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

[Julien Thierry]

Hi Bjorn,

On 06/10/17 23:24, Bjorn Helgaas wrote:
> From: Bjorn Helgaas <bhelgaas at google.com>
> 
> 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/20170901072744.2409-1-dja@axtens.net
> Tested-by: Daniel Axtens <dja at axtens.net>    # arm64, ppc64-qemu-tcg
> Signed-off-by: Bjorn Helgaas <bhelgaas at google.com>
> ---
>   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?

Cheers,

-- 
Julien Thierry