Nsryjdtyk

I use dma_alloc_coherent() in my custom driver to get both virtual and bus addresses.

res->KernelAddress = (u64)dma_alloc_coherent( &DevExt->pdev->dev, size, &res->BusAddress, GFP_ATOMIC );

When printing (%llx) the bus address (res->BusAddress), I got 80009000 as the one.
I checked the log of /proc/iomem to verify the range, but there are multiple entries.

The log of /proc/iomem is shown below:

10000000-10000fff : /pcie-controller@10003000/pci@1,0

10003000-100037ff : pcie-pads

10003800-10003fff : pcie-afi

10004000-10004fff : /pcie-controller@10003000/pci@3,0

40000000-4fffffff : pcie-config-space

50100000-57ffffff : pcie-non-prefetchable

  50800000-52ffffff : PCI Bus 0000:01

    50800000-5087ffff : 0000:01:00.0

    51000000-51ffffff : 0000:01:00.0

    52000000-52ffffff : 0000:01:00.0

58000000-7fffffff : pcie-prefetchable

  58000000-58ffffff : PCI Bus 0000:01

    58000000-58ffffff : 0000:01:00.0

80000000-d82fffff : System RAM

  80080000-810fafff : Kernel code

  8123f000-814b3fff : Kernel data

d9300000-efffffff : System RAM

f0200000-275ffffff : System RAM

276600000-2767fffff : System RAM

1. Is 80009000 valid? Which section does it belong to?


2. Is it necessary to use dma_mmap_coherent() after dma_alloc_coherent() for proper mapping?

Thanks in advance !!

From https://www.kernel.org/doc/Documentation/bus-virt-phys-mapping.txt (some of the details of this file are now obsolete but it's the best overview of the issue):

Essentially, the three ways of addressing memory are (this is "real memory",
that is, normal RAM--see later about other details):

• CPU untranslated. This is the "physical" address. Physical address
  0 is what the CPU sees when it drives zeroes on the memory bus.




• CPU translated address. This is the "virtual" address, and is
  completely internal to the CPU itself with the CPU doing the appropriate
  translations into "CPU untranslated".




• bus address. This is the address of memory as seen by OTHER devices,
  not the CPU. Now, in theory there could be many different bus
  addresses, with each device seeing memory in some device-specific way, but
  happily most hardware designers aren't actually actively trying to make
  things any more complex than necessary, so you can assume that all
  external hardware sees the memory the same way.

Now, on normal PCs the bus address is exactly the same as the physical
address, and things are very simple indeed. However, they are that simple
because the memory and the devices share the same address space, and that is
not generally necessarily true on other PCI/ISA setups.

The bottom line is that the answer to your question is architecture-dependent.

In your /proc/iomem snippet, note that that that listing is nested. The 80009000 address appears to fall into two sections because one of those sections is a subset of the other. If that address was a physical memory address, then yes, it would be a "kernel code address", which would be a strange thing to get back from dma_alloc_coherent. That leads me to believe that a physical address is not the same as a bus address on your architecture.

dma_alloc_coherent also maps the memory in kernel virtual address space so you shouldn't need to do anything else to access it from your code. (dma_mmap_coherent is used to map the memory into user virtual address space.)