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

[suijingfeng]

Hi,

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 at loongson.cn>
>>
>> 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
> drivers.
>
> 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.