[PATCH i-g-t v1 2/2] CONTRIBUTING: Add guide about igt libraries

[Kamil Konieczny]

Add some general guide about adding new library function and
a few guides for their usage outside of tests.

Signed-off-by: Kamil Konieczny <kamil.konieczny at linux.intel.com>
---
 CONTRIBUTING.md | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index b571b5e68..d85266a6c 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -38,6 +38,33 @@ The Code
 
 [igt-describe]: https://drm.pages.freedesktop.org/igt-gpu-tools/igt-gpu-tools-Core.html#igt-describe
 
+IGT libraries
+-------------
+- Tests and benchmarks are the main usage of IGT libraries, so they
+  could use test specific macros/functions, for example igt_assert,
+  igt_require, igt_skip, igt_info or igt_debug.
+
+- New library function could be written when it will have at least two
+  different users, for example if it could be used by two or more tests.
+
+- In a new library function():
+  if it has macros (for example igt_assert/igt_require/igt_skip) then
+    write also __function() counterpart without them;
+  limit also usage of prints igt_info/igt_debug, this should be a part
+    of test code.
+
+- Libraries and igt_runner
+  Runner should not use lib functions. It is crucial for CI runs so using
+  libraries puts a risk of bringing changes meant for tests which in turn
+  could break runner.
+  Note: You will find places where igt_runner uses lib functions - this will
+  be on ToDo list to be fixed.
+
+- Libraries and tools/
+  Tools should try to not use lib functions. Any abnormal condition should
+  be simply reported by printf or fprintf to stdout/stderr and then tool
+  should exit gracefully. Do not use igt_abort nor igt_assert, igt_print,
+  igt_debug nor other testing/printing macros from igt lib/
 
 Sending Patches
 ---------------
-- 
2.49.0