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Re: [Qemu-ppc] [PATCH v3 09/33] add doc about Resettable interface

From: Peter Maydell
Subject: Re: [Qemu-ppc] [PATCH v3 09/33] add doc about Resettable interface
Date: Wed, 7 Aug 2019 17:01:42 +0100

On Wed, 31 Jul 2019 at 07:33, David Gibson <address@hidden> wrote:
> On Mon, Jul 29, 2019 at 04:56:30PM +0200, Damien Hedde wrote:
> > Signed-off-by: Damien Hedde <address@hidden>
> > +For Devices and Buses there is also the corresponding helpers:
> > +void device_reset(Device *dev, bool cold)
> > +void bus_reset(Device *dev, bool cold)
> What's the semantic difference between resetting a bus and resetting
> the bridge device which owns it?

We should definitely explain this in the documentation, but
consider for instance a SCSI controller. Resetting the
SCSI controller puts all its registers back into whatever
the reset state is for the device, as well as resetting
everything on the SCSI bus. Resetting just the SCSI bus
resets the disks and so on on the bus, but doesn't change
the state of the controller itself, which remains programmed
with whatever state the guest has set up.

PCI has a similar distinction between resetting the controller
and resetting the bus.

Note that we have this distinction in the current APIs too:
qbus_reset_all() vs qdev_reset_all().

-- PMM

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