[PATCH v2 0/4] Allow non-legacy cards to be vgaarb default

[Gabriele Paoloni]

Hi Daniel many thanks for your patch

> -----Original Message-----
> From: Daniel Axtens [mailto:dja at axtens.net]
> Sent: 19 July 2017 03:15
> To: linux-pci at vger.kernel.org; linuxppc-dev at lists.ozlabs.org; linux-
> arm-kernel at lists.infradead.org
> Cc: benh at kernel.crashing.org; Liuxinliang (Matthew Liu);
> zourongrong at gmail.com; catalin.marinas at arm.com; will.deacon at arm.com;
> Gabriele Paoloni; bhelgaas at google.com; airlied at linux.ie;
> daniel.vetter at intel.com; alex.williamson at redhat.com; Daniel Axtens
> Subject: [PATCH v2 0/4] Allow non-legacy cards to be vgaarb default
> 
> [v2, in which I send the right patches. My apologies to you all.]
> 
> Hi all,
> 
> Previously I posted a patch that provided a quirk for a hibmc card
> behind a particular Huawei bridge that allowed it to be marked as the
> default device in the VGA arbiter.[0] This lead to some discussion.[1]
> It was broadly suggested that a more generic solution would be better,
> something in the style of powerpc's fixup_vga() quirk.
> 
> Here is my suggested solution:
> 
>  - Create a Kconfig option ARCH_WANT_VGA_ARB_FALLBACK and

In my opinion we could avoid depending on a Kernel config options.
I.e. we can have generic code that, after all PCI devs are enumerated:
1) check if we have a default vga device
2) if not check each registered PCI device and make default device the first
   one that is a VGA device, capable to respond to IO and MEM requests
   and that has a driver bound to it 

> 
>  - if an arch selects that option, install PCI_FIXUP_CLASS_ENABLE
>    hook. This hook fires when a card is enabled, which will require
>    that a driver has been bound.
> 
>  - if there is no default device when the hook fires, and the device
>    can control memory and I/O, mark it as default.

I am worried that the patchset you proposed has a race condition with the
VGA arbiter. In fact you see:
subsys_initcall(vga_arb_device_init) is not guaranteed to be called before
subsys_initcall(acpi_init)

acpi_init->acpi_scan_init->acpi_pci_root_init() at this stage the PCI enumeration
is done and as soon as a device is added the Kernel will look for a driver
to bind to it and therefore you quirk could be called before the VGA arbiter...
Do you agree?

What about modifying vgaarb.c to add a late_initcall() checking what I suggested
above?

Thanks
Gab

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