devnotes/hostboot-attributes.md: describe attribute sources

Signed-off-by: Sergii Dmytruk <sergii.dmytruk@3mdeb.com>

devnotes/hostboot-attributes.md

@@ -0,0 +1,47 @@
+Hostboot stores information in a tree made of targets and attributes associated
+with them. Below is a short overview.
+
+# APIs
+
+* `FAPI_ATTR_*()` macros.
+* Methods of `TARGETING::Target` like `getAttr()`, `tryGetAttr()`.
+* There might be others.
+
+# Data sources
+
+## Hostboot's code
+
+There are methods for setting attributes and Hostboot uses them, so potentially
+any value can be modified.
+
+## talos.xml
+
+Machine specification like
+[talos.xml](https://git.raptorcs.com/git/talos-xml/plain/talos.xml)
+contains most of the attributes or at least their default values.
+
+Mind that defaults can be specified at multiple levels and default closest to
+the target in question has the highest priority.
+
+## Processed talos.xml
+
+At some point during the build of Hostboot machine specification is processed by
+Perl scripts which reorganize it and fill in values of some attributes (other
+things are removed, so resulting content isn't a superset of `talos.xml`).
+
+Command for generating it:
+
+```
+$ cd talos-hostboot/src/usr/targeting/common
+$ perl -I. processMrw.pl -x ~/data/talos.xml -o talos-mrw.xml
+$ vim talos-mrw.xml
+```
+
+Some of the generated attributes: `ATTR_IPMI_SENSORS`.
+
+## Attribute-specific get/set methods
+
+At least `src/include/usr/fapi2/attribute_service.H` contains functions which
+are called on use of `FAPI_ATTR_*()` macros for certain attributes. So if you
+don't see where attribute's value is set or where its value is used, it might be
+generated/consumed by one of those functions.