linux-kernel - Re: Special handling of display/VGA devices in hotplug drivers

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Date:	Thu, 11 Dec 2014 11:32:43 -0800
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>, Dely Sy <dely.l.sy@...el.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Rajat Jain <rajatxjain@...il.com>,
	Guenter Roeck <groeck@...iper.net>,
	Kristen Carlson Accardi <kristen.c.accardi@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Special handling of display/VGA devices in hotplug drivers

On Thu, 11 Dec 2014 13:11:36 -0500
Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:

> On Thu, Dec 11, 2014 at 10:34:30AM -0700, Bjorn Helgaas wrote:
> > It looks like you added the initial pciehp driver [1], which
> > includes the following code in pciehp_disable_slot():
> > 
> > + if (class_code == PCI_BASE_CLASS_DISPLAY) {
> > + /* Display/Video adapter (not supported) */
> > + rc = REMOVE_NOT_SUPPORTED;
> > 
> > + /* If it's a bridge, check the VGA Enable bit */
> > + if ((header_type & 0x7F) == PCI_HEADER_TYPE_BRIDGE) {
> > + rc = pci_bus_read_config_byte (pci_bus, devfn,
> > PCI_BRIDGE_CONTROL, &BCR);
> > + if (rc)
> > + return rc;
> > +
> > + /* If the VGA Enable bit is set, remove isn't supported */
> > + if (BCR & PCI_BRIDGE_CTL_VGA) {
> > + rc = REMOVE_NOT_SUPPORTED;
> > 
> > I'm trying to figure out why VGA devices are handled specially.  I
> > can't find anything in the PCI specs that mentions this.  Most of
> > the other PCI hotplug drivers have similar code.  Do you remember
> > anything about this?
> 
> The PCI spec said that you were not allowed to hotplug VGA drivers.
> The big issue is that POST usually needs to run on those things, and
> there is no way to POST a PCI hotplugged device.
> 
> Does the spec not say that anymore?  I haven't looked in years at
> it...
> 
> Do you want to hot-add a VGA device?  Are these lines causing a
> problem with something?

Yeah, the legacy I/O regions get routed through the bridge with the VGA
bit set, and most legacy code probably can't handle that (whether POST,
VBIOS, or VGA drivers).

There is some code for moving the VGA routing around, so that might be
an option if you wanted to remove such a bridge.  You'd have to find a
VGA device under another bridge, and enable routing to that first, then
you could do the remove.

Jesse
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