autoload/lsp/completion.vim |  7 +++++++
 doc/lsp.txt                 | 12 ++++++++++++

diff --git a/autoload/lsp/completion.vim b/autoload/lsp/completion.vim
index 91971651eef229d686f95be0f6628c93097a5c0e..70bb2ae74269921287b7a8988ddaff15f35dea64 100644
--- a/autoload/lsp/completion.vim
+++ b/autoload/lsp/completion.vim
@@ -488,6 +488,13 @@     return res->filter((i, v) => v.word->stridx(prefix) == 0)
   endif
 enddef
 
+# For plugins that implement async completion this function indicates if
+# omnifunc is waiting for LSP response.
+def g:LspOmniCompletePending(): bool
+  var lspserver: dict<any> = buf.CurbufGetServerChecked('completion')
+  return !lspserver->empty() && lspserver.omniCompletePending
+enddef
+
 # Insert mode completion handler. Used when 24x7 completion is enabled
 # (default).
 def LspComplete()
diff --git a/doc/lsp.txt b/doc/lsp.txt
index 7d46ad1b7805bf2fd14e590580969172387dd836..0c60316f978719e33ea1690b2588fc21d654667f 100644
--- a/doc/lsp.txt
+++ b/doc/lsp.txt
@@ -1257,6 +1257,18 @@ do the filtering.  By default, case sensitive comparison is used to filter the
 returned items.  You can modify the 'completionMatcher' option to use either
 case insensitive or fuzzy comparison.
 
+In addition to the automatic completion and omni completion, it is possible to
+use external completion engines which provide asynchronous completion
+from various sources. LSP client can be used as a completion source by
+repurposing `g:LspOmniFunc` function. It needs to be invoked twice as
+described in |complete-functions|. After the first invocation a request is
+sent to the LSP server to find completion candidates. Later the function is
+called again to actually obtain the matches. If the LSP server has not replied
+with the matches `g:LspOmniFunc` could wait up to 2 seconds. To avoid blocking
+wait call `g:LspOmniCompletePending` function which returns `true` immediately
+if LSP server is not ready. `g:LspOmniFunc` should be called the second time
+only after this function returns `false`.
+
 ==============================================================================
 7. Diagnostics						*lsp-diagnostics*