Re: [RFC] Simple userspace interface for PCI drivers

[Matt Porter <mporter@embeddedalley.com>]

On Tue, Aug 29, 2006 at 11:23:38PM -0700, Greg KH wrote:
> A while ago, Thomas and I were sitting in the back of a conference
> presentation where the presenter was trying to describe what they did in
> order to add the ability to write a userspace PCI driver.  As was usual
> in a presentation like this, the presenter totaly ignored the real-world
> needs for such a framework, and only got it up and working on a single
> type of embedded system.  But the charts and graphs were quite pretty :)
> 
> Thomas and I lamented that we were getting tired of seeing stuff like
> this, and it was about time that we added the proper code to the kernel
> to provide everything that would be needed in order to write PCI drivers
> in userspace in a sane manner.  Really all that is needed is a way to
> handle the interrupt, everything else can already be done in userspace
> (look at X for an example of this...)

What about portable access to the PCI DMA API from userspace? I didn't
think we had a way to handle DMA buffer management entirely from
userspace. Looking at this code, it seems focused on one particular
problem space of industrial control. If the goal is to allow most
PCI drivers to be implemented in user space then I think it makes further
sense to not limit the subsystem to PCI but any device (like things that
are implemented as platform devices in kernel space).

> Thomas mentioned that he had code to do all of this working in some
> customer sites already and that he would get it to me.
> 
> Fast forward to OLS of this year, and I bugged Thomas to send me the
> code.  He did, and then I sat on it for a while longer...
> 
> So, here's the code.  I think it does a bit too much all at once, but it
> is an example of how this can be done.  This is working today in some
> industrial environments, successfully handling hardware controls of very
> large and scary machines.  So it is possible to use this interface to
> successfully build your own laser wielding robot, all from userspace,
> allowing you to keep your special-secret-how-to-control-the-laser
> algorithm closed source if you so desire.
> 
> In looking at the proposed kevent interface, I think a few things in
> this proposed interface can be dropped in favor of using kevents
> instead, but I haven't looked at the latest version of that code to make
> sure of this.
> 
> And the name is a bit ackward, anyone have a better suggestion?

Well, if it's focused on industrial controls like it appears from
the code here then the name is fine. If it's a starting point to
become someting more generic then User Space Driver (USD) subsystem
might be nice.

A more generic system would go a long way to get rid of the dubious
secret-sauce binary kernel modules that are popular in embedded
linux projects. 

> Thomas has also promised to come up with some userspace code that uses
> this interface to show how to use it, but seems to have forgotten.
> Consider this a reminder :)

That would be nice to see. I can't see how devices and drivers
are registered from user space with what's here. I see
iio_register_device() exported but no clue how userspace tells
the kernel to claim a device or register an iio driver.

-Matt
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