[SYCL] Document test-related processes

Author: vladimirlaz

CONTRIBUTING.md

@@ -20,6 +20,11 @@ is strongly encouraged that you submit the patch to https://llvm.org/ directly.
 See [LLVM contribution guidelines](https://llvm.org/docs/Contributing.html)
 for more information.
 
+**NB**: A change in compiler and runtime should be accompanied with
+corresponding test change.
+See [DPC++ test toolchain](sycl/doc/GetStartedGuide.md#test-dpc-toolchain)
+for more information.
+
 **Note (October, 2020)**: DPC++ runtime and compiler ABI is currently in frozen
 state. This means that no ABI-breaking changes will be accepted by default.
 Project maintainers may still approve breaking changes in some cases. Please,

sycl/doc/GetStartedGuide.md

@@ -280,7 +280,50 @@ command:
 
 ### Test DPC++ toolchain
 
-#### Run regression tests
+There are 3 types of tests which are used for DPC++ toolchain validation:
+* DPC++ in-source LIT tests including [check-llvm](../../llvm/test),
+[check-clang](../../clang/test), [check-llvm-spirv](../../llvm-spirv/test) and
+[check-sycl](../../sycl/test) targets stored in this repository. These tests
+should be not have hardware or external software dependencies (e.g OpenCL,
+Level Zero, CUDA runtimes). All test which do not follow this approach should
+be moved to DPC++ end-to-end tests.
+
+* DPC++ end-to-end (E2E) tests which are extension to
+[LLVM\* test suite](https://github.com/intel/llvm-test-suite/tree/intel/SYCL).
+These tests cover full DPC++ stack including all dependencies (e.g. OpenCL,
+Level Zero, CUDA runtimes).
+
+* SYCL-CTS are official [Khronos\* SYCL\* conformance tests](https://github.com/KhronosGroup/SYCL-CTS).
+They verify SYCL 1.2.1 specification compatibility. All implementation details
+or extensions are out of scope for the tests.
+
+#### Contribution to DPC++ tests
+Every product change should be accompanied with corresponding test modification
+(adding new test(s), extending or modifying existing test(s)).
+
+The test location is selected depending on test nature:
+
+ - A test which depends only on tools and libraries produced during build of
+the repository can be put to in-source LIT. Also the tests for a feature under
+active development requiring atomic change for tests and product can be put to
+[sycl/test/on-device](../../sycl/test/on-device) temporarily. It is developer
+responsibility to move the tests to DPC++ E2E test suite once feature is
+stabilized.
+
+ - A test which requires full stack including backend runtimes (e.g. OpenCL,
+Level Zero or CUDA) should be put to DPC++ E2E test suite following
+[CONTRIBUTING](https://github.com/intel/llvm-test-suite/blob/intel/CONTRIBUTING.md).
+
+ - If SYCL specification has changed (SYCL CTS tests conflict with recent
+version of SYCL specification) or change is required in the way the tests are
+built with DPC++ compiler (defined in
+[FindIntel_SYCL](https://github.com/KhronosGroup/SYCL-CTS/blob/SYCL-1.2.1/master/cmake/FindIntel_SYCL.cmake))
+pull request should be created under
+[KhronosGroup/SYCL-CTS](https://github.com/KhronosGroup/SYCL-CTS) with required
+patch.
+
+
+#### Run in-source LIT tests
 
 To verify that built DPC++ toolchain is working correctly, run:
 
@@ -302,6 +345,11 @@ skipped.
 If CUDA support has been built, it is tested only if there are CUDA devices
 available.
 
+#### Run DPC++ E2E test suite
+
+Follow instructions from the link below to build and run tests:
+[README](https://github.com/intel/llvm-test-suite/tree/intel/SYCL#execution)
+
 #### Run Khronos\* SYCL\* conformance test suite (optional)
 
 Khronos\* SYCL\* conformance test suite (CTS) is intended to validate