opencl map to cpu cost long time

I used opencl1.1 on Mali628(Exynos5422)。

1.

first I create a buffer

buffer = clCreateBuffer(context, CL_MEM_READ_WRITE | CL_MEM_ALLOC_HOST_PTR, 1280*720*4, NULL, &errorNumber);

next call the kernel to handle the buffer and waiting the command queue finish.

finally, map the buffer to cpu

*pCPU = (unsigned char *)clEnqueueMapBuffer(command_queue,buffer,

            CL_TRUE,    

            CL_MAP_READ,

            0,

            1280*720*4,

            0, NULL, NULL, &errorNumber);

the process buffer map to cpu takes 1843us

2.

change CL_MAP_READ to CL_MAP_WRITE

*pCPU = (unsigned char *)clEnqueueMapBuffer(command_queue,buffer,

            CL_TRUE,    

            CL_MAP_WRITE,

            0,

            1280*720*4,

            0, NULL, NULL, &errorNumber);

I though it will save time , but it still takes 1891us

In my opinion, CL_MEM_ALLOC_HOST_PTR mean map/umap the buffer will only translate the pointer (may been flush the cache) but not to copy memory.why it cost a long time.

thx!