Flushing a virtual address range

Question

I am developing a Linux kernel module that interfaces with a hardware component accessing the DDR using DMA. (running on an arm SoC)

What the driver essentially does is allocate a buffer (w/ kmalloc), and write the physical (using virt_to_phys) address of the buffer to the underlying hardware

When reading the buffer, the old data seems to get read, which points to a cache problem (when inspecting the DDR directly, bypassing Linux, the new data is read).

I can't seem to understand how cache flushing / invalidation works.

What is the best approach?

Using kernel 3.14

Answer 1

You have 2 options:

1. use coherent mapping that is allocate memory using dma_alloc_coherent

2. use streaming mapping that is map the buffer for dma using dma_map_single telling the kernel the DMA direction and unmap it after you finish