[Patch V2] Build spec: update description for build handles PCDs for SKU support

[Yonghong Zhu <yonghong.zhu@intel.com>]

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Kevin W Shaw <kevin.w.shaw@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
---
 2_design_discussion/27_sku_support.md | 129 +++++++++++++++++++++++++++++-----
 1 file changed, 111 insertions(+), 18 deletions(-)

diff --git a/2_design_discussion/27_sku_support.md b/2_design_discussion/27_sku_support.md
index 18d40b7..891fc93 100644
--- a/2_design_discussion/27_sku_support.md
+++ b/2_design_discussion/27_sku_support.md
@@ -1,9 +1,9 @@
 <!--- @file
   2.7 SKU Support
 
-  Copyright (c) 2008-2017, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2008-2018, Intel Corporation. All rights reserved.<BR>
 
   Redistribution and use in source (original document form) and 'compiled'
   forms (converted to PDF, epub, HTML and other formats) with or without
   modification, are permitted provided that the following conditions are met:
 
@@ -31,22 +31,115 @@
 
 ## 2.7 SKU Support
 
 The EDK II build system provides the capability of supporting multiple SKUs in
 a single firmware image. SKU selection is a runtime option that can be set from
-a platform driver. During runtime, the configuration elements, PCDs, must be
-accessed using either Dynamic or DynamicEx PCD access methods. When building
-for multiple SKU support, the FeatureFlag, FixedAtBuild and PatchableInModule
-access methods will only use the default SKU configuration settings. The set of
-SKUs that can be included is configurable either through setting the list in
-the DSC file or through setting commandline options to the build command.
-
-The SKU enabled PCD sections are a sparsely populated set of configuration
-settings; the PCD drivers will automatically return a default SKU value if no
-specific SKU value was specified.
-
-The build system also supports building a specific SKU. In this mode, it is
-possible to use SKU specific FeatureFlag, FixedAtBuild and PatchableInModule
-access method configuration elements along with the Dynamic and DynamicEx PCD
-for the specific SKU. The build tools will automatically adjust the SKU
-specific Dynamic and DynamicEx PCD values overriding the default values. This
-is equivalent of running SetSku() immediately on reset.
+a platform driver. The build system also supports building a specific SKU. The
+SKU enabled PCD sections (defined by a PCD section tag in the DSC file that
+contains a SKUID modifier that is not DEFAULT) are a sparsely populated set of
+configuration settings. The platform developer may specify one or more PCDs that
+will have different values than the PCD values specified by the default SKU.
+Additional PCDs not listed in a default PCD section may also be specified under
+a section with a SKUID modifier. 
+During runtime, the PCD drivers will automatically return a default SKU value
+if no specific SKU value was specified after a platform driver calls SetSku().
+The configuration elements, PCDs, must be accessed using either Dynamic or
+DynamicEx PCD access methods. When building and image that supports multiple
+SKUs, the Feature Flag, Fixed At Build and Patchable In Module PCDs will only
+use the default SKU configuration settings. The default configuration settings
+are identified by PCD section tags that have either a Default SKUID modifier
+or have not SKUID modifier. The set of SKUs that can be included is configurable
+either through setting the list in the DSC file [Defines] section's SKU_IDENTIFIER
+element or through setting one or more -x SKUID command-line options to the build
+command.
+When building a single SKU, it is possible to use SKU specific Feature Flag, Fixed
+At Build and Patchable In Module configuration elements along with the Dynamic and
+DynamicEx PCD for the specific SKU. The build tools will automatically adjust the
+SKU specific Dynamic and DynamicEx PCD values overriding the default values. This
+is equivalent of running SetSku immediately on reset.
+
+**Note:** If there are no PcdsDynamic or PcdsDynamicEx section tags that use a
+SkuId modifier, then only the DEFAULT values will be placed into the PCD Database.
+The platform drivers must not call SetSku() for this single SKU, as the 'DEFAULT'
+SKU will be the only SKU available when built by this method.
+
+The following examples show the three types of builds.
+DEFAULT SKUID Build
+One or more SKUID | SKUIDENTIFIER entries may appear in the DSC file's [SkuIds]
+section as in the following example:
+[SkuIds]
+  0 | DEFAULT
+  1 | ScsiSku
+  2 | SataSku
+  3 | iScsiSku
+Only the DEFAULT SKU will be built as identified by a single entry in the
+SKU_IDENTIFIER in the DSC file's [Defines] section as in the following example:
+  SKU_IDENTIFIER = DEFAULT
+The user is not required to specify:
+  -x DEFAULT
+FeatureFlag, PatchableInModule and FixedAtBuild PCD values from the values in
+the default PCD sections. Dynamic and DynamicEx PCD values from the default
+PCD sections (sections that do not contain a SkuId modifier in the section tag
+or that contain a SkuId modifier of DEFAULT) must be used. These values will
+then be placed in the DEFAULT table of the PCD Database.
+**WARNING:** The platform drivers must not call SetSku() for this single SKU, as
+the 'DEFAULT' SKU will be the only SKU available when built by this method.
+
+Single SKUID Build
+This method is useful for debugging as well as for size-optimization.
+More than one SKUID | SKUIDENTIFIER entry must appear in the DSC file's [SkuIds]
+section as in the following example:
+[SkuIds]
+  0 | DEFAULT
+  1 | ScsiSku
+  2 | SataSku
+  3 | iScsiSku
+Only one of the possible SKUs will be built as identified by a single entry in
+the SKU_IDENTIFIER in the DSC file's [Defines] section as in the following example:
+  SKU_IDENTIFIER = ScsiSku
+**Note:** A SKU_IDENTIFIER = DEFAULT | ScsiSku must be treated by tools as exactly
+the same as if just SKU_IDENTIFIER = ScsiSku had been specified.
+If the users specifies the following option on the build command-line, only SataSku
+SKUID will be included:
+  -x SataSku
+The -x SKUIDENTIFIER command-line option overrides the SKU_IDENTIFIER statement in
+the [Defines] section.
+FeatureFlag, PatchableInModule and FixedAtBuild PCD values from the PCD section
+that contains the matching SKUID modifier will override the values in the default
+PCD sections.
+Dynamic and DynamicEx PCD values from PCD sections that contain the matching SKUID
+modifier will override the values from the default PCD section. These values will
+then be placed in the DEFAULT table of the PCD Database. This is equivalent of
+executing a SetSku() immediately on reset/power-on.
+**WARNING:** The platform drivers must not call SetSku() for this single SKU build,
+as the 'DEFAULT' SKU will be the only SKU available when built by this method.
+
+Multiple SKUID Build
+The DEFAULT SKU is always included in a multi-SKU platform build as these are the
+default values returned by PcdGet statements until such time as a platform driver
+executes a SetSku() call.
+More than two SKUID | SKUIDENTIFIER entries must appear in the DSC file's [SkuIds]
+section as in the following example:
+[SkuIds]
+  0 | DEFAULT
+  1 | ScsiSku
+  2 | SataSku
+  3 | iScsiSku
+Only two of the possible SKUs will be built as identified by a list of SKU_IDENTIFIER
+in the DSC file's [Defines] section as in the following example:
+  SKU_IDENTIFIER = ScsiSku | SataSku
+**Note:** A SKU_IDENTIFIER = DEFAULT | ScsiSku | SataSku must be treated by tools as
+exactly the same as if this SKU_IDENTIFIER = ScsiSku | SataSku  statement had been
+specified.
+If the users specifies the following options on the build command-line, all of the
+SKUIDs will be included (DEFAULT is always included):
+  -x ScsiSku -x SataSku -x iScsiSku
+FeatureFlag, PatchableInModule and FixedAtBuild PCD values must come from the default
+PCD sections; PCD sections for these access types that have a SKUID modifier must be
+ignored by the Build Tools. If a PCD listed in a PCD section with a SKUID modifier
+is NOT listed in the default PCD section, the PCD cannot be used by any module
+included in the build.Dynamic and DynamicEx PCD values from the DEFAULT SKU as well
+as values from the specified SKUs will be put into tables in the PCD Database. Prior
+to a platform driver is calling SetSku(), drivers accessing the PCD database will get
+values from the DEFAULT SKU. Once a platform driver calls SetSku(), the values for
+the specific SKU will be returned (unless there is no entry for the PCD in the
+specific SKU table, in which case, the value will come from the DEFAULT SKU table).
-- 
2.6.1.windows.1