Re: [Qemu-devel] [RFC] Plan for moving forward with QOM

[Edgar E. Iglesias]

On Wed, Sep 14, 2011 at 03:22:29PM -0500, Anthony Liguori wrote:
> On 09/14/2011 03:00 PM, Edgar E. Iglesias wrote:
> >On Wed, Sep 14, 2011 at 01:04:00PM -0500, Anthony Liguori wrote:
> >>Hi,
> >>
> >>I spent a couple hours today writing up some comparisons and an
> >>initial task list for converting qdev to QOM.  The main location of
> >>this is the wiki[1] but I thought I would inline it here for easier
> >>commenting.
> >>
> >>I decided to do this because I wanted to avoid a massively long 00
> >>patch explaining the rationale for the first batch of changes that
> >>I'm going to send out.
> >>
> >>There is so much complexity in qdev and the device model in general
> >>that it's hard to come up with a concise document.  I'd really
> >>appreciate suggestions for topics to write up more rationale as that
> >>would help me avoid writing a book on the topic :-)
> >
> >Thanks Anthony,
> >
> >I'd appreciate if you could elaborate more on the backlinks. Also,
> >tiny code examples would help. Maybe you've got a git repo with
> >more code to link to?
> 
> Message-Id: <address@hidden>
> 
> Or
> 
> http://repo.or.cz/w/qemu/aliguori.git/shortlog/refs/heads/qom
> 
> >
> >Regarding the composition, a problem I face (which I mostly just
> >hack my way around) is that inter device connections may exist
> >at various layers at the same time. For example, a device that is
> >burried under a couple of busses/bridges may somehow be tighly
> >connected to another device at a very different location in
> >the bus hierarchy by means of another overlayed interconnect (e.g
> >data channels, timing generation, IRQs etc). Maybe you could
> >clarify a bit more on how QOM handles these topics.
> 
> Links are meant to handle this.  You can have something like:
> 
> + PlatformDevice1
> + PlatformDevice2
> + PciBus
>   |
>   +- PciDevice1
>   +- PciDevice2
>      |
>      + I2CBus
>        |
>        +- I2CDevice1
>           |- pd2: link<PlatformDevice2>
> 
> So you have I2CDevice that sites behind multiple levels of
> hierarchy, but has a direct relationship (via a link) to a platform
> device much higher in the hierarchy.  This is an simple
> demonstration of how you can create a graph in QOM.
> 
> qdev doesn't allow graphs in the object model which is the problem
> that you're having today.  That's because its a tree with
> alternating levels.  The top level is always a bus, the next level
> is always devices, the next level are always busses, etc.
> 
> With QOM, there are only devices, and devices can link to 0 or more other 
> devices.


Great, thanks for clarifying!

Maybe you can copy+paste this into your book :p

Cheers