Re: Alloc and lock down large amounts of memory

From: Mike Galbraith (
Date: Wed Aug 21 2002 - 09:35:57 EST

Roland Kuhn wrote:

>On Wed, 21 Aug 2002, Mike Galbraith wrote:
>>At 03:08 PM 8/20/2002 -0500, Bhavana Nagendra wrote:
>>>>Curiosity: why do you want to do device DMA buffer
>>>>allocation from userland?
>>>I need 256M memory for a graphics operation. It's a requiremment,
>>>can't change it. There will be other reasonably sized allocs in kernel
>>>space, this is a special case that will be done from userland. As
>>>discussed earlier in this thread, there's no good way of alloc()ing
>>>and pinning that much in DMA memory space, is there?
>>Not that I know of. It seems to me that any interface that tried
>>to provide this would have to know what kind of device is going
>>to DMA from/to that ram.
>>Usually, when someone needs a large gob of contiguous ram,
>>folks suggest doing the allocation in kernel, and early.
>BTW: What is the limit for pci_alloc_consistent and friends? Can it really
>provide 256MB?

Dunno. The page allocator however (lowest) can deliver 1 << MAX_ORDER
pages per request.. unless fragmentation gets you that is, so I doubt
it's remotely possible
to get 256MB of _physically_ contiguous ram without doing early
allocation of some sort.
(bootmem, bigphysarea patches or whatnot)


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