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authorNitin A Kamble <nitin.a.kamble@intel.com>2014-09-23 17:26:23 -0700
committerTom Zanussi <tom.zanussi@linux.intel.com>2014-10-21 14:39:46 -0500
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README: Add documentation for Intel-specific machine features
Start a new documentation section in the top-level README to describe Intel layer-specific machine features. Te intel-ucode machine feature is described first. In the future more such features will added to this section. Signed-off-by: Nitin A Kamble <nitin.a.kamble@intel.com> Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
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@@ -83,3 +83,114 @@ The meta-intel maintainers will do their best to review and/or pull in
a patch or patchset within 24 hours of the time it was posted. For
larger and/or more involved patches and patchsets, the review process
may take longer.
+
+
+Intel-specific machine features
+===============================
+
+The meta-intel layer makes some additional machine features available
+to BSPs. These machine features can be used in a BSP layer in the
+same way that machine features are used in other layers based on
+oe-core, via the MACHINE_FEATURES variable.
+
+Requirements
+------------
+
+The meta-intel-specific machine features are only available to a BSP
+when the meta-intel layer is included in the build configuration, and
+the meta-intel.inc file is included in the machine configuration of
+that BSP.
+
+To make these features available for your machine, you will need to:
+
+ 1. include a configuration line such as the below in bblayers.conf
+ BBLAYERS += "<local path>/meta-intel"
+ 2. include the following line in the machine configuration file
+ require conf/machine/include/meta-intel.inc
+
+Once the above requirements are met, the machine features provided by
+the meta-intel layer will be available for the BSP to use.
+
+Available machine features
+--------------------------
+
+Currently, the meta-intel layer makes the following set of
+Intel-specific machine features available:
+
+ * intel-ucode
+
+These machine features can be included by listing them in the
+MACHINE_FEATURES variable in the machine configuration file. For
+example:
+
+ MACHINE_FEATURES += "intel-ucode"
+
+Machine feature details
+-----------------------
+
+ * intel-ucode
+
+ This feature provides support for microcode updates to Intel
+ processors. The intel-ucode feature runs at early boot and uses
+ the microcode data file added by the feature into the BSP's
+ initrd. It also puts the userland microcode-updating tool,
+ iucode_tool, into the target images along with the microcode data
+ file.
+
+ Q. Why might a user want to enable the intel-ucode feature?
+
+ A. Intel releases microcode updates to correct processor behavior
+ as documented in the respective processor specification
+ updates. While the normal approach to getting such microcode
+ updates is via a BIOS upgrade, this can be an administrative
+ hassle and not always possible in the field. The intel-ucode
+ feature enables the microcode update capability present in the
+ Linux kernel. It provides an easy path for upgrading processor
+ microcode without the need to change the BIOS. If the feature
+ is enabled, it is also possible to update the existing target
+ images with a newer microcode update in the future.
+
+ Q. How would a user bundle only target-specific microcode in the
+ target image?
+
+ A. The Intel microcode data file released by Intel contains
+ microcode updates for multiple processors. If the BSP image is
+ meant to run on only a certain subset of processor types, a
+ processor-specific subset of microcode can be bundled into the
+ target image via the UCODE_FILTER_PARAMETERS variable. This
+ works by listing a sequence of iucode-tool parameters in the
+ UCODE_FILTER_PARAMETERS variable, which in this case will
+ select only the specific microcode relevant to the BSP. For
+ more information on the underlying parameters refer to the
+ iucode-tool manual page at http://manned.org/iucode-tool
+
+ To define a set of parameters for microcode-filtering via the
+ UCODE_FILTER_PARAMETERS variable, one needs to identify the
+ cpuid signatures of all the processors the BSP is meant to run
+ on. One way to determine the cpuid signature for a specific
+ processor is to build and run an intel-ucode-feature-enabled
+ image on the target hardware, without first assigning any value
+ to the UCODE_FILTER_PARAMETERS variable, and then once the
+ image is booted, run the "ucode_tool -S" command to have the
+ ucode tool scan the system for processor signatures. These
+ signatures can then be used in the UCODE_FILTER_PARAMETERS
+ variable in conjunction with -s parameter. For example, for
+ the fri2 BSP, the cpuid can be determined as such:
+
+ [root@fri2 ~]# iucode_tool -S
+ iucode_tool: system has processor(s) with signature 0x00020661
+
+ Given that output, a suitable UCODE_FILTER_PARAMETERS variable
+ definition could be specified in the machine configuration as
+ such:
+
+ UCODE_FILTER_PARAMETERS = "-s 0x00020661"
+
+ Q. Are there any reasons a user might want to disable the
+ intel-ucode feature?
+
+ A. The microcode data file and associated tools occupy a small
+ amount of space (a few KB) on the target image. BSPs which are
+ highly sensitive to target image size and which are not
+ experiencing microcode-related issues might consider not
+ enabling this feature.