Re: [Qemu-devel] [PATCH for-1.4] qapi: Improve chardev-add documentation

[Markus Armbruster]

Gerd Hoffmann <address@hidden> writes:

>   Hi,
>
>> Now let's take another step back: a character device is just a file.
>> Why can't we use plain ChardevFile for it?
>
> It's not.

A Unix character device is a special file.

> First, the file backend allows input and output being different files,
> and the input file is optional.  That doesn't make sense for the
> parallel/serial port case.
>
> Second, it's actually more than a file.  The guest can change device
> parameters such as the baudrate on the virtual device and qemu will
> apply those changes to the host device.
>
> IMO it is different enough to have its own type.

Special files support some special operations.  I didn't propose to drop
support for the special operations and treat special files exactly like
regular files.  I asked whether we really need a completely separate way
to configure them.

The argument for separate configuration is file's capability to split
input and output.  Okay.

But why nested discriminators?

    regular files: type=file
    serial       : type=port, data.type=serial
    parallel     : type=port, data.type=parallel

Simpler, and closer to existing -chardev:

    regular files: type=file
    serial       : type=serial
    parallel     : type=parallel

I also dislike the pointless '"data" : {}' required by type pty and
null, but I can't figure out how to express 'void' in the schema.

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