Bret> Jes Sorensen [mailto:Jes.Sorensen@cern.ch] wrote:
>> This goal is already achieved, if the PPC uses bus_to_virt() it is
>> doing it wrong. The correct way is to use ioremap(), however what
>> ioremap() returns what could be referred to as a _bus_ _virtual_
>> address.
Bret> I think it would help a lot of people started talking about
Bret> three different addresses: 1. The Bus physical address,
Bret> i.e. what is in the PCI configuration registers. 2. The System
Bret> physical address, i.e. what ioremap() uses and is found in the
Bret> struct pci_dev base_address[] array. It is the physical address
Bret> used by the CPU. On systems like SPARC, this address would then
Bret> go through the IOMMU to generate the correct bus physical
Bret> address. 3. The System virtual address, i.e. what the
Bret> programmer uses to reference the address space. It goes through
Bret> the normal kernel virtual address translation.
Dont' forget that the virtual address you get from ioremap() is not a
real pointer in the sense that you are allowed to directly reference
the memory it points at.
>> This one is not allowed to be accessed directly by the CPU but must
>> use the readl/writel macros, or __raw_readl/__raw_writel ones.
Bret> Where in the heck did you find that?
By following Linux kernel.
Bret> I was under the impression that the readb/w/l and writeb/w/l
Bret> were only for spaces marked as I/O space in PCI. I thought that
Bret> if it was in PCI memory space you could directly reference the
Bret> address after you used ioremap().
I/O space as in I/O ports registers or as in PCI shared memory?
Bret> If this is true, you can't just directly access the virtual
Bret> address, then somebody should really add this information to the
Bret> Documentation/pci.txt file.
The problem here is that some PCI implementations will not provide a
direct linear mapping of the PCI shared memory space, like on some
Alphas which come with sparse memory.
>> Remember that the base_address[] value are not some that we just
>> make out of nowhere, they are assigned by the hardware/bios at boot
>> time and we read them out of registers.
Bret> Actually, my understanding is that the base_address[] is
Bret> assigned in the PCI configuration.
Correct, maybe I wasn't clear enough on this one.
Bret> If people really want to get the device drivers all working
Bret> correctly, you should probably start by agreeing on AND
Bret> DOCUMENTING exactly what 'correct' is. Without such information
Bret> available, people are going to keep on having problems writing
Bret> device drivers.
I thought it was already documented, but I guess you are
right. Documentation/pci.txt still refers to the old and deprecated
pcibios_foo() functions and Documentation/IO-mapping.txt doesn't
mention sparse memory directly, though it does state that one should
use readl/writel.
Jes
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