difference between pci_alloc_consistent and dma_alloc_coherent

Question

I am working on pcie based network driver. Different examples use one of pci_alloc_consistent or dma_alloc_coherent to get memory for transmission and reception descriptors. Which one is better if any and what is the difference between the two?

Answer 1

The difference is subtle but quite important. pci_alloc_consistent() is the older function of the two and legacy drivers still use it. Nowadays, pci_alloc_consistent() just calls dma_alloc_coherent().

The difference? The type of the allocated memory.

• pci_alloc_consistent() - Allocates memory of type GFP_ATOMIC. Allocation does not sleep, for use in e.g. interrupt handlers, bottom halves.

• dma_alloc_coherent()- You specify yourself what type of memory to allocate. You should not use the high priority GFP_ATOMIC memory unless you need it and in most cases, you will be fine with GFP_KERNEL allocations.

Kernel 3.18 definition of pci_alloc_consistent() is very simple, namely:

 static inline void *
 pci_alloc_consistent(struct pci_dev *hwdev, size_t size,
                      dma_addr_t *dma_handle)
 {
         return dma_alloc_coherent(hwdev == NULL ? NULL : &hwdev->dev, size, dma_handle, GFP_ATOMIC);
 }

In short, use dma_alloc_coherent().