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2 How to access I/O mapped memory from within device drivers
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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10 superseded by the functionality provided by the PCI DMA interface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
17 [ This is a mail message in response to a query on IO mapping, thus the
18 strange format for a "document" ]
20 The AHA-1542 is a bus-master device, and your patch makes the driver give the
21 controller the physical address of the buffers, which is correct on x86
22 (because all bus master devices see the physical memory mappings directly).
24 However, on many setups, there are actually **three** different ways of looking
25 at memory addresses, and in this case we actually want the third, the
26 so-called "bus address".
28 Essentially, the three ways of addressing memory are (this is "real memory",
29 that is, normal RAM--see later about other details):
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.
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".
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.
45 Now, on normal PCs the bus address is exactly the same as the physical
46 address, and things are very simple indeed. However, they are that simple
47 because the memory and the devices share the same address space, and that is
48 not generally necessarily true on other PCI/ISA setups.
50 Now, just as an example, on the PReP (PowerPC Reference Platform), the
51 CPU sees a memory map something like this (this is from 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)
57 Now, that looks simple enough. However, when you look at the same thing from
58 the viewpoint of the devices, you have the reverse, and the physical memory
59 address 0 actually shows up as address 2 GB for any IO master.
61 So when the CPU wants any bus master to write to physical memory 0, it
62 has to give the master address 0x80000000 as the memory address.
64 So, for example, depending on how the kernel is actually mapped on the
65 PPC, you can end up with a setup like this::
68 virtual address: 0xC0000000
69 bus address: 0x80000000
71 where all the addresses actually point to the same thing. It's just seen
72 through different translations..
74 Similarly, on the Alpha, the normal translation is::
77 virtual address: 0xfffffc0000000000
78 bus address: 0x40000000
80 (but there are also Alphas where the physical address and the bus address
83 Anyway, the way to look up all these translations, you do::
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);
92 Now, when do you need these?
94 You want the **virtual** address when you are actually going to access that
95 pointer from the kernel. So you can have something like this::
98 * this is the hardware "mailbox" we use to communicate with
99 * the controller. The controller sees this directly.
108 unsigned char * retbuffer;
110 /* get the address from the controller */
111 retbuffer = bus_to_virt(mbox.bufstart);
112 switch (retbuffer[0]) {
116 on the other hand, you want the bus address when you have a buffer that
117 you want to give to the controller::
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);
123 notify_controller(&mbox);
125 And you generally **never** want to use the physical address, because you can't
126 use that from the CPU (the CPU only uses translated virtual addresses), and
127 you can't use it from the bus master.
129 So why do we care about the physical address at all? We do need the physical
130 address in some cases, it's just not very often in normal code. The physical
131 address 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
133 be remapped as measured in units of pages, a.k.a. the pfn (the memory
134 management layer doesn't know about devices outside the CPU, so it
135 shouldn't need to know about "bus addresses" etc).
139 The above is only one part of the whole equation. The above
140 only talks about "real memory", that is, CPU memory (RAM).
142 There is a completely different type of memory too, and that's the "shared
143 memory" on the PCI or ISA bus. That's generally not RAM (although in the case
144 of a video graphics card it can be normal DRAM that is just used for a frame
145 buffer), but can be things like a packet buffer in a network card etc.
147 This memory is called "PCI memory" or "shared memory" or "IO memory" or
148 whatever, and there is only one way to access it: the readb/writeb and
149 related functions. You should never take the address of such memory, because
150 there is really nothing you can do with such an address: it's not
151 conceptually in the same memory space as "real memory" at all, so you cannot
152 just dereference a pointer. (Sadly, on x86 it **is** in the same memory space,
153 so on x86 it actually works to just deference a pointer, but it's not
156 For such memory, you can do things like:
161 * read first 32 bits from ISA memory at 0xC0000, aka
162 * C000:0000 in DOS terms
164 unsigned int signature = isa_readl(0xC0000);
166 - remapping and writing::
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.
174 void __iomem *baseptr = ioremap(0xFC000000, 1024*1024);
176 /* write a 'A' to the offset 10 of the area */
177 writeb('A',baseptr+10);
179 /* unmap when we unload the driver */
182 - copying and clearing::
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);
191 OK, that just about covers the basics of accessing IO portably. Questions?
192 Comments? You may think that all the above is overly complex, but one day you
193 might find yourself with a 500 MHz Alpha in front of you, and then you'll be
194 happy that your driver works ;)
196 Note that kernel versions 2.0.x (and earlier) mistakenly called the
197 ioremap() function "vremap()". ioremap() is the proper name, but I
198 didn't think straight when I wrote it originally. People who have to
199 support both can do something like::
201 /* support old naming silliness */
202 #if LINUX_VERSION_CODE < 0x020100
203 #define ioremap vremap
204 #define iounmap vfree
207 at the top of their source files, and then they can use the right names
208 even on 2.0.x systems.
210 And the above sounds worse than it really is. Most real drivers really
211 don't do all that complex things (or rather: the complexity is not so
212 much in the actual IO accesses as in error handling and timeouts etc).
213 It's generally not hard to fix drivers, and in many cases the code
214 actually looks better afterwards::
216 unsigned long signature = *(unsigned int *) 0xC0000;
218 unsigned long signature = readl(0xC0000);
220 I think the second version actually is more readable, no?