[PATCH v12 5/6] i2c: tegra: Add support for SW mutex register

Akhil R
2025-11-15T04:28:22Z

From: Kartik Rajput <kkartik@nvidia.com>

Add support for SW mutex register introduced in Tegra264 to provide
an option to share the interface between multiple firmwares and/or
VMs. This involves following steps:

- A firmware/OS writes its unique ID to the mutex REQUEST field.
- Ownership is established when reading the GRANT field returns the
same ID.
- If GRANT shows a different non-zero ID, the firmware/OS retries
until timeout.
- After completing access, it releases the mutex by writing 0.

However, the hardware does not ensure any protection based on the
values. The driver/firmware should honor the peer who already holds
the mutex.

Signed-off-by: Kartik Rajput <kkartik@nvidia.com>
Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
---
v7 -> v8:
* Use `bool` instead of `int` for `locked` variable in
tegra_i2c_mutex_lock() function.
v6 -> v7:
* Return bool from tegra_i2c_mutex_acquired() and
tegra_i2c_mutex_trylock() functions.
* Move `has_mutex` check inside tegra_i2c_mutex_lock/unlock
functions.
* Remove redundant empty line added in tegra_i2c_xfer() in v6.
* Fix pm_runtime_put() not getting called if mutex unlock fails.
* In tegra_i2c_mutex_lock() simplify the logic to check if the
mutex is acquired or not by checking the value of `ret`
variable.
* Update commit message to describe the functioning of SW mutex
feature.
v4 -> v6:
* Guard tegra_i2c_mutex_lock() and tegra_i2c_mutex_unlock() to
ensure that they are called on platforms which support SW
mutex.
v3 -> v4:
* Update timeout logic of tegra_i2c_mutex_lock() to use
read_poll_timeout APIs for improving timeout logic.
* Add tegra_i2c_mutex_acquired() to check if mutex is acquired
or not.
* Rename I2C_SW_MUTEX_ID as I2C_SW_MUTEX_ID_CCPLEX.
* Function tegra_i2c_poll_register() was moved unnecessarily, it
has now been moved to its original location.
* Use tegra_i2c_mutex_lock/unlock APIs in the tegra_i2c_xfer()
function. This ensures proper propagation of error in case
mutex lock fails.
Please note that as the function tegra_i2c_xfer() is
already guarded by the bus lock operation there is no need of
additional lock for the tegra_i2c_mutex_lock/unlock APIs.
v2 -> v3:
* Update tegra_i2c_mutex_trylock and tegra_i2c_mutex_unlock to
use readl and writel APIs instead of i2c_readl and i2c_writel
which use relaxed APIs.
* Use dev_warn instead of WARN_ON if mutex lock/unlock fails.
v1 -> v2:
* Fixed typos.
* Fix tegra_i2c_mutex_lock() logic.
* Add a timeout in tegra_i2c_mutex_lock() instead of polling for
mutex indefinitely.
---
drivers/i2c/busses/i2c-tegra.c | 93 ++++++++++++++++++++++++++++++++++
1 file changed, 93 insertions(+)

diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
index 9ebeb6a2eaf5..135883388c1e 100644
--- a/drivers/i2c/busses/i2c-tegra.c
+++ b/drivers/i2c/busses/i2c-tegra.c
@@ -137,6 +137,14 @@

#define I2C_MASTER_RESET_CNTRL 0x0a8

+#define I2C_SW_MUTEX 0x0ec
+#define I2C_SW_MUTEX_REQUEST GENMASK(3, 0)
+#define I2C_SW_MUTEX_GRANT GENMASK(7, 4)
+#define I2C_SW_MUTEX_ID_CCPLEX 9
+
+/* SW mutex acquire timeout value in microseconds. */
+#define I2C_SW_MUTEX_TIMEOUT_US (25 * USEC_PER_MSEC)
+
/* configuration load timeout in microseconds */
#define I2C_CONFIG_LOAD_TIMEOUT 1000000

@@ -214,6 +222,7 @@ enum msg_end_type {
* @has_interface_timing_reg: Has interface timing register to program the tuned
* timing settings.
* @has_hs_mode_support: Has support for high speed (HS) mode transfers.
+ * @has_mutex: Has mutex register for mutual exclusion with other firmwares or VMs.
*/
struct tegra_i2c_hw_feature {
bool has_continue_xfer_support;
@@ -244,6 +253,7 @@ struct tegra_i2c_hw_feature {
u32 setup_hold_time_hs_mode;
bool has_interface_timing_reg;
bool has_hs_mode_support;
+ bool has_mutex;
};

/**
@@ -388,6 +398,76 @@ static void i2c_readsl(struct tegra_i2c_dev *i2c_dev, void *data,
readsl(i2c_dev->base + tegra_i2c_reg_addr(i2c_dev, reg), data, len);
}

+static bool tegra_i2c_mutex_acquired(struct tegra_i2c_dev *i2c_dev)
+{
+ unsigned int reg = tegra_i2c_reg_addr(i2c_dev, I2C_SW_MUTEX);
+ u32 val, id;
+
+ val = readl(i2c_dev->base + reg);
+ id = FIELD_GET(I2C_SW_MUTEX_GRANT, val);
+
+ return id == I2C_SW_MUTEX_ID_CCPLEX;
+}
+
+static bool tegra_i2c_mutex_trylock(struct tegra_i2c_dev *i2c_dev)
+{
+ unsigned int reg = tegra_i2c_reg_addr(i2c_dev, I2C_SW_MUTEX);
+ u32 val, id;
+
+ val = readl(i2c_dev->base + reg);
+ id = FIELD_GET(I2C_SW_MUTEX_GRANT, val);
+ if (id != 0 && id != I2C_SW_MUTEX_ID_CCPLEX)
+ return false;
+
+ val = FIELD_PREP(I2C_SW_MUTEX_REQUEST, I2C_SW_MUTEX_ID_CCPLEX);
+ writel(val, i2c_dev->base + reg);
+
+ return tegra_i2c_mutex_acquired(i2c_dev);
+}
+
+static int tegra_i2c_mutex_lock(struct tegra_i2c_dev *i2c_dev)
+{
+ bool locked;
+ int ret;
+
+ if (!i2c_dev->hw->has_mutex)
+ return 0;
+
+ if (i2c_dev->atomic_mode)
+ ret = read_poll_timeout_atomic(tegra_i2c_mutex_trylock, locked, locked,
+ USEC_PER_MSEC, I2C_SW_MUTEX_TIMEOUT_US,
+ false, i2c_dev);
+ else
+ ret = read_poll_timeout(tegra_i2c_mutex_trylock, locked, locked, USEC_PER_MSEC,
+ I2C_SW_MUTEX_TIMEOUT_US, false, i2c_dev);
+
+ if (ret)
+ dev_warn(i2c_dev->dev, "failed to acquire mutex\n");
+
+ return ret;
+}
+
+static int tegra_i2c_mutex_unlock(struct tegra_i2c_dev *i2c_dev)
+{
+ unsigned int reg = tegra_i2c_reg_addr(i2c_dev, I2C_SW_MUTEX);
+ u32 val, id;
+
+ if (!i2c_dev->hw->has_mutex)
+ return 0;
+
+ val = readl(i2c_dev->base + reg);
+
+ id = FIELD_GET(I2C_SW_MUTEX_GRANT, val);
+ if (id && id != I2C_SW_MUTEX_ID_CCPLEX) {
+ dev_warn(i2c_dev->dev, "unable to unlock mutex, mutex is owned by: %u\n", id);
+ return -EPERM;
+ }
+
+ writel(0, i2c_dev->base + reg);
+
+ return 0;
+}
+
static void tegra_i2c_mask_irq(struct tegra_i2c_dev *i2c_dev, u32 mask)
{
u32 int_mask;
@@ -1443,6 +1523,10 @@ static int tegra_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[],
return ret;
}

+ ret = tegra_i2c_mutex_lock(i2c_dev);
+ if (ret)
+ return ret;
+
for (i = 0; i < num; i++) {
enum msg_end_type end_type = MSG_END_STOP;

@@ -1472,6 +1556,7 @@ static int tegra_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[],
break;
}

+ ret = tegra_i2c_mutex_unlock(i2c_dev);
pm_runtime_put(i2c_dev->dev);

return ret ?: i;
@@ -1551,6 +1636,7 @@ static const struct tegra_i2c_hw_feature tegra20_i2c_hw = {
.setup_hold_time_hs_mode = 0x0,
.has_interface_timing_reg = false,
.has_hs_mode_support = false,
+ .has_mutex = false,
};

static const struct tegra_i2c_hw_feature tegra30_i2c_hw = {
@@ -1580,6 +1666,7 @@ static const struct tegra_i2c_hw_feature tegra30_i2c_hw = {
.setup_hold_time_hs_mode = 0x0,
.has_interface_timing_reg = false,
.has_hs_mode_support = false,
+ .has_mutex = false,
};

static const struct tegra_i2c_hw_feature tegra114_i2c_hw = {
@@ -1609,6 +1696,7 @@ static const struct tegra_i2c_hw_feature tegra114_i2c_hw = {
.setup_hold_time_hs_mode = 0x0,
.has_interface_timing_reg = false,
.has_hs_mode_support = false,
+ .has_mutex = false,
};

static const struct tegra_i2c_hw_feature tegra124_i2c_hw = {
@@ -1638,6 +1726,7 @@ static const struct tegra_i2c_hw_feature tegra124_i2c_hw = {
.setup_hold_time_hs_mode = 0x0,
.has_interface_timing_reg = true,
.has_hs_mode_support = false,
+ .has_mutex = false,
};

static const struct tegra_i2c_hw_feature tegra210_i2c_hw = {
@@ -1667,6 +1756,7 @@ static const struct tegra_i2c_hw_feature tegra210_i2c_hw = {
.setup_hold_time_hs_mode = 0,
.has_interface_timing_reg = true,
.has_hs_mode_support = false,
+ .has_mutex = false,
};

static const struct tegra_i2c_hw_feature tegra186_i2c_hw = {
@@ -1696,6 +1786,7 @@ static const struct tegra_i2c_hw_feature tegra186_i2c_hw = {
.setup_hold_time_hs_mode = 0,
.has_interface_timing_reg = true,
.has_hs_mode_support = false,
+ .has_mutex = false,
};

static const struct tegra_i2c_hw_feature tegra194_i2c_hw = {
@@ -1727,6 +1818,7 @@ static const struct tegra_i2c_hw_feature tegra194_i2c_hw = {
.setup_hold_time_hs_mode = 0x090909,
.has_interface_timing_reg = true,
.has_hs_mode_support = true,
+ .has_mutex = false,
};
static const struct tegra_i2c_hw_feature tegra256_i2c_hw = {
.has_continue_xfer_support = true,
@@ -1757,6 +1849,7 @@ static const struct tegra_i2c_hw_feature tegra256_i2c_hw = {
.setup_hold_time_hs_mode = 0x030303,
.has_interface_timing_reg = true,
.has_hs_mode_support = true,
+ .has_mutex = true,
};

static const struct of_device_id tegra_i2c_of_match[] = {
--
2.50.1