Merge pull request #1841 from igchor/l0_v2_readme

[L0 v2] Add readme

Author: pbalcer

source/adapters/level_zero/v2/README.md

@@ -0,0 +1,14 @@
+# L0 v2 adapter
+
+This is the home directory for L0 v2 adapter sources. This is a redesigned version of the L0 adapter that focuses on maximizing the performance of each queue mode individually (immediate/batched, in-order/out-of-order).
+
+L0 v2 adapter can be enabled by setting `UR_L0_USE_QUEUE_V2=1` env variable. If the variable is not set, legacy path will be used.
+
+# Code structure
+
+v2 adapter only rewrites certain functions (mostly urEnqueue* functions) while reusing the rest. `ur_queue_handle_t` has become an abstract class and each enqueue function a virtual function.
+
+Legacy enqeue path is implemented in `ur_queue_handle_legacy_t` which inherits from `ur_queue_handle_t`. For new, optimized path, each queue mode will be implemented as a separate queue class (e.g. `v2::ur_queue_immediate_in_order_t`) inheriting from `ur_queue_handle_t`.
+
+`ur_queue_handle_t` is auto-generated by `make generate-code` - for every API function that accepts `ur_queue_handle_t` as a first parameter, new pure virtual method is created. The API function is then
+auto-implemented (see ../queue_api.cpp) by dispatching to that virtual method. Developer is only responsbile for implementing that virtual function for every queue base class.