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Date: Fri, 13 Dec 2002 21:24:42 -0800
From: Andrew Morton <akpm@digeo.com>
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To: Alan Cox <alan@lxorguk.ukuu.org.uk>
CC: Jason Howard <lists@spectsoft.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: DMA from SCSI controller to PCI frame buffer memory.
References: <1039806910.21196.29.camel@bmagic.spectsoft.com> <1039837312.25121.115.camel@irongate.swansea.linux.org.uk>
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Alan Cox wrote:
> 
> On Fri, 2002-12-13 at 19:15, Jason Howard wrote:
> > Hello All,
> >
> > I am wondering if the functionality exists in the Linux kernel to DMA
> > from a SCSI controller directly into frame buffer memory of a PCI video
> > card?  Is there a standard method for this (similar to sendfile) or will
> > it require some hacking?
> 
> In theory you can mmap the frame buffer memory, then do O_DIRECT I/O
> into it. In practice it will buffer (I hope it still does).

An O_DIRECT disk read into a mmapped device region will fail (if
it doesn't, it'll oops ;)).  O_DIRECT requires valid pages inside
mem_map[].

See the `vma->vm_flags & VM_IO' test in get_user_pages(), and
the VALID_PAGE() test in get_page_map().

Hacking is required to do this.
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