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

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Date:	Wed, 19 Oct 2011 15:54:14 +0530
From:	RKK <kulkarni.ravi4@...il.com>
To:	"Nikita V. Youshchenko" <yoush@...msu.su>
Cc:	linux-kernel@...r.kernel.org, stefan@....cs.msu.su
Subject: Re: high-order allocation + highmem + streaming DMA - possible?

Hi Nikita

On Wed, Oct 19, 2011 at 12:15 PM, Nikita V. Youshchenko <yoush@...msu.su> wrote:
> 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
  May i know why u want to mmap to userspace instead of using a
device? performance is affected by a large amount if u use mmap
> 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?
I dont think DMA is available for highmem? Not sure though . never saw
a usecase for it.
> Or in this situation buffers in highmem can't be used?
>



-- 
Warm Regards,
Ravi .
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