[wdmaudiodev] Re: Doubt about MDL
- From: <daniel@xxxxxxxxxxxxxxxx>
- To: <wdmaudiodev@xxxxxxxxxxxxx>
- Date: Thu, 22 Nov 2012 07:21:28 +0100
I will be allocating a buffer and create corresponding MDL by using
function *AllocatePagesForMdl This method always allocates an integral
number of pages from nonpaged pool.
It does not. It allocates physical memory, not virtual memory or pool. Even
if at some point you will get virtual addresses for them, that will not be
charged against nonpaged pool.
Does this mean that if i call *MmGetMdlByteOffset* with this MDL as i/p ,
the byteoffset returned by *MmGetMdlByteOffset* will always be zero?
Yes, it says it allocates an integral number of pages. That means a full
first page.
Is there any possibility of getting non zero offset indicating that event
though buffer is of say n pages the valid data starts at some non zero
offset of first page?
No, it says it allocates an integral number of pages only.
Also when i allocate buffer using *AllocatePagesForMdl* is it guaranteed
that all rest of the pages are fully allocated for my buffer
No. It does not guarantee that it allocates all pages that you request. If
there are not enough free physical pages, it returns an MDL with the memory
it could allocate. Therefore you need to check the returned MDL with the
MmGetMdlByteCount function to see how much memory you actually received.
or do i need to traverse *PMDL**Next *element until mdl ends and find byte
offset within each page using *MmGetMdlByteOffset* ?
That doesnât sound like a sensible thing to do.
//Daniel
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