Re: [PATCH v2 0/9] PCI/VGA: Rework default VGA device selection

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jul 23, 2021 at 05:53:36PM +0800, Huacai Chen wrote:
> Hi, Bjorn,
> 
> On Fri, Jul 23, 2021 at 5:29 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
> >
> > From: Bjorn Helgaas <bhelgaas@google.com>
> >
> > This is a little bit of rework and extension of Huacai's nice work at [1].
> >
> > It moves the VGA arbiter to the PCI subsystem, fixes a few nits, and breaks
> > a few pieces off Huacai's patch to make the main patch a little smaller.
> >
> > That last patch is still not very small, and it needs a commit log, as I
> > mentioned at [2].
> >
> > All comments welcome!
> >
> > [1] https://lore.kernel.org/dri-devel/20210705100503.1120643-1-chenhuacai@loongson.cn/
> > [2] https://lore.kernel.org/r/20210720221923.GA43331@bjorn-Precision-5520
> Thank you for your splitting. Your two questions are answered in the following.
> 
> (1) explain why your initcall ordering is unusual.
> The original problem happens on MIPS. vga_arb_device_init() and
> pcibios_init() are both wrapped by subsys_initcall(). The order of
> functions in the same level depends on the Makefile.
> 
> TOP level Makefile:
> drivers-y       := drivers/ sound/
> ....
> include arch/$(SRCARCH)/Makefile
> 
> drivers/Makefile:
> obj-$(CONFIG_ACPI)              += acpi/
> ....
> obj-y                           += gpu/
> 
> arch/mips/Makefile:
> drivers-$(CONFIG_PCI)           += arch/mips/pci/
> 
> This makes pcibios_init() in arch/mips/pci/ placed after
> vga_arb_device_init() in drivers/gpu. ACPI-based systems have no
> problems because acpi_init() in drivers/acpi is placed before
> vga_arb_device_init().

Thanks for the above; that was helpful.  To summarize:

  - On your system, the AST2500 bridge [1a03:1150] does not implement
    PCI_BRIDGE_CTL_VGA [1].  This is perfectly legal but means the
    legacy VGA resources won't reach downstream devices unless they're
    included in the usual bridge windows.

  - vga_arb_select_default_device() will set a device below such a
    bridge as the default VGA device as long as it has PCI_COMMAND_IO
    and PCI_COMMAND_MEMORY enabled.

  - vga_arbiter_add_pci_device() is called for every VGA device,
    either at boot-time or at hot-add time, and it will also set the
    device as the default VGA device, but ONLY if all bridges leading
    to it implement PCI_BRIDGE_CTL_VGA.

  - This difference between vga_arb_select_default_device() and
    vga_arbiter_add_pci_device() means that a device below an AST2500
    or similar bridge can only be set as the default if it is
    enumerated before vga_arb_device_init().

  - On ACPI-based systems, PCI devices are enumerated by acpi_init(),
    which runs before vga_arb_device_init().

  - On non-ACPI systems, like your MIPS system, they are enumerated by
    pcibios_init(), which typically runs *after*
    vga_arb_device_init().

So I think the critical change is actually that you made
vga_arb_update_default_device(), which you call from
vga_arbiter_add_pci_device(), set the default device even if it does
not own the VGA resources because an upstream bridge doesn't implement
PCI_BRIDGE_CTL_VGA, i.e.,

  (vgadev->owns & VGA_RSRC_LEGACY_MASK) != VGA_RSRC_LEGACY_MASK

Does that seem right?

[1] https://lore.kernel.org/r/CAAhV-H4pn53XC7qVvwM792ppkQRnjWpPDwmrhBv8twgQu0eabQ@mail.gmail.com

> (2) explain the approach, which IIUC is basically to add the
> vga_arb_select_default_device() functionality to
> vga_arbiter_add_pci_device().
> vga_arb_select_default_device() has only one chance to be called, we
> want to make it be called every time a new vga device is added. So
> rename it to vga_arb_update_default_device() and move the callsite to
> vga_arbiter_add_pci_device().
> 
> I think you know all the information which you need now. And you can
> reorganize the commit message based on the existing one. As English is
> not my first language, the updated commit message written by me may
> still not be as good as you want.:)
> 
> Huacai
> 
> > Bjorn Helgaas (4):
> >   PCI/VGA: Move vgaarb to drivers/pci
> >   PCI/VGA: Replace full MIT license text with SPDX identifier
> >   PCI/VGA: Use unsigned format string to print lock counts
> >   PCI/VGA: Remove empty vga_arb_device_card_gone()
> >
> > Huacai Chen (5):
> >   PCI/VGA: Move vga_arb_integrated_gpu() earlier in file
> >   PCI/VGA: Prefer vga_default_device()
> >   PCI/VGA: Split out vga_arb_update_default_device()
> >   PCI/VGA: Log bridge control messages when adding devices
> >   PCI/VGA: Rework default VGA device selection
> >
> >  drivers/gpu/vga/Kconfig           |  19 ---
> >  drivers/gpu/vga/Makefile          |   1 -
> >  drivers/pci/Kconfig               |  19 +++
> >  drivers/pci/Makefile              |   1 +
> >  drivers/{gpu/vga => pci}/vgaarb.c | 269 ++++++++++++------------------
> >  5 files changed, 126 insertions(+), 183 deletions(-)
> >  rename drivers/{gpu/vga => pci}/vgaarb.c (90%)
> >
> > --
> > 2.25.1
> >