man/man3jedec/libjedec_spd.3jedec

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.Dd September 22, 2023
.Dt LIBJEDEC_SPD 3JEDEC
.Os
.Sh NAME
.Nm libjedec_spd
.Nd parse DIMM SPD data
.Sh LIBRARY
.Lb libjedec
.Sh SYNOPSIS
.In libjedec.h
.Ft "nvlist_t *"
.Fo libjedec_spd
.Fa "const uint8_t *buf"
.Fa "size_t buflen"
.Fa "spd_error_t *errp
.Fc
.Sh DESCRIPTION
The
.Fn libjedec_spd
function parses a binary payload of SPD
.Pq serial presence detect
data and transforms it into a
.Vt nvlist_t
that can be used by callers to inspect the data.
.Pp
SPD data is defined by JEDEC and used in various DDR DRAM standards.
This information describes everything from the size and organization of
the DIMM and its dies,
timing information, electrical properties, manufacturing data, and more.
A DRAM module
.Pq the thing we plug into a computer
is made up of a number of different components.
For example, a DDR5 module
.Pq which is plugged into a slot
contains not only a number of different DRAM dies, but also power
controllers, registers, clock drivers, and more.
The SPD data describes all of this information.
.Pp
There are two major properties of SPD information that determine which
keys are present in it:
.Bl -enum -width Ds
.It
The DDR version that the device implements.
Examples of the version information include DDR4, LPDDR5, DDR3, and many
others.
.It
The module's type.
Examples of this include whether it is an RDIMM
.Pq registered DIMM ,
UDIMM
.Pq unregistered DIMM ,
soldered down, etc.
.El
.Pp
While each DDR version has different SPD information and the module's
type may further modify parts of it, the
.Vt nvlist_t
that is returned attempts to normalize this information as much as
possible.
Each key that is returned is defined in
.In libjedec.h
as a macro.
The key space is dot
.Pq Sq \&.
delineated.
The following key spaces are currently used:
.Bl -tag -width Ds
.It Dq meta
This contains information about the SPD ROM itself, the encoding
revision, the DRAM standard in use, the type of module, etc.
This section also contains all of the CRC values and information that
affects how the rest of data is parsed
.Pq such as whether or not the package is monolithic .
.It Dq dram
This section contains information that is specific to the SDRAM dies
that are present.
This includes addressing information such as the number of rows,
columns, banks, and bank groups that are on each die.
It also includes information such as the supported voltages of the dies
themselves and then describes a lot of the different timing properties
such as the minimum times that a controller must wait between certain
actions.
.It Dq ddr4 , Dq ddr5
This section contains information that is specific to a given DDR
standard and cannot otherwise share a common definition.
This section has a few additional subdivisions of information to cover
items related to the module's type or are otherwise logically grouped
together such as refresh management.
For example, the library uses namespaces such as
.Dq ddr4.rdimm
and
.Dq ddr5.lrdimm
for properties that would be specific to DDR4 RDIMMs and DDR5 LRDIMMs
respectively.
.It Dq module
This section relates to information about the physical module which
includes information such as its physical dimensions, the types of
components that are present on it, signal mapping, and related.
.It Dq mfg
This section contains information about the module manufacturing.
This is where things like the module's serial number, the manufacturer,
the part number, and related can be found.
Generally this information is found in a different part of the SPD ROM
and therefore may not always be present.
.It Dq errors
This section is an embedded
.Vt nvlist_t
that contains keys for each value that couldn't be parsed.
For more information, see the
.Sx Parsing Errors
section for more information.
.El
.Ss Data Types
The library attempts to use a fixed number of data types when performing
conversions and follows the following guidelines:
.Bl -bullet
.It
By default, integer values use a
.Vt uint32_t
to try and make things more uniform where possible and to allow for
future changes.
A
.Vt uint64_t
is used when the types contain data that could naturally overflow a
.Vt uint32_t
such as when counting the number of bits, bytes, or measuring time
.Pq which is normalized to picoseconds .
.Pp
All integers are denoted as such explicitly with their size: either
.Sq uint32_t
or
.Sq uint64_t .
Some banks of keys all have the same type and are denoted as such in the
introductory block comment.
Use
.Xr nvlist_lookup_uint32 3NVPAIR
or
.Xr nvlist_lookup_uint64 3NVPAIR
to retrieve these keys.
.It
Strings, which are stored as traditional
.Vt char *
values should only contain printable characters.
.Pp
All strings are denoted as such by using the comment
.Sq string .
Use
.Xr nvlist_lookup_string 3NVPAIR
to retrieve these keys.
.It
There are many values which represent enumerations.
The raw type is stored as a
.Vt uint32_t .
In general, these enumerations, unless otherwise indicated should not be
assumed to have the identical values of a given specification.
This is done because the values are not always uniform from
specification to specification, except in rare cases.
Not all DDR specifications can result in the same set of enumeration
values.
.Pp
For example, consider the
.Vt spd_temp_type_t
which is an enumeration that describes the kind of temperature
monitoring device present.
The enumeration contains values from DDR3, DDR4, and DDR5; however, each
standard has its own defined type of SPD temperature sensor.
.Pp
All enumerations are denoted as such by using the comment
.Sq uint32_t Pq enum .
Use
.Xr nvlist_lookup_uint32 3NVPAIR
to retrieve these keys.
.It
Several items are used to indicate a fact, but do not have any actual
data associated with them.
Their mere presence is sufficient to indicate a unique property of the
DIMM.
Examples of this include if the package is non-monolithic, the DIMM has
asymmetrical ranks, etc.
.Pp
All such items are denoted as such by using the comment
.Sq key .
Use
.Xr nvlist_lookup_boolean 3NVPAIR
to retrieve these keys.
.It
A few types use arrays of
.Vt uint32_t
values to denote information.
Some of the keys have a fixed number of entries that are expected, while
others are variable and will depend on the parsed data.
For example, a JEDEC ID is always encoded as two
.Vt uint32_t
values, the continuation and the underlying ID itself.
Other values, such as the number of supported voltages or CAS latencies
will vary.
.Pp
All fixed size arrays denote their size by using the comment
.Sq uint32_t [4] ,
where the number four is replaced with the actual size.
All variable sized arrays elide the number and are denoted by the
comment
.Sq uint32_t [] .
Use
.Xr nvlist_lookup_uint32_array 3NVPAIR
to retrieve these keys.
.El
.Ss Parsing Errors
There are a number of different issues that can arise when trying to
parse the SPD data that a device returns.
The library attempts to parse as much as it can and breaks errors into
two large categories:
.Bl -enum -width Ds
.It
Errors which prevent us from doing anything at all, which are noted in
the
.Sx ERRORS
section.
The main items that relate to data
.Pq as opposed to issues like running out of memory
include when we encounter a DDR specification that we don't support or
that the data has an encoding version we don't understand.
This can also occur if there's not enough data for us to figure out what
kind of SPD information is encoded.
.It
Errors that mean we cannot parse a specific piece of SPD data.
This is by far the most common form of error that we encounter and these
are the entries that make up the
.Dq errors
section of the returned
.Vt nvlist_t
that was mentioned earlier.
.El
.Pp
The
.Dq errors
section is a nested
.Vt nvlist_t
that can be retrieved by using the
.Dv SPD_KEY_ERRS
key.
This keys in this nvlist use the same name as the normal data keys.
For example, if we were unable to properly parse the manufacturer's
JEDEC ID, then the key
.Dv SPD_KEY_MFG_MOD_MFG_ID
.Pq Dq mfg.module-mfg-id
would not be present in the top-level
.Vt nvlist_t
and would instead be in the error
.Vt nvlist_t .
.Pp
Each item present in the error
.Vt nvlist_t
is itself a
.Vt nvlist_t
which contains the following keys:
.Bl -tag -width Ds
.It Dv SPD_KEY_ERRS_CODE
The error code, a
.Vt uint32_t,
indicates a class of issue that occurred and its values are defined by
the
.Vt spd_error_kind_t
enumeration.
The following errors are defined:
.Bl -tag -width Ds
.It SPD_ERROR_NO_XLATE
This indicates that the library did not know how to interpret a specific
binary value.
For example, if a given particular field was using what we believed to
be a reserved value then we would set this error.
.It SPD_ERROR_UNPRINT
This indicates that we encountered a non-ASCII or otherwise unprintable
character that was not allowed in the SPD string character set.
.It SPD_ERROR_NO_DATA
This indicates that there was no data for a given key.
For example, this would be a string that consisted solely of space
characters which are used to represent padding.
.It SPD_ERROR_INTERNAL
This indicates that something went wrong inside the library that was
unexpected.
.It SPD_ERROR_BAD_DATA
This indicates that we've encountered something strange about the data
in question.
For example, this would be used for an invalid CRC or if the combination
of values would cause an underflow in a calculation.
.El
.It Dv SPD_KEY_ERRS_MSG
This is an error message that is intended for a person to understand and
contains more specific information than the code above.
.El
.Pp
Finally, there is one last top-level key that we will set if we find
that the set of data that we had was incomplete.
Rather than tag every single item in the
.Dq errors
.Vt nvlist_t ,
we instead insert the key
.Dv SPD_KEY_INCOMPLETE
on the top-level nvlist.
The key is a
.Vt uint32_t
that contains the starting offset of the
.Sy key
that we were unable to parse, which may be less than the total amount of
data that was provided.
For example, if we had 100 bytes of data, but there was a 20 byte key
starting at byte 90, the this key would have a value of 90 as that's
what we were unable to parse.
.Sh RETURN VALUES
Upon successful completion, the
.Fn libjedec_spd
function returns an allocated
.Vt nvlist_t
that contains the parsed SPD data.
There may be individual SPD fields that were unparsable which will be
found in the
.Dv SPD_KEY_ERRS
field of the nvlist still.
Otherwise
.Dv NULL
is returned
and
.Fa errp
is set with a value that indicates why the function was unable to parse
any data.
.Sh EXAMPLES
.Sy Example 1
Printing SPD DRAM and Module Type
.Pp
The following example shows how to parse the SPD information and print
out the type of DRAM and module's type.
This example assumes that one has already obtained the raw SPD file from
somewhere and just prints the raw values.
.Bd -literal -offset 2
#include <stdio.h>
#include <stdint.h>
#include <libjedec.h>

void
dump_dram_module_type(const uint8_t *buf, size_t len)
{
	nvlist_t *nvl;
	spd_error_t err;
	uint32_t ddr, mod;

	nvl = libjedec_spd(buf, len, &err);
	if (nvl == NULL) {
		(void) fprintf(stderr, "failed to parse SPD data: 0x%x\en",
		    err);
		return;
	}

	if (nvlist_lookup_pairs(nvl, 0,
	    SPD_KEY_DRAM_TYPE, DATA_TYPE_UINT32, &ddr,
	    SPD_KEY_MOD_TYPE, DATA_TYPE_UINT32, &mod,
	    NULL) != 0) {
		nvlist_free(nvl);
		(void) fprintf(stderr, "failed to look up keys\en");
		return;
	}

	nvlist_free(nvl);
	(void) printf("Found DDR type 0x%x, module type 0x%x\en", ddr,
	    mod);
}
.Ed
.Sh ERRORS
Upon returning from the
.Fn libjedec_spd
function, the
.Fa errp
pointer will be set to one of the following values:
.Bl  -tag -width Dv
.It Er LIBJEDEC_SPD_OK
This indicates that the system was able to parse SPD data into a valid
.Vt nvlist_t
and return it to the caller.
This code should never appear when the function fails and returns
.Dv NULL .
Callers must still cehck the contents of the
.Dv SPD_KEY_ERRS
nvlist.
.It Dv LIBJEDEC_SPD_NOMEM
This indicates that the system ran out of memory while trying to
construct the
.Vt nvlist_t
to return.
.It Dv LIBJEDEC_SPD_TOOSHORT
This indicates that not enough SPD data was present as indicated by
.Fa buf
and
.Fa buflen
for the given type of SPD information and therefore we were unable to
successfully parse basic information.
.It Dv LIBJEDEC_SPD_UNSUP_TYPE
This indicates that the type of SPD data present, e.g. DDR4 SDRAM,
LPDDR3, etc., that was found is not currently supported by the library.
.It Dv LIBJEDEC_SPD_UNSUP_REV
This indicates that while the library is familiar with the specific type
of SPD data present, the encoding level of the data
.Pq similar to a major version
is unsupported by the library.
.El
.Sh INTERFACE STABILITY
.Sy Uncommitted
.Sh MT-LEVEL
.Sy MT-Safe
.Sh SEE ALSO
.Xr libjedec 3LIB ,
.Xr nvlist_lookup_boolean 3NVPAIR ,
.Xr nvlist_lookup_string 3NVPAIR ,
.Xr nvlist_lookup_uint32 3NVPAIR ,
.Xr nvlist_lookup_uint32_array 3NVPAIR ,
.Xr nvlist_lookup_uint64 3NVPAIR
.Pp
.Rs
.%Q JEDEC Solid State Technology Association
.%T Serial Presence Detect (SPD), General Standard
.%N 21-C
.Re
.Rs
.%Q JEDEC Solid State Technology Association
.%T DDR5 Serial Presence Detect (SPD) Contents
.%N JESD400-5A.01
.%D January 2023
.Re