During the integration of the RTL8239 POE chip + its frontend MCU, it was noticed that multi-byte operations were basically broken in the current driver.
Tests using SMBus Block Writes showed that the data (after the Wr + Ack marker) was mixed up on the wire. At first glance, it looked like an endianness problem. But for transfers were the number of count + data bytes was not divisible by 4, the last bytes were not looking like an endianness problem because they were were in the wrong order but not for example 0 - which would be the case for an endianness problem with 32 bit registers. At the end, it turned out to be a the way how i2c_write tried to add the bytes to the send registers.
Each 32 bit register was used similar to a shift register - shifting the various bytes up the register while the next one is added to the least significant byte. But the I2C controller expects the first byte of the tranmission in the least significant byte of the first register. And the last byte (assuming it is a 16 byte transfer) in the most significant byte of the fourth register.
While doing these tests, it was also observed that the count byte was missing from the SMBus Block Writes. The driver just removed them from the data->block (from the I2C subsystem). But the I2C controller DOES NOT automatically add this byte - for example by using the configured transmission length.
The RTL8239 MCU is not actually an SMBus compliant device. Instead, it expects I2C Block Reads + I2C Block Writes. But according to the already identified bugs in the driver, it was clear that the I2C controller can simply be modified to not send the count byte for I2C_SMBUS_I2C_BLOCK_DATA. The receive part, just needs to write the content of the receive buffer to the correct position in data->block.
While the on-wire formwat was now correct, reads were still not possible against the MCU (for the RTL8239 POE chip). It was always timing out because the 2ms were not enough for sending the read request and then receiving the 12 byte answer.
These changes were originally submitted to OpenWrt. But there are plans to migrate OpenWrt to the upstream Linux driver. As result, the pull request was stopped and the changes were redone against this driver.
For reasons of transparency: The work on I2C_SMBUS_I2C_BLOCK_DATA support for the RTL8239-MCU was done on RTL931xx. All problem were therefore detected with the patches from Jonas Jelonek [1] and not the vanilla Linux driver. But looking through the code, it seems like these are NOT regressions introduced by the RTL931x patchset.
I've picked up Alex Guo's patch [2] to reduce conflicts between pending fixes.
[1] https://patchwork.ozlabs.org/project/linux-i2c/cover/20250727114800.3046-1-j... [2] https://lore.kernel.org/r/20250615235248.529019-1-alexguo1023@gmail.com
Signed-off-by: Sven Eckelmann sven@narfation.org --- Changes in v3: - integrated patch https://lore.kernel.org/r/20250615235248.529019-1-alexguo1023@gmail.com to avoid conflicts in the I2C_SMBUS_BLOCK_DATA code - added Fixes and stable@vger.kernel.org to Alex Guo's patch - added Chris Packham's Reviewed-by/Acked-by - Link to v2: https://lore.kernel.org/r/20250803-i2c-rtl9300-multi-byte-v2-0-9b7b759fe2b6@...
Changes in v2: - add the missing transfer width and read length increase for the SMBus Write/Read - Link to v1: https://lore.kernel.org/r/20250802-i2c-rtl9300-multi-byte-v1-0-5f687e0098e2@...
--- Alex Guo (1): i2c: rtl9300: Fix out-of-bounds bug in rtl9300_i2c_smbus_xfer
Harshal Gohel (2): i2c: rtl9300: Fix multi-byte I2C write i2c: rtl9300: Implement I2C block read and write
Sven Eckelmann (2): i2c: rtl9300: Increase timeout for transfer polling i2c: rtl9300: Add missing count byte for SMBus Block Ops
drivers/i2c/busses/i2c-rtl9300.c | 51 ++++++++++++++++++++++++++++++++++------ 1 file changed, 44 insertions(+), 7 deletions(-) --- base-commit: 0ae982df67760cd08affa935c0fe86c8a9311797 change-id: 20250802-i2c-rtl9300-multi-byte-edaa1fb0872c
Best regards,