linux-kernel - high-order allocation + highmem + streaming DMA

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Message-Id: <201110191045.42613@zigzag.lvk.cs.msu.su>
Date:	Wed, 19 Oct 2011 10:45:40 +0400
From:	"Nikita V. Youshchenko" <yoush@...msu.su>
To:	linux-kernel@...r.kernel.org
Cc:	stefan@....cs.msu.su
Subject: high-order allocation + highmem + streaming DMA - possible?

Hello.

We are writing a driver for a custom PCI device that receives large amount 
of data.

Device is capable of DMAing received data in 64k chunks.

We have been going to:
- use physically-continuous 64k-sized memory areas using alloc_pages() with 
order=4,
- allocate large number of such areas on driver init,
- implement mmap() to map those all to userspace for zero-copy processing,
- use streaming DMA to actually receive data

However I can't find a way to use streaming DMA for large highmem areas.
- pci_map_single() is for linear mapping and can't work with highmem
- pci_map_page() can work with highmem but is limited to one page
- pci_map_sg() is for s/g lists, not for single 64k blocks.

Is there a way to use streaming DMA with large memory areas located in 
highmem?
Or in this situation buffers in highmem can't be used?

Nikita
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