Subject: Re: [PATCH 1/2] vgaarb: Select a default VGA device even if there's no legacy VGA

From: Lothar Waßmann <[email-masked]>
Date: Thu, 12 Oct 2017 14:05:48 +0200

Hi,

On Thu, 12 Oct 2017 12:24:10 +0100 Julien Thierry wrote:
> Hi Bjorn,
> 
> On 06/10/17 23:24, Bjorn Helgaas wrote:
> > From: Bjorn Helgaas <[email-masked]>
> > 
> > Daniel Axtens reported that on the HiSilicon D05 board, the VGA device is
> > behind a bridge that doesn't support PCI_BRIDGE_CTL_VGA, so the VGA arbiter
> > never selects it as the default, which means Xorg auto-detection doesn't
> > work.
> > 
> > VGA is a legacy PCI feature: a VGA device can respond to addresses, e.g.,
> > [mem 0xa0000-0xbffff], [io 0x3b0-0x3bb], [io 0x3c0-0x3df], etc., that are
> > not configurable by BARs.  Consequently, multiple VGA devices can conflict
> > with each other.  The VGA arbiter avoids conflicts by ensuring that those
> > legacy resources are only routed to one VGA device at a time.
> > 
> > The arbiter identifies the "default VGA" device, i.e., a legacy VGA device
> > that was used by boot firmware.  It selects the first device that:
> > 
> >    - is of PCI_CLASS_DISPLAY_VGA,
> >    - has both PCI_COMMAND_IO and PCI_COMMAND_MEMORY enabled, and
> >    - has PCI_BRIDGE_CTL_VGA set in all upstream bridges.
> > 
> > Some systems don't have such a device.  For example, if a host bridge
> > doesn't support I/O space, PCI_COMMAND_IO probably won't be enabled for any
> > devices below it.  Or, as on the HiSilicon D05, the VGA device may be
> > behind a bridge that doesn't support PCI_BRIDGE_CTL_VGA, so accesses to the
> > legacy VGA resources will never reach the device.
> > 
> > This patch extends the arbiter so that if it doesn't find a device that
> > meets all the above criteria, it selects the first device that:
> > 
> >    - is of PCI_CLASS_DISPLAY_VGA and
> >    - has PCI_COMMAND_IO or PCI_COMMAND_MEMORY enabled
> > 
> > If it doesn't find even that, it selects the first device that:
> > 
> >    - is of class PCI_CLASS_DISPLAY_VGA.
> > 
> > Such a device may not be able to use the legacy VGA resources, but most
> > drivers can operate the device without those.  Setting it as the default
> > device means its "boot_vga" sysfs file will contain "1", which Xorg (via
> > libpciaccess) uses to help select its default output device.
> > 
> > This fixes Xorg auto-detection on some arm64 systems (HiSilicon D05 in
> > particular; see the link below).
> > 
> > It also replaces the powerpc fixup_vga() quirk, albeit with slightly
> > different semantics: the quirk selected the first VGA device we found, and
> > overrode that selection with any enabled VGA device we found.  If there
> > were several enabled VGA devices, the *last* one we found would become the
> > default.
> > 
> > The code here instead selects the *first* enabled VGA device we find, and
> > if none are enabled, the first VGA device we find.
> > 
> > Link: http://lkml.kernel.org/r/[email-masked]
> > Tested-by: Daniel Axtens <[email-masked]>    # arm64, ppc64-qemu-tcg
> > Signed-off-by: Bjorn Helgaas <[email-masked]>
> > ---
> >   arch/powerpc/kernel/pci-common.c |   12 ------------
> >   drivers/gpu/vga/vgaarb.c         |   25 +++++++++++++++++++++++++
> >   2 files changed, 25 insertions(+), 12 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/pci-common.c b/arch/powerpc/kernel/pci-common.c
> > index 02831a396419..0ac7aa346c69 100644
> > --- a/arch/powerpc/kernel/pci-common.c
> > +++ b/arch/powerpc/kernel/pci-common.c
> > @@ -1740,15 +1740,3 @@ static void fixup_hide_host_resource_fsl(struct pci_dev *dev)
> >   }
> >   DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_MOTOROLA, PCI_ANY_ID, fixup_hide_host_resource_fsl);
> >   DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_FREESCALE, PCI_ANY_ID, fixup_hide_host_resource_fsl);
> > -
> > -static void fixup_vga(struct pci_dev *pdev)
> > -{
> > -	u16 cmd;
> > -
> > -	pci_read_config_word(pdev, PCI_COMMAND, &cmd);
> > -	if ((cmd & (PCI_COMMAND_IO | PCI_COMMAND_MEMORY)) || !vga_default_device())
> > -		vga_set_default_device(pdev);
> > -
> > -}
> > -DECLARE_PCI_FIXUP_CLASS_FINAL(PCI_ANY_ID, PCI_ANY_ID,
> > -			      PCI_CLASS_DISPLAY_VGA, 8, fixup_vga);
> > diff --git a/drivers/gpu/vga/vgaarb.c b/drivers/gpu/vga/vgaarb.c
> > index 76875f6299b8..aeb41f793ed4 100644
> > --- a/drivers/gpu/vga/vgaarb.c
> > +++ b/drivers/gpu/vga/vgaarb.c
> > @@ -1468,6 +1468,31 @@ static int __init vga_arb_device_init(void)
> >   			vgaarb_info(dev, "no bridge control possible\n");
> >   	}
> >   
> > +	if (!vga_default_device()) {
> > +		list_for_each_entry(vgadev, &vga_list, list) {
> > +			struct device *dev = &vgadev->pdev->dev;
> > +			u16 cmd;
> > +
> > +			pdev = vgadev->pdev;
> > +			pci_read_config_word(pdev, PCI_COMMAND, &cmd);
> > +			if (cmd & (PCI_COMMAND_IO | PCI_COMMAND_MEMORY)) {
> > +				vgaarb_info(dev, "setting as boot device (VGA legacy resources not available)\n");
> > +				vga_set_default_device(pdev);
> > +				break;
> > +			}
> > +		}
> > +	}
> > +
> > +	if (!vga_default_device()) {
> > +		vgadev = list_first_entry_or_null(&vga_list,
> > +						  struct vga_device, list);
> > +		if (vgadev) {
> > +			struct device *dev = &vgadev->pdev->dev;
> > +			vgaarb_info(dev, "setting as boot device (VGA legacy resources not available)\n");
> > +			vga_set_default_device(pdev);
> 
> Isn't 'pdev' NULL here? shouldn't it be vgadev->pdev instead?
> 
That cannot not happen, though it isn't quite obvious.
'vgadev' will only be non-NULL, when the vga_list isn't empty and in
that case pdev has been set up in the list_for_each_entry() loop above.

Lothar Waßmann