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dma_alloc_coherent memory with mmap

Question asked by jack ou on Feb 14, 2018
Latest reply on Feb 18, 2018 by jack ou


I am writing a pcie driver for LS1021a to get data from the pcie card.    

In driver, memory is allocated by:

   virt_mem = dma_alloc_coherent(&dev, size, &dma_mem, GFP_KERNEL);

Everything works fine, I get virt_mem for kernel and dma_mem for DMA. 

This memory is mmapped to user application by:

    remap_pfn_range(vma, vma->vm_start,  virt_to_phys(dma_mem)>>PAGE_SHIFT,  vma->vm_end-vma->vm_start, vma->vm_page_prot);


The mmap is successful in user application.  

During self test, the driver (instead of DMA) writes value to this memory, and user application reads it out.  Driver writes 1 -> being read, writes 2 ->being read, ......    user application is supposed to get data in right sequence, 1 2 3 4 ...      


Issue:   User application doesn't get the right sequence of data.  Sometimes it gets right number, sometimes gets old value.   

The memory should be non-cacheable or coherent.  The driver written data should be visible to user application.  I am thinking it might be due to something like write-buffer somewhere.   I tried using mb() or __asm("dsb") after each data write.  But no help, application still gets wrong data sequence.    

I then tried allocating the memory using kmalloc (instead of dma_alloc_coherent), and use remap_pfn_range again for mmap.   Everything works fine this time.  I believe because it is cacheable memory, the driver-write data is correctly read by application.   


Can't figure out what goes wrong.   Is remap_pfn_range the right choice for dma_alloc_coherent memory ?  why mb() seems no help at all ? 

Thanks !