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

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Date:	Tue, 15 Dec 2009 16:20:56 -0800
From:	Earl Chew <earl_chew@...lent.com>
To:	"Hans J. Koch" <hjk@...utronix.de>
CC:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, gregkh@...e.de,
	linux-mm <linux-mm@...ck.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/1] Userspace I/O (UIO): Add support for userspace DMA

Hans J. Koch wrote:
> One example: An A/D converter has an on-chip 32k buffer. It causes an
> interrupt as soon as the buffer is filled up to a certain high-water mark.
> Such cases would easily fit into the current UIO system. The UIO core could
> simply DMA the data to one of the mappings. A new flag for that mapping and
> a few other changes are all it takes. After the DMA transfer is complete, the
> interrupt is passed on to userspace, which would find the buffer already
> filled with the desired data. Just a thought, unfortunately I haven't got
> such hardware to try it.

Hans,

Is this case already covered by the pre-existing UIO_MEM_LOGICAL
option ?

I'm thinking that since the memory is statically defined, it can be
described using one of the existing struct uio_mem mem[] slots in
struct uio_info and marked as UIO_MEM_LOGICAL.

The userspace program can map that into its process space using the
existing mmap() interface.

What am I missing?

> When it comes to dynamically allocated DMA buffers, it might well be possible
> to add a new directory in sysfs besides the "mem" directory, e.g. something
> like /sys/class/uio/uioN/dma-mem/. This would save us the trouble of creating
> a new device. Maybe the example above would better fit in here, too. Who knows.

I looked at the 2.6.32 source at

http://lxr.linux.no/#linux+v2.6.32/drivers/uio/uio.c

and didn't see any reference to /sys/class/uio/uioN/mem .  Perhaps
you're referring to something new.

In any case, I think you're describing adding

/sys/class/uio/uioN/dma-mem

as a means to control /dev/uioN .  Presumably writing to
/sys/class/uio/uioN/dma-mem would create additional dynamic
DMA buffers.

I can't yet see a way to make this request-response. When requesting
a dynamic buffer I need to indicate the size that I want, and in
return I need to obtain a handle to the buffer (either its mapping
number, address, etc). Once I have that, I can query other
interesting information (eg its bus address).

Earl

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