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List:       linux-newbie
Subject:    Trying to alloc many continous pages
From:       "EXTERNAL Werner Philipp (Praktikant; CR/AEM5)"
Date:       2008-11-21 15:17:17
Message-ID: BF92A053E4A99B4E8EAE0B9EBD7EB203025E1650 () si-mail15 ! de ! bosch ! com
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Hi,

I'm writing a driver using DMA to transfer data to a performance critical device. I \
allocate a temporary DMA buffer which will be freed immediately after the DMA is \
completed. If there is enough continous memory available for all data I would like to \
transfer, I use it. Otherwise I split up the transfer in multiple transfers.

What's the right way to allocate the continous buffer? I didn't find how to "try \
allocating memory" without having an effect, if the memory in not available.

If I use the following code and there is not enough free memory, my linux hangs for \
several minutes after the DMA has finished.


unsigned int order = get_order(data_size);
while (1) {
     	kbuf = (void *) __get_free_pages(__GFP_NORETRY | __GFP_NOWARN, order);
	if (kbuf)
		break;		// success: we got a buffer
	if (order == 0)
		return -ENOMEM;
	order--;			// retry with a smaller order
}
buffer_size = PAGE_SIZE << order;



I think the memory management tries to reduce fragmentation in memory, but I would \
like to avoid this behaviour. It's not really a problem if I can't get a huge buffer, \
because in this case I will use a smaller one. But if there is a huge chunk of free \
memory I would like to use it, because it yields performance benefits.

I just want to look if I can alloc a number of continous pages, and if there aren't \
enough I want to handle it myself... How can I do that?


Best regards 
Philipp Werner 
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