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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Message-ID: <42c907fe-a8c3-5a07-a792-737e45f8134e@loongson.cn>
Date:   Wed, 6 Sep 2023 10:14:28 +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

Hi,


On 2023/9/5 23:05, Thomas Zimmermann wrote:
> However, on modern Linux systems the primary display does not really 
> exist.


No, it do exist.  X server need to know which one is the primary GPU.
The '*' character at the of (4@0:0:0) PCI device is the Primary.
The '*' denote primary, see the log below.

(II) xfree86: Adding drm device (/dev/dri/card2)
(II) xfree86: Adding drm device (/dev/dri/card0)
(II) Platform probe for 
/sys/devices/pci0000:00/0000:00:1c.5/0000:003:00.0/0000:04:00.0/drm/card0
(II) xfree86: Adding drm device (/dev/dri/card3)
(II) Platform probe for 
/sys/devices/pci0000:00/0000:00:1c.6/0000:005:00.0/drm/card3
(--) PCI: (0@0:2:0) 8086:3e91:8086:3e91 rev 0, Mem @ 
0xdb000000/167777216, 0xa0000000/536870912, I/O @ 0x0000f000/64, BIOS @ 
0x????????/131072
(--) PCI: (1@0:0:0) 1002:6771:1043:8636 rev 0, Mem @ 
0xc0000000/2688435456, 0xdf220000/131072, I/O @ 0x0000e000/256, BIOS @ 
0x????????/131072
(--) PCI:*(4@0:0:0) 1a03:2000:1a03:2000 rev 48, Mem @ 
0xde000000/166777216, 0xdf020000/131072, I/O @ 0x0000c000/128, BIOS @ 
0x????????/131072
(--) PCI: (5@0:0:0) 10de:1288:174b:b324 rev 161, Mem @ 
0xdc000000/116777216, 0xd0000000/134217728, 0xd8000000/33554432, I/O @ 
0x0000b000/128, BIOS @@0x????????/524288

The modesetting driver of X server will create framebuffer on the primary video adapter.
If a 2D video adapter (like the aspeed BMC) is not the primary, then it probably will not
be used. The only chance to be able to display something is to functional as a output slave.
But the output slave technology need the PRIME support for cross driver buffer sharing.

So, there do have some difference between the primary and non-primary video adapters.


> 'Primary' is the device that is available via VGA, VESA or EFI. Our 
> drivers don't use these interfaces, but the native registers. As you 
> said yourself, these firmware devices (VGA, VESA, EFI) are removed 
> ASAP by the native drivers.