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SubjectProblem with AMD chipsets. Was "Re: problems doing direct dma from a pci device to pci-e device"
The original thread is a year or so old but I never actually figured out 
what the problem was. It's still an issue I need to try to understand so
I thought I would try again. As then, I know this is a PCI thing but
have CC'd LMKL just in case. Sorry for the noise LMKL.

Briefly, we have a couple of PCI cards that talk to each other over the
pci bus. One is a reflective memory type (rms) card and the other is a
special I/O (gpiohsd) card. These are both regular PCI cards. The way
this gpiohsd writes and reads data to and from this rms card mamory and
also to regular application SHM memory is by way of internal page tables
in the gpiohsd containing bus addresses of the rms cards memory or
application SHM memory. We stuff the gpiohsd page tables with bus
addresses we get from the page_to_phys call. We used to use
virt_to_bus. The gpiohsd may do a single read or write, or a large DMA
read or write. Software never directly tells this gpiohsd card to do any
xfer, DMA or not. I've looked at the kernel DMA Documentation and don't
really see anything there that will help me with my problem (which I
will explain below). Here is a blurp of the code in one of our GPL
drivers we use for getting bus addresses for our rms card memory and/or
our application SHM memory that we stuff into the gpiohsd page tables.
FYI, a device connected to this gpiohsd card can, by its self, cause the
gpiohsd to initiate these data transfers.

/*
* Get a physical/bus address from our virtual address
*/
down_read(&current->mm->mmap_sem);

/*
* Get around the mlock fix/change in get_user_pages that forces
* the call to fail if the VM_IO bit is set in vma->vm_flags
*
* As of 2.6.16 kernels get_user_pages also fails when the
* new VM_PFNMAP bit in vm->flags is set. It is OK to
* reset this bit also as long as we return the bit to
* its original set condition.
*/

VM_flags = 0;
vma = find_vma(current->mm, (unsigned long)pte_info.virt_addr);
VM_flags = (vma->vm_flags & (VM_IO | VM_PFNMAP));
vma->vm_flags &= (~VM_IO & ~VM_PFNMAP);

stat = get_user_pages(current, current->mm,
(unsigned long)pte_info.virt_addr,
1, // one page
1, // write access
1, // force
&pages, // page struct
NULL); //

vma->vm_flags |= VM_flags; // Set vm_flags back to the way we
found them

up_read(&current->mm->mmap_sem);

if (stat < 0) {
ret = -EFAULT;
goto out;
} else {
phys_addr = page_to_phys(pages); // on x86 phys = bus
page_cache_release(pages);
}

pci_address = phys_addr;
pte_info.pcimsa = 0; // We are running 32 bit
pte_info.pcilsa = pci_address;
if (PAGE_SIZE < 8192) // MPX page size
pte_info.pagesize = PAGE_SIZE;
else
pte_info.pagesize = 8192;
if (copy_to_user((lcrs_pte_struct_t *) arg,
&pte_info, sizeof(lcrs_pte_struct_t))) {
ret = -EFAULT;
goto out;
}


We come into the above code with a virtual address (page aligned) of
either the rms memory page, via mmap, or our SHM memory page, using the
standard shm API. This has all worked just fine for years. And it still
does except for when we are using a more recent MB with an AMD chipset
and the data to/from the gpiohsd has to cross a pci-e bridge to get to
the rms memory or to our SHM memory. An example of that configuration
would be when the rms card is plugged into the MB and the gpiohsd is in
a regular PCI expansion rack and the expansion rack interface card is a
pci-e card plugged into a pci-e slot. It appears we are unable to cross
over pci-e bridges for some reason using the bus addresses obtained
using the method described above.

We have no problem with this same configuration using a MB with an
nvidia chipset. I suspect it might have something to do with the the MB
that usines the AMD chipset having an IOMMU, but I really don't know for
sure. I've also read something in the AMD chipset docs about some type
of restrictions on peer to peer transfers but again I really have no
idea if this is related to why I'm having this problem.

Any pointers from anyone (even an AMD guy) out there would again be
appreciated.

Thanks and regards
Mark


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