1========================================================== 2How to access I/O mapped memory from within device drivers 3========================================================== 4 5:Author: Linus 6 7.. warning:: 8 9 The virt_to_bus() and bus_to_virt() functions have been 10 superseded by the functionality provided by the PCI DMA interface 11 (see :doc:`/core-api/dma-api-howto`). They continue 12 to be documented below for historical purposes, but new code 13 must not use them. --davidm 00/12/12 14 15:: 16 17 [ This is a mail message in response to a query on IO mapping, thus the 18 strange format for a "document" ] 19 20The AHA-1542 is a bus-master device, and your patch makes the driver give the 21controller the physical address of the buffers, which is correct on x86 22(because all bus master devices see the physical memory mappings directly). 23 24However, on many setups, there are actually **three** different ways of looking 25at memory addresses, and in this case we actually want the third, the 26so-called "bus address". 27 28Essentially, the three ways of addressing memory are (this is "real memory", 29that is, normal RAM--see later about other details): 30 31 - CPU untranslated. This is the "physical" address. Physical address 32 0 is what the CPU sees when it drives zeroes on the memory bus. 33 34 - CPU translated address. This is the "virtual" address, and is 35 completely internal to the CPU itself with the CPU doing the appropriate 36 translations into "CPU untranslated". 37 38 - bus address. This is the address of memory as seen by OTHER devices, 39 not the CPU. Now, in theory there could be many different bus 40 addresses, with each device seeing memory in some device-specific way, but 41 happily most hardware designers aren't actually actively trying to make 42 things any more complex than necessary, so you can assume that all 43 external hardware sees the memory the same way. 44 45Now, on normal PCs the bus address is exactly the same as the physical 46address, and things are very simple indeed. However, they are that simple 47because the memory and the devices share the same address space, and that is 48not generally necessarily true on other PCI/ISA setups. 49 50Now, just as an example, on the PReP (PowerPC Reference Platform), the 51CPU sees a memory map something like this (this is from memory):: 52 53 0-2 GB "real memory" 54 2 GB-3 GB "system IO" (inb/out and similar accesses on x86) 55 3 GB-4 GB "IO memory" (shared memory over the IO bus) 56 57Now, that looks simple enough. However, when you look at the same thing from 58the viewpoint of the devices, you have the reverse, and the physical memory 59address 0 actually shows up as address 2 GB for any IO master. 60 61So when the CPU wants any bus master to write to physical memory 0, it 62has to give the master address 0x80000000 as the memory address. 63 64So, for example, depending on how the kernel is actually mapped on the 65PPC, you can end up with a setup like this:: 66 67 physical address: 0 68 virtual address: 0xC0000000 69 bus address: 0x80000000 70 71where all the addresses actually point to the same thing. It's just seen 72through different translations.. 73 74Similarly, on the Alpha, the normal translation is:: 75 76 physical address: 0 77 virtual address: 0xfffffc0000000000 78 bus address: 0x40000000 79 80(but there are also Alphas where the physical address and the bus address 81are the same). 82 83Anyway, the way to look up all these translations, you do:: 84 85 #include <asm/io.h> 86 87 phys_addr = virt_to_phys(virt_addr); 88 virt_addr = phys_to_virt(phys_addr); 89 bus_addr = virt_to_bus(virt_addr); 90 virt_addr = bus_to_virt(bus_addr); 91 92Now, when do you need these? 93 94You want the **virtual** address when you are actually going to access that 95pointer from the kernel. So you can have something like this:: 96 97 /* 98 * this is the hardware "mailbox" we use to communicate with 99 * the controller. The controller sees this directly. 100 */ 101 struct mailbox { 102 __u32 status; 103 __u32 bufstart; 104 __u32 buflen; 105 .. 106 } mbox; 107 108 unsigned char * retbuffer; 109 110 /* get the address from the controller */ 111 retbuffer = bus_to_virt(mbox.bufstart); 112 switch (retbuffer[0]) { 113 case STATUS_OK: 114 ... 115 116on the other hand, you want the bus address when you have a buffer that 117you want to give to the controller:: 118 119 /* ask the controller to read the sense status into "sense_buffer" */ 120 mbox.bufstart = virt_to_bus(&sense_buffer); 121 mbox.buflen = sizeof(sense_buffer); 122 mbox.status = 0; 123 notify_controller(&mbox); 124 125And you generally **never** want to use the physical address, because you can't 126use that from the CPU (the CPU only uses translated virtual addresses), and 127you can't use it from the bus master. 128 129So why do we care about the physical address at all? We do need the physical 130address in some cases, it's just not very often in normal code. The physical 131address is needed if you use memory mappings, for example, because the 132"remap_pfn_range()" mm function wants the physical address of the memory to 133be remapped as measured in units of pages, a.k.a. the pfn (the memory 134management layer doesn't know about devices outside the CPU, so it 135shouldn't need to know about "bus addresses" etc). 136 137.. note:: 138 139 The above is only one part of the whole equation. The above 140 only talks about "real memory", that is, CPU memory (RAM). 141 142There is a completely different type of memory too, and that's the "shared 143memory" on the PCI or ISA bus. That's generally not RAM (although in the case 144of a video graphics card it can be normal DRAM that is just used for a frame 145buffer), but can be things like a packet buffer in a network card etc. 146 147This memory is called "PCI memory" or "shared memory" or "IO memory" or 148whatever, and there is only one way to access it: the readb/writeb and 149related functions. You should never take the address of such memory, because 150there is really nothing you can do with such an address: it's not 151conceptually in the same memory space as "real memory" at all, so you cannot 152just dereference a pointer. (Sadly, on x86 it **is** in the same memory space, 153so on x86 it actually works to just deference a pointer, but it's not 154portable). 155 156For such memory, you can do things like: 157 158 - reading:: 159 160 /* 161 * read first 32 bits from ISA memory at 0xC0000, aka 162 * C000:0000 in DOS terms 163 */ 164 unsigned int signature = isa_readl(0xC0000); 165 166 - remapping and writing:: 167 168 /* 169 * remap framebuffer PCI memory area at 0xFC000000, 170 * size 1MB, so that we can access it: We can directly 171 * access only the 640k-1MB area, so anything else 172 * has to be remapped. 173 */ 174 void __iomem *baseptr = ioremap(0xFC000000, 1024*1024); 175 176 /* write a 'A' to the offset 10 of the area */ 177 writeb('A',baseptr+10); 178 179 /* unmap when we unload the driver */ 180 iounmap(baseptr); 181 182 - copying and clearing:: 183 184 /* get the 6-byte Ethernet address at ISA address E000:0040 */ 185 memcpy_fromio(kernel_buffer, 0xE0040, 6); 186 /* write a packet to the driver */ 187 memcpy_toio(0xE1000, skb->data, skb->len); 188 /* clear the frame buffer */ 189 memset_io(0xA0000, 0, 0x10000); 190 191OK, that just about covers the basics of accessing IO portably. Questions? 192Comments? You may think that all the above is overly complex, but one day you 193might find yourself with a 500 MHz Alpha in front of you, and then you'll be 194happy that your driver works ;) 195 196Note that kernel versions 2.0.x (and earlier) mistakenly called the 197ioremap() function "vremap()". ioremap() is the proper name, but I 198didn't think straight when I wrote it originally. People who have to 199support both can do something like:: 200 201 /* support old naming silliness */ 202 #if LINUX_VERSION_CODE < 0x020100 203 #define ioremap vremap 204 #define iounmap vfree 205 #endif 206 207at the top of their source files, and then they can use the right names 208even on 2.0.x systems. 209 210And the above sounds worse than it really is. Most real drivers really 211don't do all that complex things (or rather: the complexity is not so 212much in the actual IO accesses as in error handling and timeouts etc). 213It's generally not hard to fix drivers, and in many cases the code 214actually looks better afterwards:: 215 216 unsigned long signature = *(unsigned int *) 0xC0000; 217 vs 218 unsigned long signature = readl(0xC0000); 219 220I think the second version actually is more readable, no? 221