(ubuntu 18.04) opencv 4.0.1 cv::ocl::setUseOpenCL(true); does not work

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Hello, i try to use the T-API in a features detection & matching app. i succesfully install opencl drive & compile opencv with opencl support. when i take a look at cv::ocl::haveOpenCL(); it return true (so i guess my opencl installation/linking is ok) but cv::ocl::useOpenCL() return false even if i use cv::ocl::setUseOpenCL(true) first.

my app run well with UMat but all fonction are used in plain implementation (logs : norm: Plain implementation is running arithm_op: Plain implementation is running)

clinfo command give me following result :

Number of platforms 1

Platform Name Intel(R) CPU Runtime for OpenCL(TM)

Applications Platform Vendor

Intel(R) Corporation Platform

Version

OpenCL 2.1 LINUX Platform Profile

FULL_PROFILE Platform Extensions

cl_khr_icd

cl_khr_global_int32_base_atomics

cl_khr_global_int32_extended_atomics

cl_khr_local_int32_base_atomics

cl_khr_local_int32_extended_atomics

cl_khr_byte_addressable_store

cl_khr_depth_images

cl_khr_3d_image_writes

cl_intel_exec_by_local_thread

cl_khr_spir cl_khr_fp64

cl_khr_image2d_from_buffer

cl_intel_vec_len_hint Platform Host

timer resolution 1ns

Platform Extensions function suffix

INTEL

Platform Name
Intel(R) CPU Runtime for OpenCL(TM) Applications Number of devices
1 Device Name
Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz Device Vendor Intel(R) Corporation Device Vendor ID
0x8086 Device Version
OpenCL 2.1 (Build 0) Driver Version 18.1.0.0920 Device OpenCL C Version OpenCL C 2.0 Device Type
CPU Device Profile
FULL_PROFILE Device Available
Yes Compiler Available
Yes Linker Available
Yes Max compute units
4 Max clock frequency
2400MHz Device Partition
(core) Max number of sub-devices 4 Supported partition types by counts, equally, by names (Intel)
Max work item dimensions
3 Max work item sizes
8192x8192x8192 Max work group size
8192 Preferred work group size multiple 128 Max sub-groups per work group
1 Preferred / native vector sizes

char                                                 1 / 32      
short                                                1 / 16      
int                                                  1 / 8       
long                                                 1 / 4       
half                                                 0 / 0        (n/a)
float                                                1 / 8       
double                                               1 / 4        (cl_khr_fp64)

Half-precision Floating-point support (n/a) Single-precision Floating-point support (core) Denormals Yes Infinity and NANs Yes Round to nearest Yes Round to zero No Round to infinity No IEEE754-2008 fused multiply-add No Support is emulated in software No Correctly-rounded divide and sqrt operations No Double-precision Floating-point support
(cl_khr_fp64) Denormals Yes Infinity and NANs Yes Round to nearest Yes Round to zero Yes Round to infinity Yes IEEE754-2008 fused multiply-add Yes Support is emulated in software No Address bits
64, Little-Endian Global memory size 16675332096 (15.53GiB) Error Correction support
No Max memory allocation
4168833024 (3.883GiB) Unified memory for Host and Device Yes
Shared Virtual Memory (SVM) capabilities (core) Coarse-grained buffer sharing Yes Fine-grained buffer sharing Yes Fine-grained system sharing Yes Atomics Yes Minimum alignment for any data type 128 bytes Alignment of base address
1024 bits (128 bytes) Preferred alignment for atomics
SVM 64 bytes Global 64 bytes Local 0 bytes Max size for global variable 65536 (64KiB) Preferred total size of global vars 65536 (64KiB) Global Memory cache type
Read/Write Global Memory cache size 262144 (256KiB) Global Memory cache line size 64 bytes
Image support
Yes Max number of samplers per kernel 480 Max size for 1D images from buffer 260552064 pixels Max 1D ...