Re: [PATCH v1] virtio-mmio: Specify wait needed in driver during reset

[Jason Wang <jasowang@redhat.com>]

在 2021/7/27 上午3:03, Michael S. Tsirkin 写道:
> On Mon, Jul 26, 2021 at 02:17:08PM +0000, Srivatsa Vaddagiri wrote:
> > > On Mon, Jul 26, 2021 at 01:36:56PM +0200, Cornelia Huck wrote:
> > > > On Mon, Jul 26 2021, Srivatsa Vaddagiri<svaddagi@qti.qualcomm.com>  wrote:
> > > >
> > > > > > > +indicated by a read of \field{Status} returning 0. Such a device MUST
> > also
> > > > > > > fail to accept
> > > > > > > +FEATURES_OK bit if driver does not negotiate
> > VIRTIO_F_MMIO_RESET_WAIT.
> > > > > > So, this basically means that an older driver that does not know the new
> > > > > > feature bit will not work with devices offering this bit, even if it did
> > > > > > poll? This seems acceptable, I just wanted to spell it out.
> > > > > Yes I believe that's the desired result. Device has no way to know if
> > driver
> > > > > will poll for reset completion or not unless it accepts the feature.
> > > > > Are you suggesting we explicitly put in some text in spec to that
> > > > > effect?
> > > > Not sure whether it is needed (do others have an opinion?), but probably
> > > > not.
> > > What about resets before FEATURES_OK? How are these handled?
> >  From device perspective, it's reset logic will always be same, independent of
> > when reset was performed by driver (before or after feature negotiation). A
> > driver that does not wait for reset completion will see undefined behavior
> > after
> > reset until it discovers that feature negotiation has failed?
> >
> > - vatsa
> Hmm. that doesn't sound too good. Makes using
> feature negotiation for this kind of useless ...


Yes, the tricky part is that the reset happens before the features 
negotiation.


> Device can actually detect a read from status, right?


I'm not sure if MMIO can always behave like a register. E.g can area 
just plain DRAM?


> Maybe if it sees status was not read it can just
> stay in reset state and not exit it?

Or I wonder we can use increase the version instead.

Thanks