将内存从一个内核模块映射到另一个模块中的DMA缓冲区

Question

I am working on a project where module A has a memory buffer that has DMA content and calls module B 's function to perform DMA operation. To simplfy it looks like below:

Module A:

void get_info()
{
    void *outBuffer = kmalloc(10);
    void *inBuffer = kmalloc(10);

    perform_dma(outBuffer);       // function from module B

    read_output(&inBuffer);      // function from module B
}

Module B:

void perform_dma(void *outBuffer)
{
    void *dma_buffer = dma_alloc_coherent()  // <-- allocate a new DMA buffer
    memcpy(dma_buffer, outBuffer, 10);

    do_dma();  // <-- after this is done, dma_buffer has the content module A needs.
}

void read_output(void **inBuffer)
{
    memcpy(*inBuffer, dma_buffer, 10);
}

I understand this might not be efficient but I can't change module B and can use only provided APIs. I am allowed to change B 's perform_dma(void *outBuffer) prototype to perform_dma(void **outBuffer) in best case. It works most of time but in a situation (due to all interrupts/queue events involved in the whole project) I won't be able to call read_output() explicitly and provide an inBuffer to read the content back but can only access outBuffer address after sending it to B .

Is there a way I can map outBuffer form module A to the dma_buffer allocated in module B to be able to read the output back from same outBuffer ?

Answer 1

I have figured out that dma_map_single()/dma_unmap_single() work in my case. There is no need to change function prototype at all. Just use dma_map_single(outBuffer...., DMA_BIDIRECTIONAL) inside perform_dma() to obtain a bus address and pass it to dma controller, so after DMA is done outBuffer would have the content returned from dma controller.