linux-kernel - Re: [PATCH 1/1] Userspace I/O (UIO): Add support for userspace DMA (corrected)

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Date:	Thu, 25 Dec 2008 16:20:15 +0100
From:	"Hans J. Koch" <hjk@...utronix.de>
To:	Joerg Roedel <joro@...tes.org>
Cc:	"Hans J. Koch" <hjk@...utronix.de>,
	Edward Estabrook <edward.estabrook.lkml@...il.com>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	gregkh@...e.de, edward.estabrook@...il.com,
	edward_estabrook@...lent.com
Subject: Re: [PATCH 1/1] Userspace I/O (UIO): Add support for userspace DMA
	(corrected)

On Thu, Dec 25, 2008 at 01:30:04PM +0100, Joerg Roedel wrote:
> On Thu, Dec 11, 2008 at 01:33:41AM +0100, Hans J. Koch wrote:
> > On Wed, Dec 03, 2008 at 05:51:30PM -0800, Edward Estabrook wrote:
> > > From: Edward Estabrook <Edward_Estabrook@...lent.com>
> > > 
> > > Here is a patch that adds the ability to dynamically allocate and use
> > > coherent DMA
> > > from userspace by extending the Userspace IO driver.  This patch applies against
> > > 2.6.28-rc6.
> > 
> > Hi Edward,
> > sorry I didn't answer sooner. It was not lack of interest in your work, I
> > just had to think about it (and had quite a lot of other work, too).
> > I consider your idea of having DMA coherent memory allocatable from userspace
> > a valid demand. However, the interface should be implemented differently.
> > 
> > As a general requirement, all information you need to pass to userspace has
> > to go through sysfs. This is a UIO design decision and should not be changed
> > lightly. Please don't fill information into the allocated memory.
> > 
> > We shouldn't mix dynamically allocated DMA regions with the already existing
> > static memory mappings. There should be an extra directory, maybe called
> > /sys/class/uio/uioN/dyn_dma_mem/ containing information about this kind of
> > DMA memory. This directory would not exist by default, it's only created if
> > some user actually allocates such memory.
> > 
> > In the UIO core, these mappings should be handled with a dynamic list, not
> > a static array. The number of possible mappings has to be limited to a
> > sensible maximum, maybe 50 or 100. The code handling the existing static
> > mappings could mostly stay as-is.
> 
> I like that idea of a seperate directory in sysfs. How about allocating
> DMA memory by creating files in this directory.

Nice idea, but AFAICT sysfs doesn't support creating files from userspace.

> The memory itself can be
> accessed via mmap() and the dma address can be gathered using read().
> When a file is closed the dma memory is unmapped.
> This would work great as long as /sys/class/uio/uioN/ belongs only to
> one physical device.

That's guaranteed.

Thanks,
Hans
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