Custom CMA reserved memory on i.MX8MM mini

[matt67]

Hi there,

I have a custom design with 1GB of RAM on i.MX8MM however the dtsi file is setting the shared-dma-pool with a quiet huge size.

  /* global autoconfigured region for contiguous allocations */
  linux,cma {
   compatible = "shared-dma-pool";
   reusable;
   size = <0 0x28000000>;
   alloc-ranges = <0 0x40000000 0 0x60000000>;
   linux,cma-default;
  };
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I don't have video processing, how could I decrease it to its minimal value in a good way and with which values ?

Thank you

[b36401]

It is application dependent.
CMA is required for DMA operations.
So you need to estimate how many buffers is required for the interfacing (SD card, camera, LCD, etc).
Alctually we did not measure the minimum of CMA.

[matt67]

Thanks Victor. Can you explain me the alloc-range property. Is that <address, size, address size > like binding documentation tells? So why overlapping? 

[b36401]

Here is a documentation regardign to device tree usage:
https://elinux.org/Device_Tree_Usage