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

[Huacai Chen]

Hi, Bjorn,

On Sat, Jul 24, 2021 at 8:10 AM Bjorn Helgaas <helgaas at kernel.org> wrote:
>
> 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 at kernel.org> wrote:
> > >
> > > From: Bjorn Helgaas <bhelgaas at 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?
Yes, that's right.

Huacai
>
> [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
> > >