There are currently 4 known similar but incompatible definitions of the
command sets that can be sent to an NVDIMM through ACPI. It is also
clear that future platform generations (ACPI or not) will continue to
revise and extend the DIMM command set as new devices and use cases
arrive.
It is obviously untenable to continue to proliferate divergence
of these command definitions, and to that end a standardization process
has begun to provide for a unified specification. However, that leaves a
problem about what to do with this first generation where vendors are
already shipping divergence.
The Linux kernel can support these initial diverged platforms without
giving platform-firmware free reign to continue to diverge and compound
kernel maintenance overhead. The kernel implementation can encourage
standardization in two ways:
1/ Require that any function code that userspace wants to send be
explicitly white-listed in the implementation. For ACPI this means
function codes marked as supported by acpi_check_dsm() may
only be invoked if they appear in the white-list. A function must be
publicly documented before it is added to the white-list.
2/ The above restrictions can be trivially bypassed by using the
"vendor-specific" payload command. However, since vendor-specific
commands are by definition not publicly documented and have the
potential to corrupt the kernel's view of the dimm state, we provide a
toggle to disable vendor-specific operations. Enabling undefined
behavior is a policy decision that can be made by the platform owner
and encourages firmware implementations to choose public over
private command implementations.
Based on an initial patch from Jerry Hoemann
Cc: Jerry Hoemann <jerry.hoemann@hpe.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
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