Write Combining (Write Coalescing) on memory mapped I/O on IA64.

  Steffen Persvold               Systems Engineer
  Email : mailto:[email protected]     Scali AS (http://www.scali.com)
  Tlf   : (+47) 22 62 89 50      Olaf Helsets vei 6
  Fax   : (+47) 22 62 89 51      N-0621 Oslo, Norway

  Steffen Persvold               Systems Engineer
  Email : mailto:[email protected]     Scali AS (http://www.scali.com)
  Tlf   : (+47) 22 62 89 50      Olaf Helsets vei 6
  Fax   : (+47) 22 62 89 51      N-0621 Oslo, Norway


Hi,

I have a PCI adapter which does a lot of PIO (sort of network adapter using
host CPU to send data). On IA32 architechtures we could enable Write Combining
to get large burst on the PCI bus, but on IA64 I get at max 8 byte stores (ld8,
st8) at a time.

The driver for this adapter also exports the memory to user-space (with mmap())
and when doing this I do (inside the mmap function) :

int ssci_mmap(struct file *file, struct vm_area_struct *vma)
{
.....
#ifdef __ia64__      
   vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
#endif
   map_page_range(file, vma, vm_start,physaddr,
		      contlen, vma->vm_page_prot);

   remap_page_range(vma->vm_start,
		       phys_addr,
		       vma->vm_end - vma->vm_start,
		       vma->vm_page_prot))
....
}

This way we get Write Combining on IA64, but only for userspace mappings.
Kernel space clients use ioremap() and don't get Write Combining (ioremap on
IA64 use the uncacheable region directly).

To solve this I've tried to make my own __ia64_ioremap() (created
arch/ia64/mm/ioremap.c) wich has a 'flags' input paramenter so that I can set
the _PAGE_MA_WC attribute. I've taken a look on how other architechtures does
this (e.g i386 or s390), and made it nearly identical, but it doesn't seem to
work. The mapping gets set up, but if I try to read or write something to this
address I get a "Unable to handle kernel paging request at virtual address
0xa00000000015c000" which is the address I'm accessing.


I've attached the patch I made. Please let me know if this is doable on IA64 at
all. Feedback greatly appreciated.

Thanks,
-- 
  Steffen Persvold               Systems Engineer
  Email : mailto:[email protected]     Scali AS (http://www.scali.com)
  Tlf   : (+47) 22 62 89 50      Olaf Helsets vei 6
  Fax   : (+47) 22 62 89 51      N-0621 Oslo, Norway