linux-imx

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How to access I/O mapped memory from within device drivers

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:Author: Linus

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.. warning::

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    The virt_to_bus() and bus_to_virt() functions have been

    superseded by the functionality provided by the PCI DMA interface

    (see Documentation/DMA-API-HOWTO.txt).  They continue

    to be documented below for historical purposes, but new code

    must not use them. --davidm 00/12/12

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::

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  [ This is a mail message in response to a query on IO mapping, thus the

    strange format for a "document" ]

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The AHA-1542 is a bus-master device, and your patch makes the driver give the

controller the physical address of the buffers, which is correct on x86

(because all bus master devices see the physical memory mappings directly). 

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However, on many setups, there are actually **three** different ways of looking

at memory addresses, and in this case we actually want the third, the

so-called "bus address". 

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Essentially, the three ways of addressing memory are (this is "real memory",

that is, normal RAM--see later about other details): 

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 - CPU untranslated.  This is the "physical" address.  Physical address 

   0 is what the CPU sees when it drives zeroes on the memory bus.

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 - 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". 

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 - 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. 

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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. 

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Now, just as an example, on the PReP (PowerPC Reference Platform), the 

CPU sees a memory map something like this (this is from memory)::

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    0-2 GB      "real memory"

    2 GB-3 GB   "system IO" (inb/out and similar accesses on x86)

    3 GB-4 GB   "IO memory" (shared memory over the IO bus)

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Now, that looks simple enough. However, when you look at the same thing from

the viewpoint of the devices, you have the reverse, and the physical memory

address 0 actually shows up as address 2 GB for any IO master.

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So when the CPU wants any bus master to write to physical memory 0, it 

has to give the master address 0x80000000 as the memory address.

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So, for example, depending on how the kernel is actually mapped on the 

PPC, you can end up with a setup like this::

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 physical address:  0

 virtual address:   0xC0000000

 bus address:       0x80000000

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where all the addresses actually point to the same thing.  It's just seen 

through different translations..

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Similarly, on the Alpha, the normal translation is::

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 physical address:  0

 virtual address:   0xfffffc0000000000

 bus address:       0x40000000

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(but there are also Alphas where the physical address and the bus address

are the same). 

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Anyway, the way to look up all these translations, you do::

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    #include <asm/io.h>

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    phys_addr = virt_to_phys(virt_addr);

    virt_addr = phys_to_virt(phys_addr);

     bus_addr = virt_to_bus(virt_addr);

    virt_addr = bus_to_virt(bus_addr);

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Now, when do you need these?