comparing_stm32_i2c_v2_multi_master.md

comparing

• (a) https://patchwork.kernel.org/patch/10205401/ and
• (b) https://github.com/DWagenk/zephyr/tree/stm32_i2c_v2_multi_master

relevant for zephyrproject-rtos/zephyr#5224

relevant functions

(a)	(b)
stm32f7_i2c_xfer	i2c_stm32_transfer (i2c_ll_stm32.c)
stm32f7_i2c_xfer_msg	msg_init
stm32f7_i2c_isr_event	stm32_i2c_event_isr
stm32f7_i2c_isr_error	stm32_i2c_error_isr
stm32f7_i2c_slave_isr_event	stm32_i2c_slave_event and stm32_i2c_slave_error

• (a) switches to master_mode in stm32f7_i2c_xfer_msg, directly before generating start condition on bus

  (b) does it already when preparing the messages in i2c_stm32_transfer

• (a) exits master_mode in case of arbitration lost or when a stop condition is issued on the bus, both directly inside the isr

  (b) exits master_mode in case of arbitration lost or when all messages that were passed to i2c_stm32_transfer have been transfered.

• (a) returns from stm32f7_i2c_xfer with -EAGAIN in case of arbitration lost

  (b) returns from i2c_stm32_transfer with -EIO in case of arbitration lost

• both (a) and (b) could loose arbitration in the middle of multiple messages passed to stm32f7_i2c_xfer/i2c_stm32_transfer, if there's a stop condition somewhere inbetween them. Caller of stm32f7_i2c_xfer/i2c_stm32_transfer will not know if none, or just parts of the messages have been transfered.

  --> this should be declared undefined behaviour! A stop condition is only allowed in the last message passed to stm32f7_i2c_xfer/i2c_stm32_transfer

• (a) uses SlaveByteControl mode to ack/nack each byte, (b) doesn't

• (a) flushes RX buffer when master mode read is finished, (b) doesn't

Conclusions for zephyr:

• move the switching ot master_mode directly before issuing start condition and disable it directly after issuing stop condition on the bus
• move the switching out of master_mode directly after arbitration lost is detected
• return -EAGAIN in case of arbitration lost