linux-kernel - Re: DMA with PCIe and very large DMA transfers

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Date:	Fri, 25 Jul 2008 09:33:11 +0200
From:	Clemens Ladisch <clemens@...isch.de>
To:	Alex <arghness@...il.com>
CC:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	linux-kernel@...r.kernel.org
Subject: Re: DMA with PCIe and very large DMA transfers

Jesse Barnes wrote:
> On Thursday, July 24, 2008 8:06 am Alex wrote:
> > I'm also interested in knowing if any drivers perform very large DMA
> > transfers. I'm putting together a driver for a specialist high-speed
> > data acquisition device that typically might need a DMA buffer of
> > 100-500MB (ouch!) in the low 32 bit address space (or possibly 36 bit
> > address space, but I'm not sure if this is possible to allocate
> > without allocating as much as possible and then discarding?) but only
> > supports a very limited number of scatter/gather entries (between 1
> > and 4). [...]
>
> It sounds like you'll probably have fairly special purpose configurations.  In
> that case, it may be reasonable to reserve your large DMA buffers at boot
> time, assuming you need large, contiguous chunks of physical memory.

Most of the sound drivers do this because few chips support SG.

> > I assume that to allocate that much memory in physical contiguous
> > addresses will require a driver to be loaded as soon as possible at
> > startup. I was thinking about trying to grab a lot of high-order pages
> > and try and make them one contiguous block - is that feasible?
> > Browsing the archives, I found references to early allocation for
> > large buffers, but no direct links to existing examples or recommended
> > techniques on how to stitch pages together in to a single buffer.

Have a look into sound/core/memalloc.c.  It tries to get a contiguous
block from the kernel; I don't think that it's possible to do this
manually if the kernel has failed.


Regards,
Clemens
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