edac: Improve the comments to better describe the memory concepts
The Computer memory terminology has changed with time since EDAC was
originally written: new concepts were introduced, and some things have
different meanings, depending on the memory architecture.
Improve the definition of all related terms.
Also, describe each memory type in a more detailed fashion.
No functional changes. Just comments were touched.
Acked-by: Borislav Petkov <borislav.petkov@amd.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/include/linux/edac.h b/include/linux/edac.h
index 1cd3947..0714d67 100644
--- a/include/linux/edac.h
+++ b/include/linux/edac.h
@@ -66,25 +66,64 @@
#define DEV_FLAG_X32 BIT(DEV_X32)
#define DEV_FLAG_X64 BIT(DEV_X64)
-/* memory types */
+/**
+ * enum mem_type - memory types. For a more detailed reference, please see
+ * http://en.wikipedia.org/wiki/DRAM
+ *
+ * @MEM_EMPTY Empty csrow
+ * @MEM_RESERVED: Reserved csrow type
+ * @MEM_UNKNOWN: Unknown csrow type
+ * @MEM_FPM: FPM - Fast Page Mode, used on systems up to 1995.
+ * @MEM_EDO: EDO - Extended data out, used on systems up to 1998.
+ * @MEM_BEDO: BEDO - Burst Extended data out, an EDO variant.
+ * @MEM_SDR: SDR - Single data rate SDRAM
+ * http://en.wikipedia.org/wiki/Synchronous_dynamic_random-access_memory
+ * They use 3 pins for chip select: Pins 0 and 2 are
+ * for rank 0; pins 1 and 3 are for rank 1, if the memory
+ * is dual-rank.
+ * @MEM_RDR: Registered SDR SDRAM
+ * @MEM_DDR: Double data rate SDRAM
+ * http://en.wikipedia.org/wiki/DDR_SDRAM
+ * @MEM_RDDR: Registered Double data rate SDRAM
+ * This is a variant of the DDR memories.
+ * A registered memory has a buffer inside it, hiding
+ * part of the memory details to the memory controller.
+ * @MEM_RMBS: Rambus DRAM, used on a few Pentium III/IV controllers.
+ * @MEM_DDR2: DDR2 RAM, as described at JEDEC JESD79-2F.
+ * Those memories are labed as "PC2-" instead of "PC" to
+ * differenciate from DDR.
+ * @MEM_FB_DDR2: Fully-Buffered DDR2, as described at JEDEC Std No. 205
+ * and JESD206.
+ * Those memories are accessed per DIMM slot, and not by
+ * a chip select signal.
+ * @MEM_RDDR2: Registered DDR2 RAM
+ * This is a variant of the DDR2 memories.
+ * @MEM_XDR: Rambus XDR
+ * It is an evolution of the original RAMBUS memories,
+ * created to compete with DDR2. Weren't used on any
+ * x86 arch, but cell_edac PPC memory controller uses it.
+ * @MEM_DDR3: DDR3 RAM
+ * @MEM_RDDR3: Registered DDR3 RAM
+ * This is a variant of the DDR3 memories.
+ */
enum mem_type {
- MEM_EMPTY = 0, /* Empty csrow */
- MEM_RESERVED, /* Reserved csrow type */
- MEM_UNKNOWN, /* Unknown csrow type */
- MEM_FPM, /* Fast page mode */
- MEM_EDO, /* Extended data out */
- MEM_BEDO, /* Burst Extended data out */
- MEM_SDR, /* Single data rate SDRAM */
- MEM_RDR, /* Registered single data rate SDRAM */
- MEM_DDR, /* Double data rate SDRAM */
- MEM_RDDR, /* Registered Double data rate SDRAM */
- MEM_RMBS, /* Rambus DRAM */
- MEM_DDR2, /* DDR2 RAM */
- MEM_FB_DDR2, /* fully buffered DDR2 */
- MEM_RDDR2, /* Registered DDR2 RAM */
- MEM_XDR, /* Rambus XDR */
- MEM_DDR3, /* DDR3 RAM */
- MEM_RDDR3, /* Registered DDR3 RAM */
+ MEM_EMPTY = 0,
+ MEM_RESERVED,
+ MEM_UNKNOWN,
+ MEM_FPM,
+ MEM_EDO,
+ MEM_BEDO,
+ MEM_SDR,
+ MEM_RDR,
+ MEM_DDR,
+ MEM_RDDR,
+ MEM_RMBS,
+ MEM_DDR2,
+ MEM_FB_DDR2,
+ MEM_RDDR2,
+ MEM_XDR,
+ MEM_DDR3,
+ MEM_RDDR3,
};
#define MEM_FLAG_EMPTY BIT(MEM_EMPTY)
@@ -162,8 +201,9 @@
#define OP_OFFLINE 0x300
/*
- * There are several things to be aware of that aren't at all obvious:
+ * Concepts used at the EDAC subsystem
*
+ * There are several things to be aware of that aren't at all obvious:
*
* SOCKETS, SOCKET SETS, BANKS, ROWS, CHIP-SELECT ROWS, CHANNELS, etc..
*
@@ -172,36 +212,61 @@
* creating a common ground for discussion, terms and their definitions
* will be established.
*
- * Memory devices: The individual chip on a memory stick. These devices
- * commonly output 4 and 8 bits each. Grouping several
- * of these in parallel provides 64 bits which is common
- * for a memory stick.
+ * Memory devices: The individual DRAM chips on a memory stick. These
+ * devices commonly output 4 and 8 bits each (x4, x8).
+ * Grouping several of these in parallel provides 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 in parallel. This is the atomic
- * memory component that is purchaseable by Joe consumer
- * and loaded into a memory socket.
+ * memory devices in parallel. In general, this is the
+ * Field Replaceable Unit (FRU) which gets replaced, in
+ * the case of excessive errors. Most often it is also
+ * called DIMM (Dual Inline Memory Module).
*
- * Socket: A physical connector on the motherboard that accepts
- * a single memory stick.
+ * Memory Socket: A physical connector on the motherboard that accepts
+ * a single memory stick. Also called as "slot" on several
+ * datasheets.
*
- * Channel: Set of memory devices on a memory stick that must be
- * grouped in parallel with one or more additional
- * channels from other memory sticks. This parallel
- * grouping of the output from multiple channels are
- * necessary for the smallest granularity of memory access.
- * Some memory controllers are capable of single channel -
- * which means that memory sticks can be loaded
- * individually. Other memory controllers are only
- * capable of dual channel - which means that memory
- * sticks must be loaded as pairs (see "socket set").
+ * Channel: A memory controller channel, responsible to communicate
+ * with a group of DIMMs. Each channel has its own
+ * independent control (command) and data bus, and can
+ * be used independently or grouped with other channels.
*
- * Chip-select row: All of the memory devices that are selected together.
- * for a single, minimum grain of memory access.
- * This selects all of the parallel memory devices across
- * all of the parallel channels. Common chip-select rows
- * for single channel are 64 bits, for dual channel 128
- * bits.
+ * Branch: It is typically the highest hierarchy on a
+ * Fully-Buffered DIMM memory controller.
+ * Typically, it contains two channels.
+ * Two channels at the same branch can be used in single
+ * mode or in lockstep mode.
+ * When lockstep is enabled, the cacheline is doubled,
+ * but it generally brings some performance penalty.
+ * Also, it is generally not possible to point to just one
+ * memory stick when an error occurs, as the error
+ * correction code is calculated using two DIMMs instead
+ * of one. Due to that, it is capable of correcting more
+ * errors than on single mode.
+ *
+ * Single-channel: The data accessed by the memory controller is contained
+ * into one dimm only. E. g. if the data is 64 bits-wide,
+ * the data flows to the CPU using one 64 bits parallel
+ * access.
+ * Typically used with SDR, DDR, DDR2 and DDR3 memories.
+ * FB-DIMM and RAMBUS use a different concept for channel,
+ * so this concept doesn't apply there.
+ *
+ * Double-channel: The data size accessed by the memory controller is
+ * interlaced into two dimms, accessed at the same time.
+ * E. g. if the DIMM is 64 bits-wide (72 bits with ECC),
+ * the data flows to the CPU using a 128 bits parallel
+ * access.
+ *
+ * Chip-select row: This is the name of the DRAM signal used to select the
+ * DRAM ranks to be accessed. Common chip-select rows for
+ * single channel are 64 bits, for dual 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 to it from the Memory Controller.
*
* Single-Ranked stick: A Single-ranked stick has 1 chip-select row of memory.
* Motherboards commonly drive two chip-select pins to
@@ -214,8 +279,8 @@
*
* Double-sided stick: DEPRECATED TERM, see Double-Ranked stick.
* A double-sided stick has two chip-select rows which
- * access different sets of memory devices. The two
- * rows cannot be accessed concurrently. "Double-sided"
+ * access different sets of memory devices. The two
+ * rows cannot be accessed concurrently. "Double-sided"
* is irrespective of the memory devices being mounted
* on both sides of the memory stick.
*