Re: [Qemu-devel] Trying to write a new device / virtio-i2c ?

[Andreas Färber]

Am 14.02.2014 16:58, schrieb Paolo Bonzini:
> Il 13/02/2014 14:26, Alex David ha scritto:
>> After reading code, documentation and available things, I've been trying
>> to write something like a "virtio-i2c" : I wrote a virtio-i2c module for
>> my kernel (I used some examples from virtio-pci and virtio-console), it
>> seems that it created a "i2c-1" device in /dev,
>>
>> My device that I launch with QEMU (-chardev
>> socket,path=/tmp/test0,server,nowait,id=bob -device
>> virtio-i2c,chardev=bob) doesn't seem to be recognized by the kernel
>> driver : my probe function doesn't run.
> 
> i2c is a bus, not directly a device.  Do you want to pass an entire
> adapter down to the guest?  Or just a slave?
> 
> QEMU has i2c emulation but it is bus-based, so you need one device per
> slave + 1 for the adapter. x86 already has an I2C bus (actually it's
> SMBus) that you may be able to use.  So you probably want something like
> 
>     -object i2c-linux,file=/dev/i2c-1,id=i2c-backend
>     -device i2c-host,backend=i2c-backend,hostaddr=0x40,address=0x40
>     -device i2c-host,backend=i2c-backend,hostaddr=0x50,address=0x50
> 
> and so on.  i2c-linux would be the object issuing ioctls to /dev/i2c-1,
> multiplexing access to the device for all the i2c-host devices.

While that is certainly possible in case host passthrough was desired,
maybe virtio was mixed up with VFIO? For now I believe it is PCI-only
and there was work on platform devices at KVM Forum 2013; for s390x a
ccw backend may be needed, so maybe it's possible to extend vfio for I2C?

If however it's just about QEMU reading data from a daemon on the host,
a simple slave device might do and is then independent of host I2C devices.

Regards,
Andreas

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