linux-kernel - [PATCH 0/4] i2c: rtl9300: Fix multi-byte I2C operations

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Message-Id: <20250802-i2c-rtl9300-multi-byte-v1-0-5f687e0098e2@narfation.org>
Date: Sat, 02 Aug 2025 12:31:59 +0200
From: Sven Eckelmann <sven@...fation.org>
To: Chris Packham <chris.packham@...iedtelesis.co.nz>, 
 Andi Shyti <andi.shyti@...nel.org>
Cc: linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Jonas Jelonek <jelonek.jonas@...il.com>, 
 Harshal Gohel <hg@...onwunderlich.de>, 
 Simon Wunderlich <sw@...onwunderlich.de>, 
 Sven Eckelmann <sven@...fation.org>, stable@...r.kernel.org
Subject: [PATCH 0/4] i2c: rtl9300: Fix multi-byte I2C operations

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.

Signed-off-by: Sven Eckelmann <sven@...fation.org>
---
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 Write

 drivers/i2c/busses/i2c-rtl9300.c | 43 +++++++++++++++++++++++++++++++++-------
 1 file changed, 36 insertions(+), 7 deletions(-)
---
base-commit: b9ddaa95fd283bce7041550ddbbe7e764c477110
change-id: 20250802-i2c-rtl9300-multi-byte-edaa1fb0872c

Best regards,
-- 
Sven Eckelmann <sven@...fation.org>