Subject: Re: [PATCH v2] virtio: only reset device and restore status if needed in device resume

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 6 Nov 2024 04:28:53 -0500

On Fri, Nov 01, 2024 at 10:11:11AM +0800, Jason Wang wrote:
> On Fri, Nov 1, 2024 at 9:54 AM <[email-masked]> wrote:
> >
> > From: Qiang Zhang <[email-masked]>
> >
> > Virtio core unconditionally reset and restore status for all virtio
> > devices before calling restore method. This breaks some virtio drivers
> > which don't need to do anything in suspend and resume because they
> > just want to keep device state retained.
> 
> The challenge is how can driver know device doesn't need rest.

I actually don't remember why do we do reset on restore. Do you?

> For example, PCI has no_soft_reset which has been done in the commit
> "virtio: Add support for no-reset virtio PCI PM".
> 
> And there's a ongoing long discussion of adding suspend support in the
> virtio spec, then driver know it's safe to suspend/resume without
> reset.
> 
> >
> > Virtio GPIO is a typical example. GPIO states should be kept unchanged
> > after suspend and resume (e.g. output pins keep driving the output) and
> > Virtio GPIO driver does nothing in freeze and restore methods. But the
> > reset operation in virtio_device_restore breaks this.
> 
> Is this mandated by GPIO or virtio spec? If yes, let's quote the revelant part.
> 
> >
> > Since some devices need reset in suspend and resume while some needn't,
> > create a new helper function for the original reset and status restore
> > logic so that virtio drivers can invoke it in their restore method
> > if necessary.
> 
> How are those drivers classified?
> 
> >
> > Signed-off-by: Qiang Zhang <[email-masked]>
> 
> Thanks