Hi, Bjorn, On Wed, May 19, 2021 at 3:35 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > On Tue, May 18, 2021 at 03:13:43PM +0800, Huacai Chen wrote: > > On Tue, May 18, 2021 at 2:28 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > > On Mon, May 17, 2021 at 08:53:43PM +0800, Huacai Chen wrote: > > > > On Sat, May 15, 2021 at 5:09 PM Huacai Chen <chenhuacai@xxxxxxxxx> wrote: > > > > > On Fri, May 14, 2021 at 11:10 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > > > > > On Fri, May 14, 2021 at 04:00:25PM +0800, Huacai Chen wrote: > > > > > > > According to PCI-to-PCI bridge spec, bit 3 of Bridge Control Register is > > > > > > > VGA Enable bit which modifies the response to VGA compatible addresses. > > > > > > > > > > > > The bridge spec is pretty old, and most of the content has been > > > > > > incorporated into the PCIe spec. I think you can cite "PCIe r5.0, sec > > > > > > 7.5.1.3.13" here instead. > > > > > > > > > > > > > If the VGA Enable bit is set, the bridge will decode and forward the > > > > > > > following accesses on the primary interface to the secondary interface. > > > > > > > > > > > > *Which* following accesses? The structure of English requires that if > > > > > > you say "the following accesses," you must continue by *listing* the > > > > > > accesses. > > > > > > > > > > > > > The ASpeed AST2500 hardward does not set the VGA Enable bit on its > > > > > > > bridge control register, which causes vgaarb subsystem don't think the > > > > > > > VGA card behind the bridge as a valid boot vga device. > > > > > > > > > > > > s/hardward/bridge/ > > > > > > s/vga/VGA/ (also in code comments and dmesg strings below) > > > > > > > > > > > > From the code, it looks like AST2500 ([1a03:2000]) is a VGA device, > > > > > > since it apparently has a VGA class code. But here you say the > > > > > > AST2500 has a Bridge Control register, which suggests that it's a > > > > > > bridge. If AST2500 is some sort of combination that includes both a > > > > > > bridge and a VGA device, please outline that topology. > > > > > > > > > > > > But the hardware defect is that some bridges forward VGA accesses even > > > > > > though their VGA Enable bit is not set? The quirk should be attached > > > > > > to broken *bridges*, not to VGA devices. > > > > > > > > > > > > If a bridge forwards VGA accesses regardless of how its VGA Enable bit > > > > > > is set, that means VGA arbitration (in vgaarb.c) cannot work > > > > > > correctly, so merely setting the default VGA device once in a quirk is > > > > > > not sufficient. You would have to somehow disable any future attempts > > > > > > to use other VGA devices. Only the VGA device below this defective > > > > > > bridge is usable. Any other VGA devices in the system would be > > > > > > useless. > > > > > > > > > > > > > So we provide a quirk to fix Xorg auto-detection. > > > > > > > > > > > > > > See similar bug: > > > > > > > > > > > > > > https://patchwork.kernel.org/project/linux-pci/patch/20170619023528.11532-1-dja@xxxxxxxxxx/ > > > > > > > > > > > > This patch was never merged. If we merged a revised version, please > > > > > > cite the SHA1 instead. > > > > > > > > > > This patch has never merged, and I found that it is unnecessary after > > > > > commit a37c0f48950b56f6ef2ee637 ("vgaarb: Select a default VGA device > > > > > even if there's no legacy VGA"). Maybe this ASpeed patch is also > > > > > unnecessary. If it is still needed, I'll investigate the root cause. > > > > > > > > I found that vga_arb_device_init() and pcibios_init() are both wrapped > > > > by subsys_initcall(), which means their sequence is unpredictable. And > > > > unfortunately, in our platform vga_arb_device_init() is called before > > > > pcibios_init(), which makes vga_arb_device_init() fail to set a > > > > default vga device. This is the root cause why we thought that we > > > > still need a quirk for AST2500. > > > > > > Does this mean there is no hardware defect here? The VGA Enable bit > > > works correctly? > > > > > No, VGA Enable bit still doesn't set, but with commit > > a37c0f48950b56f6ef2ee637 ("vgaarb: Select a default VGA device even if > > there's no legacy VGA") we no longer depend on VGA Enable. > > Correct me if I'm wrong: > > - On the AST2500 bridge [1a03:1150], the VGA Enable bit is > read-only 0. > > - The AST2500 bridge never forwards VGA accesses ([mem > 0xa0000-0xbffff], [io 0x3b0-0x3bb], [io 0x3c0-0x3df]) to its > secondary bus. > > The VGA Enable bit is optional, and if both the above are true, the > bridge is working correctly per spec, and the quirk below is not the > right solution, and whatever solution we come up with should not > claim that the bridge is misbehaving. Yes, you are right, the bridge is working correctly, which is similar to HiSilicon D05. > > > > > I think the best solution is make vga_arb_device_init() be wrapped by > > > > subsys_initcall_sync(), do you think so? > > > > > > Hmm. Unfortunately the semantics of subsys_initcall_sync() are not > > > documented, so I'm not sure exactly *why* such a change would work and > > > whether we could rely on it to continue working. > > > > > > pcibios_init() isn't very consistent across arches. On some, > > > including alpha, microblaze, some MIPS platforms, powerpc, and sh, it > > > enumerates PCI devices. On others (ia64, parisc, sparc, x86), it does > > > basically nothing. That makes life a little difficult. > > > > subsys_initcall_sync() is ensured after all subsys_initcall() > > functions, so at least it can solve the problem on platforms which use > > pcibios_init() to enumerate PCI devices (x86 and other ACPI-based > > platforms are also OK, because they use acpi_init() > > -->acpi_scan_init() -->pci_acpi_scan_root() to enumerate devices). > > More details in my response to suijingfeng: > https://lore.kernel.org/r/20210518193100.GA148462@bjorn-Precision-5520 > > I'd rather not fiddle with the initcall ordering. That mechanism is > fragile and I'd prefer something more robust. > > I'm wondering whether it's practical to do something in the normal PCI > enumeration path, e.g., in pci_init_capabilities(). Maybe we can > detect the default VGA device as we enumerate it. Then we wouldn't > have this weird process of "find all PCI devices first, then scan for > the default VGA device, and oh, by the way, also check for VGA devices > hot-added later." If we don't want to rely on initcall order, and want to solve the hot-added case, then can we add vga_arb_select_default_device() in pci_notify() when (action == BUS_NOTIFY_ADD_DEVICE && !vga_default_device())? Huacai > > Bjorn
