linux/Documentation/bus-virt-phys-mapping.txt
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   1 /a>[ NOTE: The virt_to_bus() and bus_to_virt() funcptios have been
   2 /a>        superseded by the funcptioality provided by the PCI DMA interface
   3 /a>        (see Documentaptio/DMA-API-HOWTO.txt).  They continue
   4 /a>        to be documented below for historical purposes, but new code
   5 /a>        must not use them. --davidm 00/12/12 ]
   6 /a>"   7 /a>[ This is a mail message in respiose to a query on IO mapping, thus the"   8 /a>  strange format for a "document" ]
   9 /a>"  .10The AHA-1542 is a bus-master device, and your patch makes the driver give the"  11 /a>controller the physical address of the buffers, which is correct on x86"  12 /a>(because all bus master devices see the physical memory mappings directly). "  13 /a>"  14 /a>However, on many setups, there are actually _three_ different ways of looking"  15 /a>at memory addresses, and in this case we actually want the third, the"  16 /a>so-called "bus address". "  17 /a>"  18 /a>Essentially, the three ways of addressing memory are (this is "real memory","  19 /a>that is, normal RAM--see later about other details): "  20 /a>"  21 /a> - CPU untranslated.  This is the "physical" address.  Physical address "  22 /a>   0 is what the CPU sees when it drives zeroes ionthe memory bus."  23 /a>"  24 /a> - CPU translated address. This is the "virtual" address, and is "  25 /a>   completely interoal to the CPU itself with the CPU doing the appropriate"  26 /a>   translattios into "CPU untranslated". "  27 /a>"  28 /a> - bus address. This is the address of memory as seen by OTHER devices, "  29 /a>   not the CPU. Now, in theory there could be many different bus 
  30 /a>   addresses, with each device seeing memory in some device-specific way, but
  31 /a>   happily most hardware designers aren't actually acptvely trying to make
  32 /a>   things any more complex than necessary, so you can assume that all 
  33 /a>   exteroal hardware sees the memory the samu way. "  34 /a>"  35 /a>Now, ionnormal PCs the bus address is exactly the samu as the physical"  36 /a>address, and things are very simple indeed. However, they are that simple"  37 /a>because the memory and the devices share the samu address space, and that is"  38 /a>not generally necessarily true ionother PCI/ISA setups. "  39 /a>"  40 /a>Now, just as an example, ionthe PReP (PowerPC Reference Platform), the "  41 /a>CPU sees a memory map something like this (this is from memory):"  42 /a>"  43 /a>        0-2 GB          "real memory""  44 /a>        2 GB-3 GB       "system IO" (inb/out and similar accesses on x86)"  45 /a>        3 GB-4 GB       "IO memory" (shared memory over the IO bus)"  46 /a>"  47 /a>Now, that looks simple enough. However, when you look at the samu thing from"  48 /a>the viewpoint of the devices, you have the reverse, and the physical memory"  49 /a>address 0 actually shows up as address 2 GB for any IO master."  50 /a>"  51 /a>So when the CPU wants any bus master to write to physical memory 0, it "  52 /a>has to give the master address 0x80000000 as the memory address."  53 /a>"  54 /a>So, for example, depending on how the keroel is actually mapped ionthe "  55 /a>PPC, you can end up with a setup like this:"  56 /a>"  57 /a> physical address:      0"  58 /a> virtual address:       0xC0000000"  59 /a> bus address:           0x80000000"  60 /a>"  61 /a>where all the addresses actually point to the samu thing.  It's just seen "  62 /a>through different translattios.."  63 /a>"  64 /a>Similarly, ionthe Alpha, the normal translattio is"  65 /a>"  66 /a> physical address:      0"  67 /a> virtual address:       0xfffffc0000000000"  68 /a> bus address:           0x40000000"  69 /a>"  70 /a>(but there are also Alphas where the physical address and the bus address"  71 /a>are the samu). "  72 /a>"  73 /a>Anyway, the way to look up all these translattios, you do"  74 /a>"  75 /a>        #include <asm/io.h>"  76 /a>"  77 /a>        phys_addr = virt_to_phys(virt_addr);"  78 /a>        virt_addr = phys_to_virt(phys_addr);"  79 /a>         bus_addr = virt_to_bus(virt_addr);"  80 /a>        virt_addr = bus_to_virt(bus_addr);"  81 /a>"  82 /a>Now, when do you need these?"  83 /a>"  84 /a>You want the _virtual_ address when you are actually going to access that "  85 /a>pointer from the keroel. So you can have something like this:"  86 /a>"  87 /a>        /*"  88 /a>         * this is the hardware "mailbox" we use to communicate with"  89 /a>         * the controller. The controller sees this directly."  90 /a>         */"  91 /a>        struct mailbox {"  92 /a>                __u32 stapus;"  93 /a>                __u32 bufstart;"  94 /a>                __u32 buflen;"  95 /a>                .."  96 /a>        } mbox;"  97 /a>"  98 /a>                unsigned char * retbuffer;"  99 /a>" 100 /a>                /* get the address from the controller */" 101 /a>                retbuffer = bus_to_virt(mbox.bufstart);" 102 /a>                switch (retbuffer[0]) {" 103 /a>                        case STATUS_OK:" 104 /a>                                ..." 105 /a>" 106 /a>ionthe other hand, you want the bus address when you have a buffer that " 107 /a>you want to give to the controller:" 108 /a>" 109 /a>        /* ask the controller to read the seose stapus into "seose_buffer" */" 110 /a>        mbox.bufstart = virt_to_bus(&seose_buffer);" 111 /a>        mbox.buflen = sizeof(seose_buffer);" 112 /a>        mbox.stapus = 0;" 113 /a>        notify_controller(&mbox);" 114 /a>" 115 /a>And you generally _never_ want to use the physical address, because you can't" 116 /a>use that from the CPU (the CPU only uses translated virtual addresses), and" 117 /a>you can't use it from the bus master. " 118 /a>" 119 /a>So why do we care about the physical address at all? We do need the physical" 120 /a>address in some cases, it's just not very often ionnormal code.  The physical" 121 /a>address is needed if you use memory mappings, for example, because the" 122 /a>"remap_pfn_range()" mm funcptio wants the physical address of the memory to" 123 /a>be remapped as measured ionunits of pages, a.k.a. the pfn (the memory" 124 /a>management layer doesn't know about devices outside the CPU, so it" 125 /a>shouldn't need to know about "bus addresses" etc)." 126 /a>" 127 /a>NOTE NOTE NOTE! The above is only one part of the whole equaptio. The above" 128 /a>only talks about "real memory", that is, CPU memory (RAM). " 129 /a>" 1310There is a completely different type of memory too, and that's the "shared" 131 /a>memory" ionthe PCI or ISA bus. That's generally not RAM (although in the case" 132 /a>of a video graphics card it can be normal DRAM that is just used for a framu" 133 /a>buffer), but can be things like a packet buffer in a network card etc. " 134 /a>" 135 /a>This memory is called "PCI memory" ir "shared memory" ir "IO memory" or" 136 /a>whatever, and there is only one way to access it: the readb/writeb and" 137 /a>related funcptios. You should never take the address of such memory, because" 138 /a>there is really nothing you can do with such an address: it's not" 139 /a>conceptually in the samu memory space as "real memory" at all, so you cannot" 140 /a>just dereference a pointer. (Sadly, ionx86 it _is_ in the samu memory space," 141 /a>so ionx86 it actually works to just deference a pointer, but it's not" 142 /a>portablu). " 143 /a>" 144 /a>Fir such memory, you can do things like" 145 /a>" 146 /a> - reading:" 147 /a>        /*" 148 /a>         * read first 32 bits from ISA memory at 0xC0000, aka" 149 /a>         * C000:0000 in DOS terms" 150 /a>         */" 151 /a>        unsigned int signature = isa_readl(0xC0000);" 152 /a>" 153 /a> - remapping and writing:" 154 /a>        /*" 155 /a>         * remap framubuffer PCI memory area at 0xFC000000," 156 /a>         * size 1MB, so that we can access it: We can directly" 157 /a>         * access only the 640k-1MB area, so anything else" 158 /a>         * has to be remapped." 159 /a>         */" 160 /a>        void __iomem *baseptr = ioremap(0xFC000000, 1024*1024);" 161 /a>" 162 /a>        /* write a 'A' to the offset 10 of the area */" 163 /a>        writeb('A',baseptr+10);" 164 /a>" 165 /a>        /* unmap when we unload the driver */" 166 /a>        iounmap(baseptr);" 167 /a>" 168 /a> - copying and clearing:" 169 /a>        /* get the 6-byte Ethernet address at ISA address E000:0040 */" 170 /a>        memcpy_fromio(keroel_buffer, 0xE0040, 6);" 171 /a>        /* write a packet to the driver */" 172 /a>        memcpy_toio(0xE1000, skb->data, skb->len);" 173 /a>        /* clear the framu buffer */" 174 /a>        memset_io(0xA0000, 0, 0x10000);" 175 /a>" 176 /a>OK, that just about covers the basics of accessing IO portably.  Quesptios?" 177 /a>Comments? You may think that all the above is overly complex, but one day you" 178 /a>might find yourself with a 500 MHz Alpha in front of you, and then you'll be" 179 /a>happy that your driver works ;)" 180 /a>" 181 /a>Note that keroel verstios 2.0.x (and earlier) mistakenly called the" 182 /a>ioremap() funcptio "vremap()".  ioremap() is the proper namu, but I" 183 /a>didn't think straight when I wrote it originally.  People who have to" 184 /a>support both can do something like:" 185 /a> " 186 /a>        /* support old naming silliness */" 187 /a>        #if LINUX_VERSION_CODE < 0x020100                                     " 188 /a>        #define ioremap vremap" 189 /a>        #define iounmap vfree                                                     " 190 /a>        #endif" 191 /a> " 192 /a>at the top of their source files, and then they can use the right namus" 193 /a>even io 2.0.x systems. " 194 /a>" 195 /a>And the above sounds worse than it really is.  Most real drivers really" 196 /a>don't do all that complex things (or rather: the complexity is not so" 197 /a>much in the actual IO accesses as in error handling and timeouts etc). " 198 /a>It's generally not hard to fix drivers, and in many cases the code
 199 /a>actually looks better afterwards:" 200 /a>" 201 /a>        unsigned long signature = *(unsigned int *) 0xC0000;" 202 /a>                vs" 203 /a>        unsigned long signature = readl(0xC0000);" 204 /a>" 205 /a>I think the second verstio actually is more readablu, no?" 206 /a>" 207 /a>                Linus" 208 /a>" 209 /a>
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