How can I allocate dma buffers in a very specific range in linux kernel?

I have a microprocessor which can access only 0xFFFFFF bytes in the DDR through its data cache. I can give it the offset where it can start to read these 0xFFFFFF the DDR

I want to allocate dma buffers for it.

I saw that I can give a mask to restrict the place these dma buffers can be allocated.

How can I know where the allocation will start for the dma_coherent_alloc to set the offset for the microprocessor?

Are there any other solutions ? Like allocating a big buffer and then do some suballocation in it (I don't know if the kernel already has some facilities for that)

Thanks in advance.