blob: c72b24a7d52c2b8b162f6f7cff2ff1c212138fdd [file] [log] [blame]
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -03001==========================================================
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
Randy Dunlap5872fb92009-01-29 16:28:02 -080010 superseded by the functionality provided by the PCI DMA interface
Mauro Carvalho Chehaba822b2e2021-06-16 08:27:23 +020011 (see Documentation/core-api/dma-api-howto.rst). They continue
Linus Torvalds1da177e2005-04-16 15:20:36 -070012 to be documented below for historical purposes, but new code
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -030013 must not use them. --davidm 00/12/12
Linus Torvalds1da177e2005-04-16 15:20:36 -070014
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -030015::
16
17 [ This is a mail message in response to a query on IO mapping, thus the
18 strange format for a "document" ]
Linus Torvalds1da177e2005-04-16 15:20:36 -070019
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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -030024However, on many setups, there are actually **three** different ways of looking
Linus Torvalds1da177e2005-04-16 15:20:36 -070025at 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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -030051CPU sees a memory map something like this (this is from memory)::
Linus Torvalds1da177e2005-04-16 15:20:36 -070052
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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -030065PPC, you can end up with a setup like this::
Linus Torvalds1da177e2005-04-16 15:20:36 -070066
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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -030074Similarly, on the Alpha, the normal translation is::
Linus Torvalds1da177e2005-04-16 15:20:36 -070075
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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -030083Anyway, the way to look up all these translations, you do::
Linus Torvalds1da177e2005-04-16 15:20:36 -070084
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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -030094You want the **virtual** address when you are actually going to access that
95pointer from the kernel. So you can have something like this::
Linus Torvalds1da177e2005-04-16 15:20:36 -070096
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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300117you want to give to the controller::
Linus Torvalds1da177e2005-04-16 15:20:36 -0700118
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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300125And you generally **never** want to use the physical address, because you can't
Linus Torvalds1da177e2005-04-16 15:20:36 -0700126use 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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300137.. 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).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700141
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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300152just dereference a pointer. (Sadly, on x86 it **is** in the same memory space,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700153so on x86 it actually works to just deference a pointer, but it's not
154portable).
155
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300156For such memory, you can do things like:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700157
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300158 - reading::
159
Linus Torvalds1da177e2005-04-16 15:20:36 -0700160 /*
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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300166 - remapping and writing::
167
Linus Torvalds1da177e2005-04-16 15:20:36 -0700168 /*
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 */
H Hartley Sweeten143724f2010-01-01 20:35:41 -0800174 void __iomem *baseptr = ioremap(0xFC000000, 1024*1024);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700175
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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300182 - copying and clearing::
183
Linus Torvalds1da177e2005-04-16 15:20:36 -0700184 /* 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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300199support both can do something like::
Linus Torvalds1da177e2005-04-16 15:20:36 -0700200
201 /* support old naming silliness */
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300202 #if LINUX_VERSION_CODE < 0x020100
Linus Torvalds1da177e2005-04-16 15:20:36 -0700203 #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
Mauro Carvalho Chehab38975e92017-05-14 08:13:48 -0300214actually looks better afterwards::
Linus Torvalds1da177e2005-04-16 15:20:36 -0700215
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?