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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 02:48:04 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

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.

IMO a device shouldnt neceserally need to know what kind of bus it's
connected to. Those kind of details should be decided by the board
instantiation (of course they might be exceptions).

Another thing that I find very wrong is th endianness model we've got
now, where devices specify their endianness. That is complety bogus.
If you've got the RTL for a device it is trivial to switch endianness,
why shouldnt QEMU boards be able to do that? Endianness should be up to
the board, not to the device itself.

Sometimes, it get the impressino we think more about synthesized
devices into phsyical chips than to the actual options that exist
when you've got the logic at hand and  can place things the way you
(read board) desires.


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