Re: [RFC PATCH 0/4] hw/i2c: i2c slave mode support

[Klaus Jensen]

On Apr  5 20:52, Peter Delevoryas wrote:
> 
> 
> > On Mar 31, 2022, at 9:57 AM, Klaus Jensen <its@irrelevant.dk> wrote:
> > 
> > From: Klaus Jensen <k.jensen@samsung.com>
> > 
> > Hi all,
> > 
> > This RFC series adds I2C "slave mode" support for the Aspeed I2C
> > controller as well as the necessary infrastructure in the i2c core to
> > support this.
> > 
> > Background
> > ~~~~~~~~~~
> > We are working on an emulated NVM Express Management Interface[1] for
> > testing and validation purposes. NVMe-MI is based on the MCTP
> > protocol[2] which may use a variety of underlying transports. The one we
> > are interested in is I2C[3].
> > 
> > The first general trickery here is that all MCTP transactions are based
> > on the SMBus Block Write bus protocol[4]. This means that the slave must
> > be able to master the bus to communicate. As you know, hw/i2c/core.c
> > currently does not support this use case.
> 
> This is great, I’m attempting to use your changes right now for the same 
> thing (MCTP).
> 

Awesome!

> > 
> > The second issue is how to interact with these mastering devices. Jeremy
> > and Matt (CC'ed) have been working on an MCTP stack for the Linux Kernel
> > (already upstream) and an I2C binding driver[5] is currently under
> > review. This binding driver relies on I2C slave mode support in the I2C
> > controller.
> > 
> > This series
> > ~~~~~~~~~~~
> > Patch 1 adds support for multiple masters in the i2c core, allowing
> > slaves to master the bus and safely issue i2c_send/recv(). Patch 2 adds
> > an asynchronous send i2c_send_async(I2CBus *, uint8) on the bus that
> > must be paired with an explicit ack using i2c_ack(I2CBus *).
> > 
> > Patch 3 adds the slave mode functionality to the emulated Aspeed I2C
> > controller. The implementation is probably buggy since I had to rely on
> > the implementation of the kernel driver to reverse engineer the behavior
> > of the controller slave mode (I do not have access to a spec sheet for
> > the Aspeed, but maybe someone can help me out with that?).
> > 
> > Finally, patch 4 adds an example device using this new API. The device
> > is a simple "echo" device that upon being sent a set of bytes uses the
> > first byte as the address of the slave to echo to.
> > 
> > With this combined I am able to boot up Linux on an emulated Aspeed 2600
> > evaluation board and have the i2c echo device write into a Linux slave
> > EEPROM. Assuming the echo device is on address 0x42:
> > 
> >  # echo slave-24c02 0x1064 > /sys/bus/i2c/devices/i2c-15/new_device
> >  i2c i2c-15: new_device: Instantiated device slave-24c02 at 0x64
> >  # i2cset -y 15 0x42 0x64 0x00 0xaa i
> >  # hexdump /sys/bus/i2c/devices/15-1064/slave-eeprom
> >  0000000 ffaa ffff ffff ffff ffff ffff ffff ffff
> >  0000010 ffff ffff ffff ffff ffff ffff ffff ffff
> >  *
> >  0000100
> 
> When I try this with my system, it seems like the i2c-echo device takes over
> the bus but never echoes the data to the EEPROM. Am I missing something to
> make this work? It seems like the “i2c_send_async” calls aren’t happening,
> which must be because the bottom half isn’t being scheduled, right? After
> the i2c_do_start_transfer, how is the bottom half supposed to be scheduled
> again? Is the slave receiving (the EEPROM) supposed to call i2c_ack or 
> something?
> 
> root@bmc-oob:~# echo 24c02 0x1064 > /sys/bus/i2c/devices/i2c-8/new_device
> [  135.559719] at24 8-1064: 256 byte 24c02 EEPROM, writable, 1 bytes/write
> [  135.562661] i2c i2c-8: new_device: Instantiated device 24c02 at 0x64
> root@bmc-oob:~# i2cset -y 8 0x42 0x64 0x00 0xaa i
> i2c_echo_event: start send
> i2c_echo_send: data[0] = 0x64
> i2c_echo_send: data[1] = 0x00
> i2c_echo_send: data[2] = 0xaa
> i2c_echo_event: scheduling bottom-half
> i2c_echo_bh: attempting to gain mastery of bus
> i2c_echo_bh: starting a send to address 0x64
> root@bmc-oob:~# hexdump -C /sys/bus/i2c/devices/8-1064/eeprom
> 00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
> *
> 00000100
> 
> Thanks again for this, it’s exactly what I needed.
> 

Hmmm. The only obvious difference I see here is that I write
"slave-24c02" and not just "24c02" to new_device. Not sure if that has
any implications? Also, looks like your EEPROM is initialized with
zeroes, mine is all ones. This hints at the device being instantiated is
different. I'm also not seeing the 'at24', which upon looking in the
kernel code is a different device?

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