From: Bjorn Helgaas <bjorn.helgaas@hp.com>
To: linux-pci@atrey.karlin.mff.cuni.cz
Subject: Re: Add a "enable" sysfs attribute to the pci devices to allow userspace (Xorg) to enable devices without doing foul direct access
Date: Thu, 4 May 2006 13:09:53 -0600
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Cc: Dave Airlie <airlied@linux.ie>, Arjan van de Ven <arjan@linux.intel.com>,
       Andrew Morton <akpm@osdl.org>, greg@kroah.com,
       linux-kernel@vger.kernel.org, pjones@redhat.com
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On Saturday 29 April 2006 03:04, Dave Airlie wrote:
> > This patch adds an "enable" sysfs attribute to each PCI device. When read it
> > shows the "enabled-ness" of the device, but you can write a "0" into it to
> > disable a device, and a "1" to enable it.
> >
> > This later is needed for X and other cases where userspace wants to enable
> > the BARs on a device (typical example: to run the video bios on a secundary
> > head). Right now X does all this "by hand" via bitbanging, that's just evil.
> > This allows X to no longer do that but to just let the kernel do this.

I'm all in favor of cleaning up X.  But making the X code prettier without
changing the underlying issues of claiming and sharing resources doesn't
help much.  In fact, I suspect the ultimate plan for X does not involve
an "enable" attribute in sysfs, so this may just introduce ABI cruft that
will be difficult to remove later.

> This would allow me to remove the issue in X where loading the DRM at X 
> startup acts differently than loading the DRM before X runs, due to Xs PCI 
> probe running in-between... with this I can just enable all VGA devices 
> and no worry whether they have a DRM or not..

This seems to be the main justification for the patch.  But I don't know
enough about X and DRM to understand it, or why this patch is the best
way to solve it.

I think Jon has a pretty convincing argument, which I *do* understand.
Can you expand on this justification?  Do you envision long-term usage
of the sysfs "enable" attribute?

Bjorn
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