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

[Rafael J. Wysocki]

On Fri, Jun 26, 2020 at 2:06 AM Dan Williams <dan.j.williams@xxxxxxxxx> 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.

Cheers!