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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Thu, 9 Jul 2020 09:10:06 -0700
Message-ID: <CAPcyv4hSPWEUih=we5QM_rdk7fLemi8phyk8_0tOd8ieL_=vPg@mail.gmail.com>
Subject: Re: [PATCH v2 11/12] PM, libnvdimm: Add 'mem-quiet' state and
 callback for firmware activation
To:     Jason Gunthorpe <jgg@mellanox.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Doug Ledford <dledford@redhat.com>,
        Pavel Machek <pavel@ucw.cz>, Len Brown <len.brown@intel.com>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Jul 9, 2020 at 8:39 AM Jason Gunthorpe <jgg@mellanox.com> wrote:
>
> 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.

Right, it's more my hunch than a known bug at this point, but in my
experience with testing server class hardware when I've reported a
power management bugs I've sometimes got the incredulous response "who
suspends / hibernates servers!?". I can drop that comment.

Are there protocol timeouts that might need to be adjusted for a 100s
of microseconds blip in memory controller response?

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

Ok, and there is still the option to race the quiesce if the effects
of the freeze are worse than a potential timeout from the quiesce.