I2C BUFFER_RW transaction does not use Repeated Start (Sr) on GB202/Blackwell via GSP firmware

[Loong0x00]

Summary

On GB202 (RTX 5090) with driver 590.48.01, all I2C read commands that should use Repeated Start (Sr) between the write phase (register address) and read phase actually use STOP + START. This breaks communication with I2C slave devices that strictly require Sr per the I2C specification, such as the Bosch BMI323 IMU.

Expected Behavior

According to ctrl402c.h (line 439-441), NV402C_CTRL_I2C_TRANSACTION_TYPE_I2C_BUFFER_RW should produce:

I2C Buffer Read: S Addr|Wr [A] cmd [A] Sr Addr|Rd [A] Data1 A...DataN P

The Sr (Repeated Start) between the write and read phases is critical for devices like BMI323 that reset their register pointer on STOP.

Actual Behavior

The actual bus sequence is:

S Addr|Wr [A] cmd [A] P    S Addr|Rd [A] Data1 A...DataN P

STOP is sent after the register address write, followed by a new START for the read. The slave device loses the register pointer and returns zeros (or default values).

Evidence

• BMI323 at I2C address 0x68 on port 1 ACKs correctly but returns [01 00 00 00] for all registers (0x00-0x7E) — identical data regardless of register address proves the register pointer is not being set
• IT8915FN at 0x2b returns real data because it tolerates STOP+START (retains register pointer across transactions)
• Both NV402C_CTRL_CMD_I2C_INDEXED (0x402c0102) and NV402C_CTRL_CMD_I2C_TRANSACTION with I2C_BUFFER_RW (type 3) produce identical results, confirming they share the same backend path through GSP
• The same hardware works correctly on Windows via NvAPI_I2CRead (GPU Tweak III reads BMI323 successfully)

Commands Tested

Command	Code	Result
NV402C_CTRL_CMD_I2C_INDEXED	0x402c0102	ACK but zeros (no Sr)
NV402C_CTRL_CMD_I2C_TRANSACTION (I2C_BUFFER_RW)	0x402c0105	ACK but zeros (no Sr)
NV2080_CTRL_CMD_I2C_READ_BUFFER	0x20800601	ACK but zeros (no Sr)
NV2080_CTRL_CMD_I2C_ACCESS	0x20800610	Returns 0x1f (INVALID_STATE)
Linux I2C_RDWR via i2c-dev	—	Zeros (master_xfer splits msgs)

Additional Note: nv-i2c.c master_xfer splits combined messages

In kernel-open/nvidia/nv-i2c.c, nv_i2c_algo_master_xfer processes each i2c_msg as a separate rm_i2c_transfer call in a for loop, which means Linux I2C combined transactions (which should use Sr) are broken at the adapter level too.

Environment

• GPU: ASUS ROG Astral RTX 5090 (GB202)
• Driver: 590.48.01 (open kernel modules)
• OS: Arch Linux, kernel 6.19.6
• GSP firmware: 570.144

Impact

This affects any I2C slave device on the GPU PCB that requires Repeated Start. Known affected: Bosch BMI323 IMU (used by ASUS ROG Astral for GPU sag detection). The sensor is completely unreadable on Linux while working fine on Windows.