Re: [PATCH v2] hw/i2c: Add remote I2C master with host CUSE bridge

[Markus Armbruster <armbru@redhat.com>]

Ilya Chichkov <ilya.chichkov.dev@gmail.com> writes:

> Add a "remote-i2c-master" device that exposes a QEMU I2C bus to the
> host system through a FUSE/CUSE character device. This lets external
> host programs and standard i2c-tools interact with I2C slaves emulated
> inside QEMU as if they were real devices attached to the host.
>
> The implementation is split into three layers:
>
>   - A non-blocking finite state machine that drives the QEMU I2C
>     master. It is pumped by a QEMU Bottom Half and uses virtual timers
>     to yield during long transfers and to model clock stretching for
>     asynchronous slaves, so the main loop is never blocked. The FSM
>     walks IDLE -> ADDR -> SEND/RECV -> WAIT_STRETCH -> END -> FINISHED
>     and handles NACKs (ENXIO), lost arbitration (EBUSY, with optional
>     back-off and retry), stretch timeouts, and manual abort/reset.
>
>   - An abstract RemoteI2CBackend QOM base class that decouples the
>     internal I2C hardware state machine (the frontend) from any
>     host-specific transport, exposing on_tx_complete and on_tx_error
>     virtual callbacks.
>
>   - A concrete remote-i2c-backend-cuse backend implementing that
>     transport over CUSE. It manages the FUSE session and integrates
>     its file descriptors into QEMU's main AioContext event loop,
>     translates Linux I2C_RDWR, I2C_SMBUS and I2C_SLAVE ioctls into
>     generic byte streams for the FSM, and formats responses back into
>     Linux I2C/SMBus structures for the FUSE driver. SMBus repeated
>     start is supported for atomic write-then-read operations.
>
> Example usage:
>
>   -device remote-i2c-master,i2cbus=i2c-bus.0,devname=i2c-33
>   -object remote-i2c-backend-cuse,id=b0,devname=i2c-33
>
> This creates /dev/i2c-33 on the host, usable with i2c-tools:
>
>   i2cdetect -y -l
>   i2cget -y <bus_id> <addr> <reg>
>
> Signed-off-by: Ilya Chichkov <ilya.chichkov.dev@gmail.com>

[...]

> diff --git a/qapi/qom.json b/qapi/qom.json
> index dd45ac1087..b515f1e4e7 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -1258,6 +1258,7 @@
>      'tls-creds-psk',
>      'tls-creds-x509',
>      'tls-cipher-suites',
> +    'remote-i2c-backend-cuse',
>      { 'name': 'x-remote-object', 'features': [ 'unstable' ] },
>      { 'name': 'x-vfio-user-server', 'features': [ 'unstable' ] }
>    ] }
> @@ -1334,6 +1335,7 @@
>        'tls-creds-psk':              'TlsCredsPskProperties',
>        'tls-creds-x509':             'TlsCredsX509Properties',
>        'tls-cipher-suites':          'TlsCredsProperties',
> +      'remote-i2c-backend-cuse':    'RemoteI2CBackendCuseProperties',
>        'x-remote-object':            'RemoteObjectProperties',
>        'x-vfio-user-server':         'VfioUserServerProperties'
>    } }
> @@ -1377,3 +1379,19 @@
>  ##
>  { 'command': 'object-del', 'data': {'id': 'str'},
>    'allow-preconfig': true }
> +
> +##
> +# @RemoteI2CBackendCuseProperties:
> +#
> +# Properties for the CUSE remote I2C backend.
> +#
> +# @devname: The CUSE device name to create (e.g., 'i2c-33').

What's a "CUSE device name"?  What exactly is created?

> +# @fuse-opts: Optional FUSE mount options.

Syntax?

> +# @debug: Whether to enable debug output.

To what exactly does @debug apply?

Blank line between member descriptions, please.

> +#
> +# Since: (your version)

Assuming this can't make 11.1:

   # Since: 11.2

> +##
> +{ 'struct': 'RemoteI2CBackendCuseProperties',
> +  'data': { 'devname': 'str',
> +            '*fuse-opts': 'str',
> +            '*debug': 'bool' } }

Should all this be 'if': 'CONFIG_LINUX'?