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

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Date:   Wed, 6 Sep 2023 10:34:52 +0800
From:   suijingfeng <suijingfeng@...ngson.cn>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Sui Jingfeng <sui.jingfeng@...ux.dev>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Koenig, Christian" <Christian.Koenig@....com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Daniel Vetter <daniel@...ll.ch>,
        "Deucher, Alexander" <Alexander.Deucher@....com>
Cc:     nouveau@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...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 23:05, Thomas Zimmermann wrote:
> Hi
>
> Am 05.09.23 um 15:30 schrieb suijingfeng:
>> Hi,
>>
>>
>> On 2023/9/5 18:45, 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
>>>
>>> If anything, the primary graphics adapter is the one initialized by 
>>> the firmware. I think our boot-up graphics also make this assumption 
>>> implicitly.
>>>
>>
>> Yes, but by the time of DRM drivers get loaded successfully,the 
>> boot-up graphics already finished.
>> Firmware framebuffer device already get killed by the 
>> drm_aperture_remove_conflicting_pci_framebuffers()
>> function (or its siblings). So, this series is definitely not to 
>> interact with the firmware framebuffer
>
> Yes and no. The helpers you mention will attempt to remove the 
> firmware framebuffer on the given PCI device. If you have multiple PCI 
> devices, the other devices would not be affected.
>
Yes and no.


For the yes part: drm_aperture_remove_conflicting_pci_framebuffers() only kill the conflict one.
But for a specific machine with the modern UEFI firmware,
there should be only one firmware framebuffer driver.
That shoudd be the EFIFB(UEFI GOP). I do have multiple PCI devices,
but I don't understand when and why a system will have more than one firmware framebuffer.

Even for the machines with the legacy BIOS, the fixed VGA aperture address range
can only be owned by one firmware driver. It is just that we need to handle the
routing, the ->set_decode() callback of vga_client_register() is used to do such
work. Am I correct?