+	int scl = software_i2c[bus]->get_scl(bus);
+	struct mono_time start;
+	timer_monotonic_get(&start);
+
+	do {
+		int old_sda = sda;
+		int old_scl = scl;
+		struct rela_time diff = current_time_from(&start);
+		us = rela_time_in_microseconds(&diff);
+
+		if (old_sda != (sda = software_i2c[bus]->get_sda(bus)))
+			spew("[SDA transitioned to %d after %dus] ", sda, us);
+		if (old_scl != (scl = software_i2c[bus]->get_scl(bus)))
+			spew("[SCL transitioned to %d after %dus] ", scl, us);
+	} while (us < timeout_us && (!for_scl || !scl));
+
+	return scl;
+}
+
+/* Waits the default DELAY_US to allow line state to stabilize. */
+static void wait(unsigned bus)
+{
+	__wait(bus, DELAY_US, 0);
+}
+
+/* Waits until SCL goes high. Prints a contextual error message on timeout. */
+static int wait_for_scl(unsigned bus, const char *error_context)
+{
+	if (!__wait(bus, TIMEOUT_US, 1)) {
+		printk(BIOS_ERR, "software_i2c(%d): ERROR: Clock stretching "
+				 "timeout %s!\n", bus, error_context);
+		return -1;
+	}
+
+	return 0;
+}
+
+static int start_cond(unsigned bus)
+{
+	spew("software_i2c(%d): Sending start condition... ", bus);
+
+	/* SDA might not yet be high if repeated start. */
+	software_i2c[bus]->set_sda(bus, 1);
+	wait(bus);
+
+	/* Might need to wait for clock stretching if repeated start. */
+	software_i2c[bus]->set_scl(bus, 1);
+	if (wait_for_scl(bus, "before start condition"))
+		return -1;
+	wait(bus);	/* Repeated start setup time, minimum 4.7us */
+
+	if (!software_i2c[bus]->get_sda(bus)) {
+		printk(BIOS_ERR, "software_i2c(%d): Arbitration lost trying "
+			"to send start condition!\n", bus);
+		return -1;
+	}
+
+	/* SCL is high, transition SDA low as first part of start condition. */
+	software_i2c[bus]->set_sda(bus, 0);
+	wait(bus);
+	assert(software_i2c[bus]->get_scl(bus));
+
+	/* Pull SCL low to finish start condition (next pulse will be data). */
+	software_i2c[bus]->set_scl(bus, 0);
+
+	spew("Start condition transmitted!\n");
+	return 0;
+}
+
+static int stop_cond(unsigned bus)
+{
+	spew("software_i2c(%d): Sending stop condition... ", bus);
+
+	/* SDA is unknown, set it to low. SCL must be low. */
+	software_i2c[bus]->set_sda(bus, 0);
+	wait(bus);
+
+	/* Clock stretching */
+	assert(!software_i2c[bus]->get_scl(bus));
+	software_i2c[bus]->set_scl(bus, 1);
+	if (wait_for_scl(bus, "before stop condition"))
+		return -1;
+	wait(bus);	/* Stop bit setup time, minimum 4us */
+
+	/* SCL is high, transition SDA high to signal stop condition. */
+	software_i2c[bus]->set_sda(bus, 1);
+	wait(bus);
+	if (!software_i2c[bus]->get_sda(bus)) {
+		printk(BIOS_WARNING, "software_i2c(%d): WARNING: SDA low after "
+			"stop condition... access by another master or line "
+			"stuck from faulty slave?\n", bus);
+		/* Could theoretically happen with multi-master, so no -1. */
+	}
+
+	spew("Stop condition transmitted\n");
+	return 0;
+}
+
+static int out_bit(unsigned bus, int bit)
+{
+	spew("software_i2c(%d): Sending a %d bit... ", bus, bit);
+
+	software_i2c[bus]->set_sda(bus, bit);
+	wait(bus);
+
+	if (bit && !software_i2c[bus]->get_sda(bus)) {
+		printk(BIOS_ERR, "software_i2c(%d): ERROR: SDA wedged low "
+			"by slave before clock pulse on transmit!\n", bus);
+		return -1;
+	}
+
+	/* Clock stretching */
+	assert(!software_i2c[bus]->get_scl(bus));
+	software_i2c[bus]->set_scl(bus, 1);
+	if (wait_for_scl(bus, "on transmit"))
+		return -1;
+	wait(bus);
+
+	if (bit && !software_i2c[bus]->get_sda(bus)) {
+		printk(BIOS_ERR, "software_i2c(%d): ERROR: SDA wedged low "
+			"by slave after clock pulse on transmit!\n", bus);
+		return -1;
+	}
+
+	assert(software_i2c[bus]->get_scl(bus));
+	software_i2c[bus]->set_scl(bus, 0);
+
+	spew("%d bit sent!\n", bit);
+	return 0;
+}
+
+static int in_bit(unsigned bus)
+{
+	int bit;
+
+	spew("software_i2c(%d): Receiving a bit... ", bus);
+
+	/* Let the slave drive data */
+	software_i2c[bus]->set_sda(bus, 1);
+	wait(bus);
+
+	/* Clock stretching */
+	assert(!software_i2c[bus]->get_scl(bus));
+	software_i2c[bus]->set_scl(bus, 1);
+	if (wait_for_scl(bus, "on receive"))
+		return -1;
+
+	/* SCL is high, now data is valid */
+	bit = software_i2c[bus]->get_sda(bus);
+	wait(bus);
+	assert(software_i2c[bus]->get_scl(bus));
+	software_i2c[bus]->set_scl(bus, 0);
+
+	spew("Received a %d!\n", bit);
+
+	return bit;
+}
+
+/* Write a byte to I2C bus. Return 0 if ack by the slave. */
+static int out_byte(unsigned bus, u8 byte)
+{
+	unsigned bit;
+	int nack;
+
+	for (bit = 0; bit < 8; bit++)
+		if (out_bit(bus, (byte >> (7 - bit)) & 0x1) < 0)
+			return -1;
+
+	nack = in_bit(bus);
+
+	if (DEBUG && nack >= 0)
+		printk(BIOS_DEBUG, "software_i2c(%d): wrote byte 0x%02x, "
+			"received %s\n", bus, byte, nack ? "NAK" : "ACK");
+
+	return nack;
+}
+
+static int in_byte(unsigned bus, int ack)
+{
+	u8 byte = 0;
+	int i;
+	for (i = 0; i < 8; ++i) {
+		int bit = in_bit(bus);
+		if (bit < 0)
+			return -1;
+		byte = (byte << 1) | bit;
+	}
+
+	if (out_bit(bus, !ack) < 0)
+		return -1;
+
+	if (DEBUG)
+		printk(BIOS_DEBUG, "software_i2c(%d): read byte 0x%02x, "
+			"sent %s\n", bus, byte, ack ? "ACK" : "NAK");
+
+	return byte;
+}
+
+int software_i2c_transfer(unsigned bus, struct i2c_seg *segments, int count)
+{
+	int i;
+	struct i2c_seg *seg;
+
+	for (seg = segments; seg - segments < count; seg++) {
+		if (start_cond(bus) < 0)
+			return -1;
+		if (out_byte(bus, seg->chip << 1 | !!seg->read) < 0)
+			return -1;
+		for (i = 0; i < seg->len; i++) {
+			int ret;
+			if (seg->read) {
+				ret = in_byte(bus, i < seg->len - 1);
+				seg->buf[i] = (u8)ret;
+			} else {
+				ret = out_byte(bus, seg->buf[i]);
+			}
+			if (ret < 0)
+				return -1;
+		}
+	}
+	if (stop_cond(bus) < 0)
+		return -1;
+
+	return 0;
+}
+
+void software_i2c_wedge_ack(unsigned bus, u8 chip)
+{
+	int i;
+
+	/* Start a command to 'chip'... */
+	start_cond(bus);
+
+	/* Send the address bits but don't yet read the ACK. */
+	chip <<= 1;
+	for (i = 0; i < 8; ++i)
+		out_bit(bus, (chip >> (7 - i)) & 0x1);
+
+	/* Let the slave drive it's ACK but keep the clock high forever. */
+	software_i2c[bus]->set_sda(bus, 1);
+	wait(bus);
+	software_i2c[bus]->set_scl(bus, 1);
+	wait_for_scl(bus, "on wedge_ack()");
+
+	printk(BIOS_INFO, "software_i2c(%d): wedged address write on slave "
+		"ACK. SDA %d, SCL %d\n", bus, software_i2c[bus]->get_sda(bus),
+		software_i2c[bus]->get_scl(bus));
+}
+
+void software_i2c_wedge_read(unsigned bus, u8 chip, u8 reg, int bits)
+{
+	int i;
+
+	/* Start a command to 'chip'... */
+	start_cond(bus);
+	out_byte(bus, chip << 1);
+	/* ...for register 'reg'. */
+	out_byte(bus, reg);
+
+	/* Start a read command... */
+	start_cond(bus);
+	out_byte(bus, chip << 1 | 1);
+
+	/* Read bit_count bits and stop */
+	for (i = 0; i < bits; ++i)
+		in_bit(bus);
+
+	/* Let the slave drive SDA but keep the clock high forever. */
+	software_i2c[bus]->set_sda(bus, 1);
+	wait(bus);
+	software_i2c[bus]->set_scl(bus, 1);
+	wait_for_scl(bus, "on wedge_read()");
+
+	printk(BIOS_INFO, "software_i2c(%d): wedged data read after %d bits. "
+		"SDA %d, SCL %d\n", bus, bits, software_i2c[bus]->get_sda(bus),
+		software_i2c[bus]->get_scl(bus));
+}
+
+void software_i2c_wedge_write(unsigned bus, u8 chip, u8 reg, int bits)
+{
+	int i;
+
+	/* Start a command to 'chip'... */
+	start_cond(bus);
+	out_byte(bus, chip << 1);
+
+	/* Write bit_count register bits and stop */
+	for (i = 0; i < bits; ++i)
+		out_bit(bus, (reg >> (7 - i)) & 0x1);
+
+	/* Pretend to write another 1 bit but keep the clock high forever. */
+	software_i2c[bus]->set_sda(bus, 1);
+	wait(bus);
+	software_i2c[bus]->set_scl(bus, 1);
+	wait_for_scl(bus, "on wedge_write()");
+
+	printk(BIOS_INFO, "software_i2c(%d): wedged data write after %d bits. "
+		"SDA %d, SCL %d\n", bus, bits, software_i2c[bus]->get_sda(bus),
+		software_i2c[bus]->get_scl(bus));
+}