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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@axtens.net> # arm64, ppc64-qemu-tcg Tested-by: Zhou Wang <wangzhou1@hisilicon.com> # D05 Hisi Hip07, Hip08 Signed-off-by: Bjorn Helgaas <bhelgaas@google.com> Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Reviewed-by: Daniel Axtens <dja@axtens.net> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> Link: https://patchwork.freedesktop.org/patch/msgid/20171013034721.14630.65913.stgit@bhelgaas-glaptop.roam.corp.google.com |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.