[Nouveau] [RFC, drm-misc-next v4 0/9] PCI/VGA: Allowing the user to select the primary video adapter at boot time

[suijingfeng]

Hi,


On 2023/9/6 16:05, Thomas Zimmermann wrote:
> Hi
>
> Am 05.09.23 um 17:59 schrieb suijingfeng:
> [...]
>>> 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,
>
> Please look at the history and discussion around this parameter. To my 
> knowledge, 'modeset' got introduced when modesetting with still done 
> in userspace. It was an easy way of disabling the kernel driver if the 
> system's Xorg did no yet support kernel mode setting.
>
> Fast forward a few years and all Linux' use kernel modesetting, which 
> make the modeset parameters obsolete. We discussed and decided to keep 
> them in, because many articles and blog posts refer to them. We didn't 
> want to invalidate them. BUT modeset is deprecated and not allowed in 
> new code. If you look at existing modeset usage, you will eventually 
> come across the comment at [1].
>

OK, no problem. I agree what you said.


> There's 'nomodeset', which disables all native drivers. It's useful 
> for debugging or as a quick-fix if the graphics driver breaks. If you 
> want to disable a specific driver, please use one of the options for 
> blacklisting.
>
Yeah, the 'nomodeset' disables all native drivers,
this is a good point of it, but this is also the weak point of it.

Sometimes, when you are developing a drm driver for a new device.
You will see the pain. Its too often a programmer's modification
make the entire Linux kernel hang there. The problematic drm
driver kernel module already in the initrd. Then, the real
need to disable the ill-functional drm driver kernel module
only. While what you recommend to disable them all. There
are subtle difference.

Another limitation of the 'nomodeset' parameter is that
it is only available on recent upstream kernel. Low version
downstream kernel don't has this parameter supported yet.
So this create inconstant developing experience. I believe that
there always some people need do back-port and upstream work
for various reasons.

While (kindly, no offensive) debating, since we have the modprobe.blacklist
why we still need the 'nomodeset' parameter ?
why not try modprobe.blacklist="amdgpu,radeon,i915,ast,nouveau,gma500_gfx, ..."

:-/


But OK in overall, I will listen to your advice.


> Best regards
> Thomas
>
> [1] 
> https://elixir.bootlin.com/linux/v6.5/source/include/drm/drm_module.h#L83
>
>
>> 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.
>>
>>
>