Date:   Tue, 5 Sep 2023 23:59:54 +0800
From:   suijingfeng <suijingfeng@...ngson.cn>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Sui Jingfeng <sui.jingfeng@...ux.dev>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        "Koenig, Christian" <Christian.Koenig@....com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>
Cc:     nouveau@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org, linux-pci@...r.kernel.org
Subject: Re: [Nouveau] [RFC, drm-misc-next v4 0/9] PCI/VGA: Allowing the user
 to select the primary video adapter at boot time


On 2023/9/5 18:49, Thomas Zimmermann wrote:
> Hi
>
> Am 04.09.23 um 21:57 schrieb Sui Jingfeng:
>> From: Sui Jingfeng <suijingfeng@...ngson.cn>
>>
>> On a machine with multiple GPUs, a Linux user has no control over which
>> one is primary at boot time. This series tries to solve above mentioned
>> problem by introduced the ->be_primary() function stub. The specific
>> device drivers can provide an implementation to hook up with this 
>> stub by
>> calling the vga_client_register() function.
>>
>> Once the driver bound the device successfully, VGAARB will call back to
>> the device driver. To query if the device drivers want to be primary or
>> not. Device drivers can just pass NULL if have no such needs.
>>
>> Please note that:
>>
>> 1) The ARM64, Loongarch, Mips servers have a lot PCIe slot, and I would
>>     like to mount at least three video cards.
>>
>> 2) Typically, those non-86 machines don't have a good UEFI firmware
>>     support, which doesn't support select primary GPU as firmware stage.
>>     Even on x86, there are old UEFI firmwares which already made 
>> undesired
>>     decision for you.
>>
>> 3) This series is attempt to solve the remain problems at the driver 
>> level,
>>     while another series[1] of me is target to solve the majority of the
>>     problems at device level.
>>
>> Tested (limited) on x86 with four video card mounted, Intel UHD Graphics
>> 630 is the default boot VGA, successfully override by ast2400 with
>> ast.modeset=10 append at the kernel cmd line.
>
> FYI: per-driver modeset parameters are deprecated and not to be used. 
> Please don't promote them.


Well, please wait, I want to explain.



drm/nouveau already promote it a little bit.

Despite no code of conduct or specification guiding how the modules parameters should be.
Noticed that there already have a lot of DRM drivers support the modeset parameters,
for the modeset parameter, authors of various device driver try to make the usage not
conflict with others. I believe that this is good thing for Linux users.
It is probably the responsibility of the drm core maintainers to force various drm
drivers to reach a minimal consensus. Probably it pains to do so and doesn't pay off.
But reach a minimal consensus do benefit to Linux users.


> You can use modprobe.blacklist or initcall_blacklist on the kernel 
> command line.
>
There are some cases where the modprobe.blacklist doesn't works,
I have come cross several time during the past.
Because the device selected by the VGAARB is device-level thing,
it is not the driver's problem.

Sometimes when VGAARB has a bug, it will select a wrong device as primary.
And the X server will use this wrong device as primary and completely crash
there, due to lack a driver. Take my old S3 Graphics as an example:

$ lspci | grep VGA

  00:06.1 VGA compatible controller: Loongson Technology LLC DC (Display Controller) (rev 01)
  03:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Caicos XT [Radeon HD 7470/8470 / R5 235/310 OEM]
  07:00.0 VGA compatible controller: S3 Graphics Ltd. Device 9070 (rev 01)
  08:00.0 VGA compatible controller: S3 Graphics Ltd. Device 9070 (rev 01)

Before apply this patch:

[    0.361748] pci 0000:00:06.1: vgaarb: setting as boot VGA device
[    0.361753] pci 0000:00:06.1: vgaarb: VGA device added: decodes=io+mem,owns=io+mem,locks=none
[    0.361765] pci 0000:03:00.0: vgaarb: VGA device added: decodes=io+mem,owns=none,locks=none
[    0.361773] pci 0000:07:00.0: vgaarb: VGA device added: decodes=io+mem,owns=none,locks=none
[    0.361779] pci 0000:08:00.0: vgaarb: VGA device added: decodes=io+mem,owns=none,locks=none
[    0.361781] vgaarb: loaded
[    0.367838] pci 0000:00:06.1: Overriding boot device as 1002:6778
[    0.367841] pci 0000:00:06.1: Overriding boot device as 5333:9070
[    0.367843] pci 0000:00:06.1: Overriding boot device as 5333:9070


For known reason, one of my system select the S3 Graphics as primary GPU.
But this S3 Graphics not even have a decent drm upstream driver yet.
Under such a case, I begin to believe that only the device who has a
driver deserve the primary.

Under such a condition, I want to reboot and enter the graphic environment
with other working video cards. Either platform integrated and discrete GPU.
This don't means I should compromise by un-mount the S3 graphics card from
the motherboard, this also don't means that I should update my BIOS setting.
As sometimes, the BIOS is more worse.

With this series applied, all I need to do is to reboot the computer and
pass a command line. By force override another video card (who has a
decent driver support) as primary, I'm able to do the debugging under
graphic environment. I would like to examine what's wrong with the vgaarb
on a specific platform under X server graphic environment.

Probably try compile a driver for this card and see it works, simply reboot
without the need to change anything. It is so efficient. So this is probably
the second usage of my patch. It hand the right of control back to the
graphic developer.

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