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| -rw-r--r-- | Documentation/bus-virt-phys-mapping.txt | 220 |
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diff --git a/Documentation/bus-virt-phys-mapping.txt b/Documentation/bus-virt-phys-mapping.txt new file mode 100644 index 000000000..4bb07c2f3 --- /dev/null +++ b/Documentation/bus-virt-phys-mapping.txt @@ -0,0 +1,220 @@ +========================================================== +How to access I/O mapped memory from within device drivers +========================================================== + +:Author: Linus + +.. warning:: + + The virt_to_bus() and bus_to_virt() functions have been + superseded by the functionality provided by the PCI DMA interface + (see Documentation/DMA-API-HOWTO.txt).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:: + + 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? |