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

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Message-ID: <f87a48c6-909e-39ba-62b0-289e78798540@loongson.cn>
Date:   Thu, 20 Jul 2023 06:32:19 +0800
From:   suijingfeng <suijingfeng@...ngson.cn>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        Sui Jingfeng <sui.jingfeng@...ux.dev>
Cc:     David Airlie <airlied@...il.com>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, kvm@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-fbdev@...r.kernel.org,
        Alex Deucher <alexander.deucher@....com>,
        Christian Konig <christian.koenig@....com>,
        Pan Xinhui <Xinhui.Pan@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Hawking Zhang <Hawking.Zhang@....com>,
        Mario Limonciello <mario.limonciello@....com>,
        Lijo Lazar <lijo.lazar@....com>,
        YiPeng Chai <YiPeng.Chai@....com>,
        Bokun Zhang <Bokun.Zhang@....com>,
        Likun Gao <Likun.Gao@....com>,
        Ville Syrjala <ville.syrjala@...ux.intel.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Kevin Tian <kevin.tian@...el.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Yishai Hadas <yishaih@...dia.com>,
        Abhishek Sahu <abhsahu@...dia.com>,
        Yi Liu <yi.l.liu@...el.com>,
        Jani Nikula <jani.nikula@...el.com>
Subject: Re: [PATCH v3 4/9] PCI/VGA: Improve the default VGA device selection

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@...ngson.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.