1[ NOTE: The virt_to_bus() and bus_to_virt() functions have been 2 superseded by the functionality provided by the PCI DMA interface 3 (see Documentation/DMA-API-HOWTO.txt). They continue 4 to be documented below for historical purposes, but new code 5 must not use them. --davidm 00/12/12 ] 6 7[ This is a mail message in response to a query on IO mapping, thus the 8 strange format for a "document" ] 9 10The AHA-1542 is a bus-master device, and your patch makes the driver give the 11controller the physical address of the buffers, which is correct on x86 12(because all bus master devices see the physical memory mappings directly). 13 14However, on many setups, there are actually _three_ different ways of looking 15at memory addresses, and in this case we actually want the third, the 16so-called "bus address". 17 18Essentially, the three ways of addressing memory are (this is "real memory", 19that is, normal RAM--see later about other details): 20 21 - CPU untranslated. This is the "physical" address. Physical address 22 0 is what the CPU sees when it drives zeroes on the memory bus. 23 24 - CPU translated address. This is the "virtual" address, and is 25 completely internal to the CPU itself with the CPU doing the appropriate 26 translations into "CPU untranslated". 27 28 - bus address. This is the address of memory as seen by OTHER devices, 29 not the CPU. Now, in theory there could be many different bus 30 addresses, with each device seeing memory in some device-specific way, but 31 happily most hardware designers aren't actually actively trying to make 32 things any more complex than necessary, so you can assume that all 33 external hardware sees the memory the same way. 34 35Now, on normal PCs the bus address is exactly the same as the physical 36address, and things are very simple indeed. However, they are that simple 37because the memory and the devices share the same address space, and that is 38not generally necessarily true on other PCI/ISA setups. 39 40Now, just as an example, on the PReP (PowerPC Reference Platform), the 41CPU sees a memory map something like this (this is from memory): 42 43 0-2 GB "real memory" 44 2 GB-3 GB "system IO" (inb/out and similar accesses on x86) 45 3 GB-4 GB "IO memory" (shared memory over the IO bus) 46 47Now, that looks simple enough. However, when you look at the same thing from 48the viewpoint of the devices, you have the reverse, and the physical memory 49address 0 actually shows up as address 2 GB for any IO master. 50 51So when the CPU wants any bus master to write to physical memory 0, it 52has to give the master address 0x80000000 as the memory address. 53 54So, for example, depending on how the kernel is actually mapped on the 55PPC, you can end up with a setup like this: 56 57 physical address: 0 58 virtual address: 0xC0000000 59 bus address: 0x80000000 60 61where all the addresses actually point to the same thing. It's just seen 62through different translations.. 63 64Similarly, on the Alpha, the normal translation is 65 66 physical address: 0 67 virtual address: 0xfffffc0000000000 68 bus address: 0x40000000 69 70(but there are also Alphas where the physical address and the bus address 71are the same). 72 73Anyway, the way to look up all these translations, you do 74 75 #include <asm/io.h> 76 77 phys_addr = virt_to_phys(virt_addr); 78 virt_addr = phys_to_virt(phys_addr); 79 bus_addr = virt_to_bus(virt_addr); 80 virt_addr = bus_to_virt(bus_addr); 81 82Now, when do you need these? 83 84You want the _virtual_ address when you are actually going to access that 85pointer from the kernel. So you can have something like this: 86 87 /* 88 * this is the hardware "mailbox" we use to communicate with 89 * the controller. The controller sees this directly. 90 */ 91 struct mailbox { 92 __u32 status; 93 __u32 bufstart; 94 __u32 buflen; 95 .. 96 } mbox; 97 98 unsigned char * retbuffer; 99 100 /* get the address from the controller */ 101 retbuffer = bus_to_virt(mbox.bufstart); 102 switch (retbuffer[0]) { 103 case STATUS_OK: 104 ... 105 106on the other hand, you want the bus address when you have a buffer that 107you want to give to the controller: 108 109 /* ask the controller to read the sense status into "sense_buffer" */ 110 mbox.bufstart = virt_to_bus(&sense_buffer); 111 mbox.buflen = sizeof(sense_buffer); 112 mbox.status = 0; 113 notify_controller(&mbox); 114 115And you generally _never_ want to use the physical address, because you can't 116use that from the CPU (the CPU only uses translated virtual addresses), and 117you can't use it from the bus master. 118 119So why do we care about the physical address at all? We do need the physical 120address in some cases, it's just not very often in normal code. The physical 121address is needed if you use memory mappings, for example, because the 122"remap_pfn_range()" mm function wants the physical address of the memory to 123be remapped as measured in units of pages, a.k.a. the pfn (the memory 124management layer doesn't know about devices outside the CPU, so it 125shouldn't need to know about "bus addresses" etc). 126 127NOTE NOTE NOTE! The above is only one part of the whole equation. The above 128only talks about "real memory", that is, CPU memory (RAM). 129 130There is a completely different type of memory too, and that's the "shared 131memory" on the PCI or ISA bus. That's generally not RAM (although in the case 132of a video graphics card it can be normal DRAM that is just used for a frame 133buffer), but can be things like a packet buffer in a network card etc. 134 135This memory is called "PCI memory" or "shared memory" or "IO memory" or 136whatever, and there is only one way to access it: the readb/writeb and 137related functions. You should never take the address of such memory, because 138there is really nothing you can do with such an address: it's not 139conceptually in the same memory space as "real memory" at all, so you cannot 140just dereference a pointer. (Sadly, on x86 it _is_ in the same memory space, 141so on x86 it actually works to just deference a pointer, but it's not 142portable). 143 144For such memory, you can do things like 145 146 - reading: 147 /* 148 * read first 32 bits from ISA memory at 0xC0000, aka 149 * C000:0000 in DOS terms 150 */ 151 unsigned int signature = isa_readl(0xC0000); 152 153 - remapping and writing: 154 /* 155 * remap framebuffer PCI memory area at 0xFC000000, 156 * size 1MB, so that we can access it: We can directly 157 * access only the 640k-1MB area, so anything else 158 * has to be remapped. 159 */ 160 void __iomem *baseptr = ioremap(0xFC000000, 1024*1024); 161 162 /* write a 'A' to the offset 10 of the area */ 163 writeb('A',baseptr+10); 164 165 /* unmap when we unload the driver */ 166 iounmap(baseptr); 167 168 - copying and clearing: 169 /* get the 6-byte Ethernet address at ISA address E000:0040 */ 170 memcpy_fromio(kernel_buffer, 0xE0040, 6); 171 /* write a packet to the driver */ 172 memcpy_toio(0xE1000, skb->data, skb->len); 173 /* clear the frame buffer */ 174 memset_io(0xA0000, 0, 0x10000); 175 176OK, that just about covers the basics of accessing IO portably. Questions? 177Comments? You may think that all the above is overly complex, but one day you 178might find yourself with a 500 MHz Alpha in front of you, and then you'll be 179happy that your driver works ;) 180 181Note that kernel versions 2.0.x (and earlier) mistakenly called the 182ioremap() function "vremap()". ioremap() is the proper name, but I 183didn't think straight when I wrote it originally. People who have to 184support both can do something like: 185 186 /* support old naming silliness */ 187 #if LINUX_VERSION_CODE < 0x020100 188 #define ioremap vremap 189 #define iounmap vfree 190 #endif 191 192at the top of their source files, and then they can use the right names 193even on 2.0.x systems. 194 195And the above sounds worse than it really is. Most real drivers really 196don't do all that complex things (or rather: the complexity is not so 197much in the actual IO accesses as in error handling and timeouts etc). 198It's generally not hard to fix drivers, and in many cases the code 199actually looks better afterwards: 200 201 unsigned long signature = *(unsigned int *) 0xC0000; 202 vs 203 unsigned long signature = readl(0xC0000); 204 205I think the second version actually is more readable, no? 206 207 Linus 208 209