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-[ NOTE: 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- superseded by the functionality provided by the PCI DMA interface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 ]
-
-[ 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.
- */
- void __iomem *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
-