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Linux/Documentation/bus-virt-phys-mapping.txt

  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  2         superseded by the functionality provided by the PCI DMA interface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 
 10 The AHA-1542 is a bus-master device, and your patch makes the driver give the
 11 controller the physical address of the buffers, which is correct on x86
 12 (because all bus master devices see the physical memory mappings directly). 
 13 
 14 However, on many setups, there are actually _three_ different ways of looking
 15 at memory addresses, and in this case we actually want the third, the
 16 so-called "bus address". 
 17 
 18 Essentially, the three ways of addressing memory are (this is "real memory",
 19 that 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 
 35 Now, on normal PCs the bus address is exactly the same as the physical
 36 address, and things are very simple indeed. However, they are that simple
 37 because the memory and the devices share the same address space, and that is
 38 not generally necessarily true on other PCI/ISA setups. 
 39 
 40 Now, just as an example, on the PReP (PowerPC Reference Platform), the 
 41 CPU 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 
 47 Now, that looks simple enough. However, when you look at the same thing from
 48 the viewpoint of the devices, you have the reverse, and the physical memory
 49 address 0 actually shows up as address 2 GB for any IO master.
 50 
 51 So when the CPU wants any bus master to write to physical memory 0, it 
 52 has to give the master address 0x80000000 as the memory address.
 53 
 54 So, for example, depending on how the kernel is actually mapped on the 
 55 PPC, you can end up with a setup like this:
 56 
 57  physical address:      0
 58  virtual address:       0xC0000000
 59  bus address:           0x80000000
 60 
 61 where all the addresses actually point to the same thing.  It's just seen 
 62 through different translations..
 63 
 64 Similarly, 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
 71 are the same). 
 72 
 73 Anyway, 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 
 82 Now, when do you need these?
 83 
 84 You want the _virtual_ address when you are actually going to access that 
 85 pointer 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 
106 on the other hand, you want the bus address when you have a buffer that 
107 you 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 
115 And you generally _never_ want to use the physical address, because you can't
116 use that from the CPU (the CPU only uses translated virtual addresses), and
117 you can't use it from the bus master. 
118 
119 So why do we care about the physical address at all? We do need the physical
120 address in some cases, it's just not very often in normal code.  The physical
121 address 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
123 be remapped as measured in units of pages, a.k.a. the pfn (the memory
124 management layer doesn't know about devices outside the CPU, so it
125 shouldn't need to know about "bus addresses" etc).
126 
127 NOTE NOTE NOTE! The above is only one part of the whole equation. The above
128 only talks about "real memory", that is, CPU memory (RAM). 
129 
130 There is a completely different type of memory too, and that's the "shared
131 memory" on the PCI or ISA bus. That's generally not RAM (although in the case
132 of a video graphics card it can be normal DRAM that is just used for a frame
133 buffer), but can be things like a packet buffer in a network card etc. 
134 
135 This memory is called "PCI memory" or "shared memory" or "IO memory" or
136 whatever, and there is only one way to access it: the readb/writeb and
137 related functions. You should never take the address of such memory, because
138 there is really nothing you can do with such an address: it's not
139 conceptually in the same memory space as "real memory" at all, so you cannot
140 just dereference a pointer. (Sadly, on x86 it _is_ in the same memory space,
141 so on x86 it actually works to just deference a pointer, but it's not
142 portable). 
143 
144 For 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 
176 OK, that just about covers the basics of accessing IO portably.  Questions?
177 Comments? You may think that all the above is overly complex, but one day you
178 might find yourself with a 500 MHz Alpha in front of you, and then you'll be
179 happy that your driver works ;)
180 
181 Note that kernel versions 2.0.x (and earlier) mistakenly called the
182 ioremap() function "vremap()".  ioremap() is the proper name, but I
183 didn't think straight when I wrote it originally.  People who have to
184 support 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  
192 at the top of their source files, and then they can use the right names
193 even on 2.0.x systems. 
194 
195 And the above sounds worse than it really is.  Most real drivers really
196 don't do all that complex things (or rather: the complexity is not so
197 much in the actual IO accesses as in error handling and timeouts etc). 
198 It's generally not hard to fix drivers, and in many cases the code
199 actually looks better afterwards:
200 
201         unsigned long signature = *(unsigned int *) 0xC0000;
202                 vs
203         unsigned long signature = readl(0xC0000);
204 
205 I think the second version actually is more readable, no?
206 
207                 Linus
208 

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