On 01/09/2007 06:07 PM, Bahadir Balban wrote:
On 1/9/07, Rene Herman <rene.herman@xxxxxxxxx> wrote:
Not very clearly formulated, but "tries to access the IO region [...] in
physical" sounds like you are directly adressing using physical (bus)
addresses?
Yes that is what I meant. With the advice from a former reply, I
advanced the discussion more specifically in the arm-kernel mailing
list since the platform was ARM. But the ata code in 2.6.19 that
causes the oops is as follows, from libata-sff.c:
struct ata_pci_init_native_mode(...)
{
unsigned long bmdma;
...
bmdma = pci_resource_start(pdev, 4);
/* Memory reference to a direct physical address. */
if (inb(bmdma + 2) & 8)
...
}
Ah, I see, you were talking about inb(). The story with that is:
On x86, we have two seperate address spaces that are used for I/O. One
dedicated (16-bit) I/O address space which the CPU instructions "in" and
"out" access and the regular memory addressspace which can also be used
for I/O (which is then aptly called "memory mapped I/O").
inb() and friends access the I/O addressspace on x86 -- they translate
into "in" CPU instructions and since this I/O addressspace doesn't go
though the MMU, the addresses are non-virtual/physical/bus addresses and
can thefore be used directly. All you want on the software side is a
request_region() to mark the area as reserved for other drivers but you
don't need any address translations.
Now on ARM, you don't actually have this seperate I/O address space as
far as the CPU is concerned; all I/O is memory mapped. But for PCI (and
ISA) compatibility, the x86 I/O address-space is "emulated" in so far
that it's memory mapped 1:1 to some (possibly model specific, I don't
know ARM) fixed physical memory address. The generic code then takes
care of remapping this fixed region of physical addresses to a fixed
region of virtual addresses and an "inb(addr)" then just translates to
something like "readb(addr + VIRTUAL_BASE_OF_IO_ADDRESS_SPACE)".
So, still no MMU involved past that generic "all of the I/O space"
granularity and the addresses can still be used directly.
I'm not subscribed to the ARM kernel list so I'm missing the answers to
the question there but I hope this was useful.
Rene.
--
Kernelnewbies: Help each other learn about the Linux kernel.
Archive: http://mail.nl.linux.org/kernelnewbies/
FAQ: http://kernelnewbies.org/faq/
