Message-Id: <1331120224-27420-1-git-send-email-mchehab@redhat.com>
Date:	Wed,  7 Mar 2012 08:37:04 -0300
From:	Mauro Carvalho Chehab <mchehab@...hat.com>
To:	unlisted-recipients:; (no To-header on input)
Cc:	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Linux Edac Mailing List <linux-edac@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH] edac: Improve the comments to better describe the memory concepts

The memory terms changed along the time, since when EDAC were originally
written: new concepts were introduced, and some things have different
meanings, depending on the memory architecture.

Better define those terms.

As some concepts are different for memory controllers used by certain
memory stick types, add a per-memory type comment, and add a more verbose
description for each memory type.

No functional changes. Just comments were touched.

Signed-off-by: Mauro Carvalho Chehab <mchehab@...hat.com>
---
 include/linux/edac.h |  182 +++++++++++++++++++++++++++++++++++++------------
 1 files changed, 137 insertions(+), 45 deletions(-)

diff --git a/include/linux/edac.h b/include/linux/edac.h
index 055b248..96f996f 100644
--- a/include/linux/edac.h
+++ b/include/linux/edac.h
@@ -66,25 +66,91 @@ enum dev_type {
 #define DEV_FLAG_X32		BIT(DEV_X32)
 #define DEV_FLAG_X64		BIT(DEV_X64)
 
-/* memory types */
+/**
+ * enum mem_type - memory types
+ *
+ * @MEM_EMPTY		Empty csrow
+ * @MEM_RESERVED:	Reserved csrow type
+ * @MEM_UNKNOWN:	Unknown csrow type
+ * @MEM_FPM:		Fast page mode
+ *			An old asyncronous memory technology, encapsulated as
+ *			SIMM, popularly used between 1987-1995.
+ * @MEM_EDO:		Extended data out
+ *			Asynchronous memory technology used on early Pentium's
+ *			(1995-1998).
+ * @MEM_BEDO:		Burst Extended data out
+ *			EDO memories with performance improvement gains.
+ * @MEM_SDR:		Single data rate SDRAM
+ *			First types of syncronous ram specified by JEDEC
+ *			popular between 1998-2002. The JEDEC standards defined
+ *			3 types of sticks: PC66, PC100 and PC133. There were
+ *			also some non-official overclocked sticks like PC150
+ *			and PC166.
+ *			There are 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 single data rate SDRAM
+ * @MEM_DDR:		Double data rate SDRAM, used between 2002 and 2005.
+ *			The JEDEC standards defined sticks PC1600, PC2100,
+ *			PC2700 and PC3200. Non-official overclocked sticks
+ *			also exists.
+ *			On DDR memories, there are one or two channels. A
+ *			single-channel mode means that one x72 ECC dimm is
+ *			accessed, in order to provide 64 bits of data. On
+ *			dual-channel mode, two dimm's are used simultaneously,
+ *			in order to provide a 128 bits of data. The "cschannel"
+ *			concept used on EDAC refers to such channel type.
+ * @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
+ *			Rambus uses a high-speed multi-drop serial bus to
+ *			communicate with each RDRAM chip, used on some
+ *			machines between 2000-2002 (Pentium III and IV).
+ * @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.
+ *			A FB-DIMM channel consists of 14 unidirectional signal
+ *			pairs (Northbound path) from the memories to the MC
+ *			and 10 unidirectional signal pairs (Southbond path)
+ *			from the MC to the DIMM's.
+ *			those memories are x72 ECC DIMMs. Up to 8 DIMMs can
+ *			be connected per channel. When used with 128 bits,
+ *			two channels are needed. The grouping of those two
+ *			channels is called "branch".
+ * @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
+ *			Those memories are labed as "PC3-" to differenciate
+ *			from DDR and DDR2.
+ * @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 +228,9 @@ enum scrub_type {
 #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 +239,61 @@ enum scrub_type {
  * 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 of ECC
+ *			corrected 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) that the final consumer
+ *			cares to replace. It is typically encapsulated as DIMMs
  *
  * Socket:		A physical connector on the motherboard that accepts
- *			a single memory stick.
+ *			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 DIMM's. Each channel has its own
+ *			independent control (command) and data bus, and can
+ *			be used independently or grouped.
  *
- * 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:		Not all memory controllers have it. It is 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 cache line is higher,
+ *			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 chips to be used. 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. On devices where it is visible, it
+ *			selects the DIMM rank that it is accessed by 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 +306,8 @@ enum scrub_type {
  *
  * 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.
  *
-- 
1.7.8

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