Re: [PATCH 00/12] ACPI/NVDIMM: Runtime Firmware Activation

[Dan Williams <dan.j.williams@intel.com>]

On Fri, Jun 26, 2020 at 7:22 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Fri, Jun 26, 2020 at 2:06 AM Dan Williams <dan.j.williams@intel.com> wrote:
> >
> > Quoting the documentation:
> >
> >     Some persistent memory devices run a firmware locally on the device /
> >     "DIMM" to perform tasks like media management, capacity provisioning,
> >     and health monitoring. The process of updating that firmware typically
> >     involves a reboot because it has implications for in-flight memory
> >     transactions. However, reboots are disruptive and at least the Intel
> >     persistent memory platform implementation, described by the Intel ACPI
> >     DSM specification [1], has added support for activating firmware at
> >     runtime.
> >
> >     [1]: https://docs.pmem.io/persistent-memory/
> >
> > The approach taken is to abstract the Intel platform specific mechanism
> > behind a libnvdimm-generic sysfs interface. The interface could support
> > runtime-firmware-activation on another architecture without need to
> > change userspace tooling.
> >
> > The ACPI NFIT implementation involves a set of device-specific-methods
> > (DSMs) to 'arm' individual devices for activation and bus-level
> > 'trigger' method to execute the activation. Informational / enumeration
> > methods are also provided at the bus and device level.
> >
> > One complicating aspect of the memory device firmware activation is that
> > the memory controller may need to be quiesced, no memory cycles, during
> > the activation. While the platform has mechanisms to support holding off
> > in-flight DMA during the activation, the device response to that delay
> > is potentially undefined. The platform may reject a runtime firmware
> > update if, for example a PCI-E device does not support its completion
> > timeout value being increased to meet the activation time. Outside of
> > device timeouts the quiesce period may also violate application
> > timeouts.
> >
> > Given the above device and application timeout considerations the
> > implementation defaults to hooking into the suspend path to trigger the
> > activation, i.e. that a suspend-resume cycle (at least up to the syscore
> > suspend point) is required.
>
> Well, that doesn't work if the suspend method for the system is set to
> suspend-to-idle (for example, via /sys/power/mem_sleep), because the
> syscore callbacks are not invoked in that case.
>
> Also you probably don't need the device power state toggling that
> happens during regular suspend/resume (you may not want it even for
> some devices).
>
> The hibernation freeze/thaw may be a better match and there is some
> test support in there already that may be kind of co-opted for your
> use case.

Hmm, yes I guess freeze should be sufficient to quiesce most
device-DMA in the general case as applications will stop sending
requests. I do expect some RDMA devices will happily keep on
transmitting, but that likely will need explicit mitigation. It also
appears the suspend callback for at least one RDMA device
mlx5_suspend() is rather violent as it appears to fully teardown the
device context, not just suspend operations.

To be clear, what debug interface were you thinking I could glom onto
to just trigger firmware-activate at the end of the freeze phase?
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