In our ICD, we used mmAllocateContiguousMemory to get this kind of memory,
and mmGetPhysicalAddress to convert virtual to physical. Sometimes we went
directly to the page tables, because we found it a good idea to keep API
gobbledigook within the bounds of initialization code. This was done in the
display miniport, once.
Alberto.
-----Original Message-----
From: Mark Roddy [mailto:xxxxx@hollistech.com]
Sent: Tuesday, September 11, 2001 8:53 PM
To: NT Developers Interest List
Subject: [ntdev] RE: Scatter-Gather list across PCI bridge
Hmmm…
Hard to know where to start with this other than to agree that you are
confused 
First, lets review the difference between Physical Address and Virtual
Address. Software running on the NT platform only uses virtual addresses
for memory. Memory management hardware and software translate virtual
addresses to physical addresses in order to locate the physical memory
used for any particular virtual address. The mapping from virtual to
physical can change over time.
PCI devices accessing host memory only use Physical Addresses. If you
hand a PCI device a virtual address and instruct it to perfrom some
transfer operation in or out of that address, it is very unlikely that
your request will be performed as you intended. This is why your
‘hardware guy’ was complaining about invalid PCI addresses.
When you call ExAllocatePool you obtain a virtual address of a buffer.
That virtual address maps to some number of physical page addresses.
There is no guarantee that the physical pages are contiguous (although
if you specified nonpagedpool in general they will be contiguous.) But,
nevermind that, you need a physically contiguous buffer, and you need to
use physical, not virtual, addresses for everything that your PCI device
uses.
The correct API is AllocateCommonBuffer (or if we are talking about NT4
rather than W2K HalAllocateCommonBuffer.) This API returns to you BOTH a
virtual address and a physical address (actually the documentation
refers to it as a logical address, but lets not get into that right
now.) Better yet, the api provides a guarantee that the buffer returned
is physically contiguous.
Now you can use this buffer to construct your SGL, typically as a count
plus a variable length array of {physical address, length} elements.
Inform your PCI device of the physical address of your SGL, let it do
the indexing on its own (no need for a next pointer,) and off you go.
-----Original Message-----
From: xxxxx@lists.osr.com
[mailto:xxxxx@lists.osr.com] On Behalf Of xxxxx@aja.com
Sent: Tuesday, September 11, 2001 8:23 PM
To: NT Developers Interest List
Subject: [ntdev] Scatter-Gather list across PCI bridge
Hello, brains. :-}
I’m trying to implement Scatter/Gather DMA on a PCI device,
where the
chaining list resides in Host memory on the PC. I’m using
ExAllocatePool
to allocate a chunk of memory where I will build the list. I
write the
Host Address, Device Address, Length, and Next Descriptor
Link (always this
one +1) for each descriptor. My hardware guy tells me that
the address of
the list (and the NextDescriptor fields) is not a valid PCI
address, as
seen from the device. (The Host Address does seem to be valid.)
Perhaps I should be using a different method to allocate
the memory for
the list (I’m not really familiar with HalTranslateBusAddress
and such, so
don’t know how that fits in…). Or maybe I need to do
something extra to
the address I get from ExAllocatePool?
I’m confused. Can anyone spin me around and head me in the right
direction?
Dave Taylor
AJA Video Systems
xxxxx@aja.com
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