Project ACRN I2C mediator introduction

1. ACRN I2C Mediator Introduction
CHEN Conghui
Chen, Conghui <conghui,chen@intel.com>
04/29/2020

2. Agenda
❑ Background
❑ Architecture
❑ Topology
❑ Brief introduction for virtio-i2c FE
❑ What is put in virtqueue by virtio-i2c FE?
❑ Brief introduction for virtio-i2c BE
❑ Add description in ACPI
❑ virtio-i2c usage

3. Background
In previous code, when the Guest OS wants to access to a slave I2C device, it
should pass through the whole I2C adapter. But if the adapter has two or more
slave devices, and need to be used by different Guests, pass through cannot
meet the requirement.

4. Architecture
• Work as an I2C adapter.
• Slave devices under same
physical adapter can be
assigned to different virtio-i2c
adapters.
• Support >= 1 slave devices.
• I2C slave devices driver
needn't change.
• Slave devices under different
physical adapter can be
assigned to one virtio-i2c
adapter.
• One virtio-i2c has only 1
virtqueue. For FE, it is
synchronize mode. For BE, it
work in asynchronize mode.

5. I2c-x
virtio-pci …
Vendor id:
0x1af4/0x8086
virtio-blk virtio-audio
Device Id:
0x1000
Virtio bus
PCI bus
Device Id:
0x860a
…
virto-i2c adapter
xxx yyy
Topology
In our implementation, the virtio-i2c is an I2C adapter, which is detected by virtio bus. But the
peripherals that are connected to the I2C bus are enumerated via the ACPI.
Device Id:
0x8603

6. i2c_register_adapter
i2c_acpi_register_devices
acpi_walk_namespace(xx,
i2c_acpi_add_device)
i2c_new_device
virtio_i2c_probe
virtio
native
tdf8532_i2c_probe
tdf8532_get_state
i2c_master_send
i2c_transfer
virtio-i2c .master_xfer
ACPI match
virtio
i2cdev_write
/dev/i2c-x
i2c_dev_init
a device use i2c interface
Brief introduction for virtio-i2c FE
*Use tdf8532 as example for
client driver
virtio_xfer
kick
notify
device_register
adapter device
slave device
Add msg to virtqueue
This block diagram only picks few components from I2C
subsystem to show the possible access to virtio-i2c.

7. What is put in virtqueue by virtio-i2c FE?
virtio-i2c use one virtqueue to transfer the message, the queue size is 64.
As it is an adapter, it receive i2c_msg from I2C core layer. But in this design, the i2c_msg is not
directly added to virtqueue, instead, it was split to 3 parts:
n
Out header
n+1 n+2
addr flags len
data buffer status
in/read out/write
virtqueue
struct i2c_msg {
__u16 addr; /* slave address */
__u16 flags;
__u16 len; /* msg length */
__u8 *buf; /* pointer to msg data */
};

8. Brief introduction for virtio-i2c BE
Worker
thread
Kick handler
Dispatch
i2c_msg
/dev/i2c-x
get msg
In BE kick handler, a worker thread is created during the initiate phase, and it will wait for a
signal. In the kick handler, when there is new msg in the virtqueue, it will wakeup worker thread
to process the request.
As mentioned above, the data is not i2c_msg. So, in the worker thread, it will reformated to
i2c_msg, then call i2c API to send to kernel space. After that, it will update the result to status in
virtqueue. At last, it notifies the FE driver.
Notify
virtqueue
Shared ring/queue
/dev/i2c-x
signal
1
2
3 4

9. Add description in ACPI
In native environment, i2c slave device can be probed through ACPI table. In virtio-i2c, it will add
the description for the slave devices under the virtual i2c adapter.
virtual ACPI in acrn-dm
static struct acpi_node acpi_node_table[] = {
{"cam1", acpi_add_cam1},
{"cam2", acpi_add_cam2},
{"hdac", acpi_add_hdac},
{"default", acpi_add_default},
};

10. virtio-i2c usage
acrn-dm cmdline
acrn-dm
cmdline
-s 19,virtio-i2c,0:70@cam1:71@cam2,4:1C “4” is the i2c adapter on sos side, means”/dev/i2c-4”
“1C” is the address for slave device
connected to /dev/i2c-4.
different native adapters is split by ‘,’
Guest
OS
gordon_peak_acrn:/sys/bus/i2c/devices/i2c-6 # ls
6-a0e0 device i2c-ADV7481B:00 new_device uevent
6-a0e1 firmware_node i2c-dev power
delete_device i2c-ADV7481A:00 name subsystem
“/dev/i2c-6” is the virtio-i2c adapter.
“i2c-ADV7481A:00” is the slave device configured to
ACPI table.

11. Q & A