Error Detection And Correction (EDAC) Devices============================================= Main Concepts used at the EDAC subsystem---------------------------------------- There are several things to be aware of that aren't at all obvious, like*sockets, *socket sets*, *banks*, *rows*, *chip-select rows*, *channels*,etc... These are some of the many terms that are thrown about that don't alwaysmean what people think they mean (Inconceivable!).  In the interest ofcreating a common ground for discussion, terms and their definitionswill be established. * Memory devices The individual DRAM chips on a memory stick.  These devices commonlyoutput 4 and 8 bits each (x4, x8). Grouping several of these in parallelprovides the number of bits that the memory controller expects:typically 72 bits, in order to provide 64 bits + 8 bits of ECC data. * Memory Stick A printed circuit board that aggregates multiple memory devices inparallel.  In general, this is the Field Replaceable Unit (FRU) whichgets replaced, in the case of excessive errors. Most often it is alsocalled DIMM (Dual Inline Memory Module). * Memory Socket A physical connector on the motherboard that accepts a single memorystick. Also called as "slot" on several datasheets. * Channel A memory controller channel, responsible to communicate with a group ofDIMMs. Each channel has its own independent control (command) and databus, and can be used independently or grouped with other channels. * Branch It is typically the highest hierarchy on a Fully-Buffered DIMM memorycontroller. Typically, it contains two channels. Two channels at thesame branch can be used in single mode or in lockstep mode. Whenlockstep is enabled, the cacheline is doubled, but it generally bringssome performance penalty. Also, it is generally not possible to point tojust one memory stick when an error occurs, as the error correction codeis calculated using two DIMMs instead of one. Due to that, it is capableof correcting more errors than on single mode. * Single-channel The data accessed by the memory controller is contained into one dimmonly. E. g. if the data is 64 bits-wide, the data flows to the CPU usingone 64 bits parallel access. Typically used with SDR, DDR, DDR2 and DDR3memories. FB-DIMM and RAMBUS use a different concept for channel, sothis concept doesn't apply there. * Double-channel The data size accessed by the memory controller is interlaced into twodimms, accessed at the same time. E. g. if the DIMM is 64 bits-wide (72bits with ECC), the data flows to the CPU using a 128 bits parallelaccess. * Chip-select row This is the name of the DRAM signal used to select the DRAM ranks to beaccessed. Common chip-select rows for single channel are 64 bits, fordual channel 128 bits. It may not be visible by the memory controller,as some DIMM types have a memory buffer that can hide direct access toit from the Memory Controller. * Single-Ranked stick A Single-ranked stick has 1 chip-select row of memory. Motherboardscommonly drive two chip-select pins to a memory stick. A single-rankedstick, will occupy only one of those rows. The other will be unused. .. _doubleranked: * Double-Ranked stick A double-ranked stick has two chip-select rows which access differentsets of memory devices.  The two rows cannot be accessed concurrently. * Double-sided stick **DEPRECATED TERM**, see :ref:`Double-Ranked stick <doubleranked>`. A double-sided stick has two chip-select rows which access different setsof memory devices. The two rows cannot be accessed concurrently."Double-sided" is irrespective of the memory devices being mounted onboth sides of the memory stick. * Socket set All of the memory sticks that are required for a single memory access orall of the memory sticks spanned by a chip-select row.  A single socketset has two chip-select rows and if double-sided sticks are used thesewill occupy those chip-select rows. * Bank This term is avoided because it is unclear when needing to distinguishbetween chip-select rows and socket sets. * High Bandwidth Memory (HBM) HBM is a new memory type with low power consumption and ultra-widecommunication lanes. It uses vertically stacked memory chips (DRAM dies)interconnected by microscopic wires called "through-silicon vias," orTSVs. Several stacks of HBM chips connect to the CPU or GPU through an ultra-fastinterconnect called the "interposer". Therefore, HBM's characteristicsare nearly indistinguishable from on-chip integrated RAM. Memory Controllers------------------ Most of the EDAC core is focused on doing Memory Controller error detection.The :c:func:`edac_mc_alloc`. It uses internally the struct ``mem_ctl_info``to describe the memory controllers, with is an opaque struct for the EDACdrivers. Only the EDAC core is allowed to touch it. .. kernel-doc:: include/linux/edac.h .. kernel-doc:: drivers/edac/edac_mc.h PCI Controllers--------------- The EDAC subsystem provides a mechanism to handle PCI controllers by callingthe :c:func:`edac_pci_alloc_ctl_info`. It will use the struct:c:type:`edac_pci_ctl_info` to describe the PCI controllers. .. kernel-doc:: drivers/edac/edac_pci.h EDAC Blocks----------- The EDAC subsystem also provides a generic mechanism to report errors onother parts of the hardware via :c:func:`edac_device_alloc_ctl_info` function. The structures :c:type:`edac_dev_sysfs_block_attribute`,:c:type:`edac_device_block`, :c:type:`edac_device_instance` and:c:type:`edac_device_ctl_info` provide a generic or abstract 'edac_device'representation at sysfs. This set of structures and the code that implements the APIs for the same, provide for registering EDAC type devices which are NOT standard memory orPCI, like: - CPU caches (L1 and L2)- DMA engines- Core CPU switches- Fabric switch units- PCIe interface controllers- other EDAC/ECC type devices that can be monitored for  errors, etc. It allows for a 2 level set of hierarchy. For example, a cache could be composed of L1, L2 and L3 levels of cache.Each CPU core would have its own L1 cache, while sharing L2 and maybe L3caches. On such case, those can be represented via the following sysfsnodes:: 	/sys/devices/system/edac/.. 	pci/		<existing pci directory (if available)>	mc/		<existing memory device directory>	cpu/cpu0/..	<L1 and L2 block directory>		/L1-cache/ce_count			 /ue_count		/L2-cache/ce_count			 /ue_count	cpu/cpu1/..	<L1 and L2 block directory>		/L1-cache/ce_count			 /ue_count		/L2-cache/ce_count			 /ue_count	... 	the L1 and L2 directories would be "edac_device_block's" .. kernel-doc:: drivers/edac/edac_device.h  Heterogeneous system support---------------------------- An AMD heterogeneous system is built by connecting the data fabrics ofboth CPUs and GPUs via custom xGMI links. Thus, the data fabric on theGPU nodes can be accessed the same way as the data fabric on CPU nodes. The MI200 accelerators are data center GPUs. They have 2 data fabrics,and each GPU data fabric contains four Unified Memory Controllers (UMC).Each UMC contains eight channels. Each UMC channel controls one 128-bitHBM2e (2GB) channel (equivalent to 8 X 2GB ranks).  This creates a totalof 4096-bits of DRAM data bus. While the UMC is interfacing a 16GB (8high X 2GB DRAM) HBM stack, each UMCchannel is interfacing 2GB of DRAM (represented as rank). Memory controllers on AMD GPU nodes can be represented in EDAC thusly: 	GPU DF / GPU Node -> EDAC MC	GPU UMC           -> EDAC CSROW	GPU UMC channel   -> EDAC CHANNEL For example: a heterogeneous system with 1 AMD CPU is connected to4 MI200 (Aldebaran) GPUs using xGMI. Some more heterogeneous hardware details: - The CPU UMC (Unified Memory Controller) is mostly the same as the GPU UMC.  They have chip selects (csrows) and channels. However, the layouts are different  for performance, physical layout, or other reasons.- CPU UMCs use 1 channel, In this case UMC = EDAC channel. This follows the  marketing speak. CPU has X memory channels, etc.- CPU UMCs use up to 4 chip selects, So UMC chip select = EDAC CSROW.- GPU UMCs use 1 chip select, So UMC = EDAC CSROW.- GPU UMCs use 8 channels, So UMC channel = EDAC channel. The EDAC subsystem provides a mechanism to handle AMD heterogeneoussystems by calling system specific ops for both CPUs and GPUs. AMD GPU nodes are enumerated in sequential order based on the PCIhierarchy, and the first GPU node is assumed to have a Node ID valuefollowing those of the CPU nodes after latter are fully populated:: 	$ ls /sys/devices/system/edac/mc/		mc0   - CPU MC node 0		mc1  |		mc2  |- GPU card[0] => node 0(mc1), node 1(mc2)		mc3  |		mc4  |- GPU card[1] => node 0(mc3), node 1(mc4)		mc5  |		mc6  |- GPU card[2] => node 0(mc5), node 1(mc6)		mc7  |		mc8  |- GPU card[3] => node 0(mc7), node 1(mc8) For example, a heterogeneous system with one AMD CPU is connected tofour MI200 (Aldebaran) GPUs using xGMI. This topology can be representedvia the following sysfs entries:: 	/sys/devices/system/edac/mc/.. 	CPU			# CPU node	├── mc 0 	GPU Nodes are enumerated sequentially after CPU nodes have been populated	GPU card 1		# Each MI200 GPU has 2 nodes/mcs	├── mc 1		# GPU node 0 == mc1, Each MC node has 4 UMCs/CSROWs	│   ├── csrow 0		# UMC 0	│   │   ├── channel 0	# Each UMC has 8 channels	│   │   ├── channel 1   # size of each channel is 2 GB, so each UMC has 16 GB	│   │   ├── channel 2	│   │   ├── channel 3	│   │   ├── channel 4	│   │   ├── channel 5	│   │   ├── channel 6	│   │   ├── channel 7	│   ├── csrow 1		# UMC 1	│   │   ├── channel 0	│   │   ├── ..	│   │   ├── channel 7	│   ├── ..		..	│   ├── csrow 3		# UMC 3	│   │   ├── channel 0	│   │   ├── ..	│   │   ├── channel 7	│   ├── rank 0	│   ├── ..		..	│   ├── rank 31		# total 32 ranks/dimms from 4 UMCs	├	├── mc 2		# GPU node 1 == mc2	│   ├── ..		# each GPU has total 64 GB 	GPU card 2	├── mc 3	│   ├── ..	├── mc 4	│   ├── .. 	GPU card 3	├── mc 5	│   ├── ..	├── mc 6	│   ├── .. 	GPU card 4	├── mc 7	│   ├── ..	├── mc 8	│   ├── ..