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

Question

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.

Answer 1

If you want to allocate in a specific region, you can reserve this memory in the device tree and use it through the DMA-API (dma_alloc_coherent for example)

In your device tree:

reserved-memory {
      #address-cells = <2>;
      #size-cells = <2>;
      ranges;
  
      reserved: buffer@0 {
         compatible = "shared-dma-pool"; // to be able to access this memory throught the DMA-API
         no-map;
         reg = <0x0 0x70000000 0x0 0x10000000>;
      };
   };
  
   reserved-driver@0 {
      compatible = "xlnx,reserved-memory";
      memory-region = <&reserved>;
   };

In your device driver:

  /* Initialize reserved memory resources */
  rc = of_reserved_mem_device_init(dev);
  if(rc) {
    dev_err(dev, "Could not get reserved memory\n");
    goto error1;
  }

  /* Allocate memory */
  dma_set_coherent_mask(dev, 0xFFFFFFFF);
  lp->vaddr = dma_alloc_coherent(dev, ALLOC_SIZE, &lp->paddr, GFP_KERNEL);
  dev_info(dev, "Allocated coherent memory, vaddr: 0x%0llX, paddr: 0x%0llX\n", (u64)lp->vaddr, lp->paddr);

Great explanation of how this works: https://xilinx-wiki.atlassian.net/wiki/spaces/A/pages/18841683/Linux+Reserved+Memory