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

From: Edgar E. Iglesias
Subject: Re: [Qemu-devel] [RFC] Plan for moving forward with QOM
Date: Sat, 17 Sep 2011 04:41:31 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Fri, Sep 16, 2011 at 09:17:54PM -0500, Anthony Liguori wrote:
> On 09/16/2011 07:48 PM, Edgar E. Iglesias wrote:
> >On Fri, Sep 16, 2011 at 07:47:57PM +0300, Gleb Natapov wrote:
> >>On Thu, Sep 15, 2011 at 03:50:28PM -0500, Anthony Liguori wrote:
> >>>On 09/15/2011 03:29 PM, Gleb Natapov wrote:
> >>>>On Thu, Sep 15, 2011 at 12:51:23PM -0500, Anthony Liguori wrote:
> >>>>>On 09/15/2011 11:59 AM, Gleb Natapov wrote:
> >>>>>>On Thu, Sep 15, 2011 at 11:33:00AM -0500, Anthony Liguori wrote:
> >>>>>>>On 09/15/2011 10:38 AM, Gleb Natapov wrote:
> >>>>>>>>On Thu, Sep 15, 2011 at 10:28:52AM -0500, Anthony Liguori wrote:
> >>>>>>>>>On 09/15/2011 09:25 AM, Gleb Natapov wrote:
> >>>>>>>>>
> >>>>>>>>>There is no canonical parent link.  A device may have multiple (more
> >>>>>>>>>or less equivalent) parents.
> >>>>>>>>>
> >>>>>>>>>What should be treated as the "canonical" link depends on what
> >>>>>>>>>you're trying to do.  In the case of OF, you want to treat the bus
> >>>>>>>>>as a parent.  If a device happens to sit on multiple buses, I'm not
> >>>>>>>>>really sure what you do.
> >>>>>>>>>
> >>>>>>>>Yes, "canonical" is a link to a bus. Can you give an example of a 
> >>>>>>>>device
> >>>>>>>>that sits on multiple buses?
> >>>>>>>
> >>>>>>>Not all devices buses that they sit on.
> >>>>>>>
> >>>>>>Missing "have"? If device has no bus how do you talk to it? Who carries
> >>>>>>the signal from a cpu to a device?
> >>>>>>
> >>>>>>>A good example is our favorite one to debate--the PIIX3.  Devices
> >>>>>>PIIX3 is a collection of devices, not a device.
> >>>>>>
> >>>>>>>like the UART don't sit on a bus.  They don't have any links at all.
> >>>>>>In PC UART sits on isa bus. How device can have no links at all? It just
> >>>>>>glued to a motherboard not touching any wires?
> >>>>>
> >>>>>A bus implies a bidirectional relationship.  IOW, the device has to
> >>>>>know that it sits on a ISA bus to be an ISA device.
> >>>>>
> >>>>And ISA device with UART on it definitely knows that.
> >
> >IMO, this discussion is going nowhere, Partly because assummptions are
> >beeing made about how hardware "works".
> >
> >Hardware works the way it gets designed, and it can be desinged in pretty
> >much anyway you want.
> >
> >When it comes to devives, you can design them in a way so they become very
> >dependent on a specific bus. But you can also design them in a more genric
> >way so that they become bus agnostic. Then you just need to connect a bus
> >adaptor that hooks things up to the particular bus the device needs to be
> >hooked up to.
> >
> >Sometimes, the bus adaptor becomes more like like wrapper that is part
> >of the logic, other times, the bus adaptor is just a passthru unit.
> >
> >QEMU should allow us to model devices in a a bus agnostic way.
> And this is the problem to fix in qdev.  We need to kill buses in
> qdev.  The approach really boils down to:
> 1) Add unique names to devices (this is needed because buses must
> have names because they must be addressable).
> 2) Eliminate any device that has two bus instances in it.  I'm
> pretty sure IDE is the only example of this.
> 3) Move all BusState functionality to DeviceState and eliminate
> buses.  This is a compatibility breaker but a critical change.
> Anthony Liguori

I agree. Because busses are in the eye of the beholder. For
some devices a clock connection is it's main relationship with
the system. For others, maybe a "back" channel or link is the
only connectino. For the common device maybe the "main" bus
on which CPU's is are connected is the master bus.


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