document capability to disable asynchronous copies

ChangeLog

@@ -18,6 +18,8 @@ StarPU 1.0 (svn revision xxxx)
 ==============================================
 The extensions-again release
 
+  * Allow users to disable asynchronous data transfers between CPUs and
+	GPUs.
   * Update OpenCL driver to enable CPU devices (the environment variable
 	STARPU_OPENCL_ON_CPUS must be set to a positive value when
 	executing an application)

doc/chapters/basic-api.texi

@@ -113,7 +113,15 @@ By default, StarPU parallel tasks concurrently.
 Some parallel libraries (e.g. most OpenMP implementations) however do
 not support concurrent calls to parallel code. In such case, setting this flag
 makes StarPU only start one parallel task at a time.
-@code{STARPU_SINGLE_COMBINED_WORKER} environment variable.
+This can also be specified with the @code{STARPU_SINGLE_COMBINED_WORKER} environment variable.
+
+@item @code{int disable_asynchronous_copy} (default = 0)
+This flag should be set to 1 to disable asynchronous copies between
+CPUs and accelerators. This can also be specified with the
+@code{STARPU_DISABLE_ASYNCHRONOUS_COPY} environment variable.
+The AMD implementation of OpenCL is known to
+fail when copying data asynchronously. When using this implementation,
+it is therefore necessary to disable asynchronous data transfers.
 @end table
 @end deftp