RE: [PATCH] acpi/nfit: export read_only attribute of dimms

A few more points: 1. The full NVDIMM State Flags value is already reported for the nmem device, so reporting bit 3 separately as "read_only" would be redundant. 2. For NVDIMM-Ns, the system is not preventing writing to memory; system firmware is just warning that any writes will not be persistent across power loss. 3. Considering the different layers: * the pmem and btt drivers blocks writes themselves, so it makes sense for them to report ro at the pmem device layer. * the nmem layer doesn't block writes, so shouldn't report anything about them being restricted. * the region layer doesn't block writes, so shouldn't report anything about them being restricted. * the region layer does need to pass along the NVDIMM state flags to the pmem and btt drivers; read_only might not be the best name as a user-visible attribute with that information. Some driver or software might find temporarily writing to those locations in-place would be useful (e.g. run fsck before copying the repaired filesystem content to another storage device), so we shouldn't cut them off unnecessarily. 4. The ACPI definition is a bit open: "Bit [3] set to 1 indicates that the NVDIMM containing the NVDIMM region is not able to accept persistent writes. For an energy-source backed NVDIMM device, Bit [3] is set if it is not armed or the previous ERASE operation did not complete." Perhaps NFIT needs to distinguish between these situations: * writes result in errors - this would require special support from the CPU, memory controller, and memory bus * writes are accepted but ignored - e.g., an NVDIMM-P type device made from SCM - ignored writes create cache coherency problems * writes are accepted, but not persistent across power loss - e.g., an NVDIMM-N --- Robert Elliott, HPE Persistent Memory