linux-kernel - Re: [PATCH v9 8/9] fbcon: Use screen info to find primary device

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Message-ID: <93253093-f6a8-4c34-988d-bdc9489cf4f0@kernel.org>
Date: Tue, 22 Jul 2025 10:39:08 -0500
From: Mario Limonciello <superm1@...nel.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: David Airlie <airlied@...il.com>, Bjorn Helgaas <bhelgaas@...gle.com>,
 Alex Deucher <alexander.deucher@....com>,
 Christian König <christian.koenig@....com>,
 Simona Vetter <simona@...ll.ch>, Lukas Wunner <lukas@...ner.de>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Woodhouse <dwmw2@...radead.org>, Lu Baolu <baolu.lu@...ux.intel.com>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Robin Murphy <robin.murphy@....com>,
 Alex Williamson <alex.williamson@...hat.com>,
 Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
 "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
 open list <linux-kernel@...r.kernel.org>,
 "open list:INTEL IOMMU (VT-d)" <iommu@...ts.linux.dev>,
 "open list:PCI SUBSYSTEM" <linux-pci@...r.kernel.org>,
 "open list:VFIO DRIVER" <kvm@...r.kernel.org>,
 "open list:SOUND" <linux-sound@...r.kernel.org>,
 Daniel Dadap <ddadap@...dia.com>,
 Mario Limonciello <mario.limonciello@....com>
Subject: Re: [PATCH v9 8/9] fbcon: Use screen info to find primary device

On 7/22/25 10:33 AM, Bjorn Helgaas wrote:
> On Tue, Jul 22, 2025 at 09:45:28AM -0500, Mario Limonciello wrote:
>> On 7/22/25 9:38 AM, Bjorn Helgaas wrote:
>>> On Thu, Jul 17, 2025 at 12:38:11PM -0500, Mario Limonciello wrote:
>>>> From: Mario Limonciello <mario.limonciello@....com>
>>>>
>>>> On systems with non VGA GPUs fbcon can't find the primary GPU because
>>>> video_is_primary_device() only checks the VGA arbiter.
>>>>
>>>> Add a screen info check to video_is_primary_device() so that callers
>>>> can get accurate data on such systems.
>>>
>>> This relies on screen_info, which I think is an x86 BIOS-ism.  Isn't
>>> there a UEFI console path?  How does that compare with this?  Is that
>>> relevant or is it something completely different?
>>
>> When I created and tested this I actually did this on a UEFI system (which
>> provides a UEFI GOP driver).
> 
> I guess screen_info is actually *not* an x86 BIOS-ism, and on UEFI
> systems, we do actually rely on UEFI, e.g., in efi_setup_gop(),
> alloc_screen_info(), init_screen_info()?

Right.  This all works because of the framebuffer allocated pre-boot and 
reused by the kernel.

> 
> But this patch is x86-specific, so I'm guessing the same problem could
> occur on arm64, Loongson, or other UEFI platforms, and this series
> doesn't address those?

I've never seen a multi GPU solution on another architecture, but that 
of course doesn't preclude one being created some day.

The series lays the groundwork that if it happens on another 
architecture we can easily add an architecture specific solution for 
those.  If the solution is the same we could switch to a common helper.

> 
>>>>    bool video_is_primary_device(struct device *dev)
>>>>    {
>>>> +#ifdef CONFIG_SCREEN_INFO
>>>> +	struct screen_info *si = &screen_info;
>>>> +#endif
>>>>    	struct pci_dev *pdev;
>>>>    	if (!dev_is_pci(dev))
>>>> @@ -34,7 +38,18 @@ bool video_is_primary_device(struct device *dev)
>>>>    	pdev = to_pci_dev(dev);
>>>> -	return (pdev == vga_default_device());
>>>> +	if (!pci_is_display(pdev))
>>>> +		return false;
>>>> +
>>>> +	if (pdev == vga_default_device())
>>>> +		return true;
>>>> +
>>>> +#ifdef CONFIG_SCREEN_INFO
>>>> +	if (pdev == screen_info_pci_dev(si))
>>>> +		return true;
>>>> +#endif
>>>> +
>>>> +	return false;
>>>>    }
>>>>    EXPORT_SYMBOL(video_is_primary_device);
>>>> -- 
>>>> 2.43.0
>>>>
>>