FreeBSD/Linux Kernel Cross Reference
sys/Documentation/IO-mapping.txt

     [ NOTE: The virt_to_bus() and bus_to_virt() functions have been
             superseded by the functionality provided by the PCI DMA interface
             (see Documentation/PCI/PCI-DMA-mapping.txt).  They continue
             to be documented below for historical purposes, but new code
             must not use them. --davidm 00/12/12 ]
     
     [ This is a mail message in response to a query on IO mapping, thus the
       strange format for a "document" ]
     
    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). 
    
    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". 
    
    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. 
    
    Now, just as an example, on the PReP (PowerPC Reference Platform), the 
    CPU sees a memory map something like this (this is from memory):
    
            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)
    
    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.
    
    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.
    
    So, for example, depending on how the kernel is actually mapped on the 
    PPC, you can end up with a setup like this:
    
     physical address:      0
     virtual address:       0xC0000000
     bus address:           0x80000000
    
    where all the addresses actually point to the same thing.  It's just seen 
    through different translations..
    
    Similarly, on the Alpha, the normal translation is
    
     physical address:      0
     virtual address:       0xfffffc0000000000
     bus address:           0x40000000
    
    (but there are also Alphas where the physical address and the bus address
    are the same). 
    
    Anyway, the way to look up all these translations, you do
    
            #include <asm/io.h>
    
            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);
    
    Now, when do you need these?
    
    You want the _virtual_ address when you are actually going to access that 
    pointer from the kernel. So you can have something like this:
    
            /*
             * this is the hardware "mailbox" we use to communicate with
             * the controller. The controller sees this directly.
             */
            struct mailbox {
                    __u32 status;
                    __u32 bufstart;
                    __u32 buflen;
                    ..
            } mbox;
    
                    unsigned char * retbuffer;
    
                   /* get the address from the controller */
                   retbuffer = bus_to_virt(mbox.bufstart);
                   switch (retbuffer[0]) {
                           case STATUS_OK:
                                   ...
   
   on the other hand, you want the bus address when you have a buffer that 
   you want to give to the controller:
   
           /* ask the controller to read the sense status into "sense_buffer" */
           mbox.bufstart = virt_to_bus(&sense_buffer);
           mbox.buflen = sizeof(sense_buffer);
           mbox.status = 0;
           notify_controller(&mbox);
   
   And you generally _never_ want to use the physical address, because you can't
   use that from the CPU (the CPU only uses translated virtual addresses), and
   you can't use it from the bus master. 
   
   So why do we care about the physical address at all? We do need the physical
   address in some cases, it's just not very often in normal code.  The physical
   address is needed if you use memory mappings, for example, because the
   "remap_pfn_range()" mm function wants the physical address of the memory to
   be remapped as measured in units of pages, a.k.a. the pfn (the memory
   management layer doesn't know about devices outside the CPU, so it
   shouldn't need to know about "bus addresses" etc).
   
   NOTE NOTE NOTE! The above is only one part of the whole equation. The above
   only talks about "real memory", that is, CPU memory (RAM). 
   
   There is a completely different type of memory too, and that's the "shared
   memory" on the PCI or ISA bus. That's generally not RAM (although in the case
   of a video graphics card it can be normal DRAM that is just used for a frame
   buffer), but can be things like a packet buffer in a network card etc. 
   
   This memory is called "PCI memory" or "shared memory" or "IO memory" or
   whatever, and there is only one way to access it: the readb/writeb and
   related functions. You should never take the address of such memory, because
   there is really nothing you can do with such an address: it's not
   conceptually in the same memory space as "real memory" at all, so you cannot
   just dereference a pointer. (Sadly, on x86 it _is_ in the same memory space,
   so on x86 it actually works to just deference a pointer, but it's not
   portable). 
   
   For such memory, you can do things like
   
    - reading:
           /*
            * read first 32 bits from ISA memory at 0xC0000, aka
            * C000:0000 in DOS terms
            */
           unsigned int signature = isa_readl(0xC0000);
   
    - remapping and writing:
           /*
            * remap framebuffer PCI memory area at 0xFC000000,
            * size 1MB, so that we can access it: We can directly
            * access only the 640k-1MB area, so anything else
            * has to be remapped.
            */
           char * baseptr = ioremap(0xFC000000, 1024*1024);
   
           /* write a 'A' to the offset 10 of the area */
           writeb('A',baseptr+10);
   
           /* unmap when we unload the driver */
           iounmap(baseptr);
   
    - copying and clearing:
           /* get the 6-byte Ethernet address at ISA address E000:0040 */
           memcpy_fromio(kernel_buffer, 0xE0040, 6);
           /* write a packet to the driver */
           memcpy_toio(0xE1000, skb->data, skb->len);
           /* clear the frame buffer */
           memset_io(0xA0000, 0, 0x10000);
   
   OK, that just about covers the basics of accessing IO portably.  Questions?
   Comments? You may think that all the above is overly complex, but one day you
   might find yourself with a 500 MHz Alpha in front of you, and then you'll be
   happy that your driver works ;)
   
   Note that kernel versions 2.0.x (and earlier) mistakenly called the
   ioremap() function "vremap()".  ioremap() is the proper name, but I
   didn't think straight when I wrote it originally.  People who have to
   support both can do something like:
    
           /* support old naming silliness */
           #if LINUX_VERSION_CODE < 0x020100                                     
           #define ioremap vremap
           #define iounmap vfree                                                     
           #endif
    
   at the top of their source files, and then they can use the right names
   even on 2.0.x systems. 
   
   And the above sounds worse than it really is.  Most real drivers really
   don't do all that complex things (or rather: the complexity is not so
   much in the actual IO accesses as in error handling and timeouts etc). 
   It's generally not hard to fix drivers, and in many cases the code
   actually looks better afterwards:
   
           unsigned long signature = *(unsigned int *) 0xC0000;
                   vs
           unsigned long signature = readl(0xC0000);
   
   I think the second version actually is more readable, no?
   
                   Linus
   

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