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Based on kernel version 2.6.34. Page generated on 2010-05-31 16:03 EST.

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2		superseded by the functionality provided by the PCI DMA interface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/PCI/PCI-DMA-mapping.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
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