Re: [PATCH v2 11/12] PM, libnvdimm: Add 'mem-quiet' state and callback for firmware activation

[Jason Gunthorpe]

On Thu, Jul 09, 2020 at 04:00:51PM +0100, Christoph Hellwig wrote:
> On Mon, Jul 06, 2020 at 06:59:32PM -0700, Dan Williams wrote:
> > The runtime firmware activation capability of Intel NVDIMM devices
> > requires memory transactions to be disabled for 100s of microseconds.
> > This timeout is large enough to cause in-flight DMA to fail and other
> > application detectable timeouts. Arrange for firmware activation to be
> > executed while the system is "quiesced", all processes and device-DMA
> > frozen.
> > 
> > It is already required that invoking device ->freeze() callbacks is
> > sufficient to cease DMA. A device that continues memory writes outside
> > of user-direction violates expectations of the PM core to be to
> > establish a coherent hibernation image.
> > 
> > That said, RDMA devices are an example of a device that access memory
> > outside of user process direction.

Are you saying freeze doesn't work for some RDMA drivers? That would
be a driver bug, I think.

The consequences of doing freeze are pretty serious, but it should
still stop DMA.

Jason