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Date: Mon, 16 Aug 2021 10:37:18 -0400
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [virtio-dev] Re: [PATCH v1] virtio-mmio: Specify wait needed in
 driver during reset
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To: Srivatsa Vaddagiri <quic_svaddagi@quicinc.com>
Cc: Jason Wang <jasowang@redhat.com>, Srivatsa Vaddagiri <svaddagi@qti.qualcomm.com>, Cornelia Huck <cohuck@redhat.com>, "virtio-dev@lists.oasis-open.org" <virtio-dev@lists.oasis-open.org>, Trilok Soni <quic_tsoni@quicinc.com>, Pratik Patel <quic_pratikp@quicinc.com>
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On Mon, Aug 16, 2021 at 07:04:51PM +0530, Srivatsa Vaddagiri wrote:
> * Michael S. Tsirkin <mst@redhat.com> [2021-08-16 07:48:56]:
> 
> > On Mon, Aug 16, 2021 at 12:05:50PM +0530, Srivatsa Vaddagiri wrote:
> > > * Michael S. Tsirkin <mst@redhat.com> [2021-08-16 01:35:46]:
> > > 
> > > > So thinking about all this, quite early in the setup process we
> > > > have:
> > > > 
> > > >         /* Figure out what features the device supports. */
> > > >         device_features = dev->config->get_features(dev);
> > > > 
> > > > 
> > > > isn't this sufficient? this will flush out
> > > > all writes and device can defer responding to reads
> > > > until reset is complete.
> > > 
> > > Hmm not sure if that's possible in all MMIO devices? I think a while back Jason
> > > felt some of the MMIO devices, their registers could act as plain DRAM for
> > > reads, which means device can't block such reads?
> > > 
> > > - vatsa
> > 
> > Is it worth bothering about theoretical issues though? Jason what is
> > your take?
> 
> What about the case where device reset fails, which may require driver to
> initiate reset again or give up on device? Won't the read of features be forever
> deferred in that case?
> 
> - vatsa

That might be worth addressing but I feel your proposal does not
address it sufficiently.

> ---
>  
> Qualcomm Innovation Center, Inc. is submitting the attached "feedback" as a
> non-member to the virtio-dev mailing list for consideration and inclusion.