FWIW, on ARMv6 and later hardware, the dma_alloc_coherent() provides
writecombine memory (i.e. Normal Noncacheable), so no need for
dma_alloc_writecombine(). On earlier architectures it is creating
Strongly Ordered mappings (no writecombine).

Thanks.

Do you mean that dma_alloc_coherent() and dma_alloc_writecombine() are
not different
except some additional features of dma_alloc_coherent() in ARM?