[PATCH 2/4] i2c: tegra: Disable fair arbitration for non-MCTP buses
From: Akhil R
Date: Tue May 05 2026 - 07:29:28 EST
Recent Tegra I2C controllers have a fairness arbitration register, which
allows configuring the fair idle time required to support MCTP protocol
over I2C. It is enabled by default, adding a per-transfer latency overhead
that impacts non-MCTP I2C buses.
Disable the fairness arbitration register during controller init for buses
that are not MCTP controllers.
Signed-off-by: Akhil R <akhilrajeev@xxxxxxxxxx>
Assisted-by: Cursor:claude-4.6-opus
---
drivers/i2c/busses/i2c-tegra.c | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)
diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
index a21f6457d41b..1d274431e209 100644
--- a/drivers/i2c/busses/i2c-tegra.c
+++ b/drivers/i2c/busses/i2c-tegra.c
@@ -164,6 +164,7 @@ struct tegra_i2c_regs {
unsigned int master_reset_cntrl;
unsigned int mst_fifo_control;
unsigned int mst_fifo_status;
+ unsigned int fairness_arb;
unsigned int sw_mutex;
};
@@ -272,6 +273,7 @@ static const struct tegra_i2c_regs tegra264_i2c_regs = {
.master_reset_cntrl = 0x0a8,
.mst_fifo_control = 0x0b4,
.mst_fifo_status = 0x0b8,
+ .fairness_arb = 0x0e8,
.sw_mutex = 0x0ec,
};
@@ -300,6 +302,7 @@ static const struct tegra_i2c_regs tegra410_i2c_regs = {
.master_reset_cntrl = 0x0ac,
.mst_fifo_control = 0x0b8,
.mst_fifo_status = 0x0bc,
+ .fairness_arb = 0x0ec,
.sw_mutex = 0x0f0,
};
@@ -379,6 +382,7 @@ enum tegra_i2c_variant {
* timing settings.
* @enable_hs_mode_support: Enable support for high speed (HS) mode transfers.
* @has_mutex: Has mutex register for mutual exclusion with other firmwares or VMs.
+ * @has_fairarb_reg: Has fairness arbitration register for SMBUS/MCTP support.
* @variant: This represents the I2C controller variant.
* @regs: Register offsets for the specific SoC variant.
*/
@@ -412,6 +416,7 @@ struct tegra_i2c_hw_feature {
bool has_interface_timing_reg;
bool enable_hs_mode_support;
bool has_mutex;
+ bool has_fairarb_reg;
enum tegra_i2c_variant variant;
const struct tegra_i2c_regs *regs;
};
@@ -476,6 +481,7 @@ struct tegra_i2c_dev {
void *dma_buf;
bool multimaster_mode;
+ bool is_mctp;
bool atomic_mode;
bool dma_mode;
bool msg_read;
@@ -914,6 +920,10 @@ static int tegra_i2c_init(struct tegra_i2c_dev *i2c_dev)
if (IS_VI(i2c_dev))
tegra_i2c_vi_init(i2c_dev);
+ /* Disable fairness arbitration if not an MCTP controller */
+ if (i2c_dev->hw->has_fairarb_reg && !i2c_dev->is_mctp)
+ i2c_writel(i2c_dev, 0, i2c_dev->hw->regs->fairness_arb);
+
if (i2c_dev->hw->enable_hs_mode_support)
max_bus_freq_hz = I2C_MAX_HIGH_SPEED_MODE_FREQ;
else
@@ -1779,6 +1789,7 @@ static const struct tegra_i2c_hw_feature tegra20_i2c_hw = {
.has_interface_timing_reg = false,
.enable_hs_mode_support = false,
.has_mutex = false,
+ .has_fairarb_reg = false,
.variant = TEGRA_I2C_VARIANT_DEFAULT,
.regs = &tegra20_i2c_regs,
};
@@ -1812,6 +1823,7 @@ static const struct tegra_i2c_hw_feature tegra20_dvc_i2c_hw = {
.has_interface_timing_reg = false,
.enable_hs_mode_support = false,
.has_mutex = false,
+ .has_fairarb_reg = false,
.variant = TEGRA_I2C_VARIANT_DVC,
.regs = &tegra20_dvc_i2c_regs,
};
@@ -1845,6 +1857,7 @@ static const struct tegra_i2c_hw_feature tegra30_i2c_hw = {
.has_interface_timing_reg = false,
.enable_hs_mode_support = false,
.has_mutex = false,
+ .has_fairarb_reg = false,
.variant = TEGRA_I2C_VARIANT_DEFAULT,
.regs = &tegra20_i2c_regs,
};
@@ -1877,6 +1890,7 @@ static const struct tegra_i2c_hw_feature tegra114_i2c_hw = {
.has_interface_timing_reg = false,
.enable_hs_mode_support = false,
.has_mutex = false,
+ .has_fairarb_reg = false,
.variant = TEGRA_I2C_VARIANT_DEFAULT,
.regs = &tegra20_i2c_regs,
};
@@ -1909,6 +1923,7 @@ static const struct tegra_i2c_hw_feature tegra124_i2c_hw = {
.has_interface_timing_reg = true,
.enable_hs_mode_support = false,
.has_mutex = false,
+ .has_fairarb_reg = false,
.variant = TEGRA_I2C_VARIANT_DEFAULT,
.regs = &tegra20_i2c_regs,
};
@@ -1941,6 +1956,7 @@ static const struct tegra_i2c_hw_feature tegra210_i2c_hw = {
.has_interface_timing_reg = true,
.enable_hs_mode_support = false,
.has_mutex = false,
+ .has_fairarb_reg = false,
.variant = TEGRA_I2C_VARIANT_DEFAULT,
.regs = &tegra20_i2c_regs,
};
@@ -1974,6 +1990,7 @@ static const struct tegra_i2c_hw_feature tegra210_vi_i2c_hw = {
.has_interface_timing_reg = true,
.enable_hs_mode_support = false,
.has_mutex = false,
+ .has_fairarb_reg = false,
.variant = TEGRA_I2C_VARIANT_VI,
.regs = &tegra210_vi_i2c_regs,
};
@@ -2007,6 +2024,7 @@ static const struct tegra_i2c_hw_feature tegra186_i2c_hw = {
.has_interface_timing_reg = true,
.enable_hs_mode_support = false,
.has_mutex = false,
+ .has_fairarb_reg = false,
.variant = TEGRA_I2C_VARIANT_DEFAULT,
.regs = &tegra20_i2c_regs,
};
@@ -2041,6 +2059,7 @@ static const struct tegra_i2c_hw_feature tegra194_i2c_hw = {
.has_interface_timing_reg = true,
.enable_hs_mode_support = true,
.has_mutex = false,
+ .has_fairarb_reg = false,
.variant = TEGRA_I2C_VARIANT_DEFAULT,
.regs = &tegra20_i2c_regs,
};
@@ -2075,6 +2094,7 @@ static const struct tegra_i2c_hw_feature tegra256_i2c_hw = {
.has_interface_timing_reg = true,
.enable_hs_mode_support = true,
.has_mutex = true,
+ .has_fairarb_reg = true,
.variant = TEGRA_I2C_VARIANT_DEFAULT,
.regs = &tegra264_i2c_regs,
};
@@ -2109,6 +2129,7 @@ static const struct tegra_i2c_hw_feature tegra264_i2c_hw = {
.has_interface_timing_reg = true,
.enable_hs_mode_support = true,
.has_mutex = true,
+ .has_fairarb_reg = true,
.variant = TEGRA_I2C_VARIANT_DEFAULT,
.regs = &tegra264_i2c_regs,
};
@@ -2143,6 +2164,7 @@ static const struct tegra_i2c_hw_feature tegra410_i2c_hw = {
.has_interface_timing_reg = true,
.enable_hs_mode_support = true,
.has_mutex = true,
+ .has_fairarb_reg = true,
.variant = TEGRA_I2C_VARIANT_DEFAULT,
.regs = &tegra410_i2c_regs,
};
@@ -2175,6 +2197,7 @@ static void tegra_i2c_parse_dt(struct tegra_i2c_dev *i2c_dev)
multi_mode = device_property_read_bool(i2c_dev->dev, "multi-master");
i2c_dev->multimaster_mode = multi_mode;
+ i2c_dev->is_mctp = device_property_present(i2c_dev->dev, "mctp-controller");
}
static int tegra_i2c_init_clocks(struct tegra_i2c_dev *i2c_dev)
--
2.50.1