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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

If the default I2C bus is not good, I suggest that you write QEMU code 
to emulate an USB-I2C bridge, rather than write a virtio one.  It is not 
a performance-intensive path in all likelihood, and you won't have to 
write driver code for the guest.

Another alternative is to support the kernel's I2C-over-parallel 
interface.  You can then write a character device backend that maps 
/dev/i2c-1 read/write/ioctl to the parallel port interface that the 
kernel expects use it with a parallel port device.  It would be used 
like this:

     -chardev i2c-linux,file=/dev/i2c-1,id=i2c-backend
     -device parallel,chrdev=i2c-backend

Paolo