Re: [PATCH v3 4/9] PCI/VGA: Improve the default VGA device selection

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On 2023/7/20 03:32, Bjorn Helgaas wrote:
[+cc linux-pci (please cc in the future since the bulk of this patch
is in drivers/pci/)]

On Wed, Jul 12, 2023 at 12:43:05AM +0800, Sui Jingfeng wrote:
From: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>

Currently, the strategy of selecting the default boot on a multiple video
card coexistence system is not perfect. Potential problems are:

1) This function is a no-op on non-x86 architectures.
Which function in particular is a no-op for non-x86?

I refer to the vga_is_firmware_default() function,

I will improve the commit message at the next version. (To make it more human readable).

Thanks you point it out.

2) It does not take the PCI Bar may get relocated into consideration.
3) It is not effective for the PCI device without a dedicated VRAM Bar.
4) It is device-agnostic, thus it has to waste the effort to iterate all
    of the PCI Bar to find the VRAM aperture.
5) It has invented lots of methods to determine which one is the default
    boot device, but this is still a policy because it doesn't give the
    user a choice to override.
I don't think we need a list of *potential* problems.  We need an
example of the specific problem this will solve, i.e., what currently
does not work?

1) The selection of primary GPU on Non-x86 platform. (Arm64, risc-v, powerpc etc)

Mostly server platforms have equipped with aspeed bmc, and such hardware platforms have a lot PCIe slot.

So I think, aspeed bmc V.S (P.K) radeon(or amdgpu) is very common.

2) The ability to pass the control back to the end user.

Convert the *device driven* to the "driver driven" or "human driven".

Currently, it is the machine making the decision.

Emm, I probably will be able to give some examples at the next version.

The drm/ast and maybe drm/loongson patches are the only ones that use
the new callback, so I assume there are real problems with those

CONFIG_DRM_AST is a tristate.  We're talking about identifying the
boot-time console device.  So if CONFIG_DRM_AST=m, I guess we don't
get the benefit of the new callback unless the module gets loaded?

Since, this patch set is mostly for the user of X server.

It is actually okey if CONFIG_DRM_AST=m. (it will be works no matter CONFIG_DRM_AST=m or CONFIG_DRM_AST=y)

As the device and the driver bound at a latter time.

So we are lucky, we need this behavior to implement the override.

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