linux-kernel - Re: [PATCH v10 3/4] fbcon: Use screen info to find primary device

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Message-ID: <fb519d0e-9d7f-4835-964a-c21fd24b10e8@kernel.org>
Date: Tue, 28 Oct 2025 08:15:48 -0500
From: Mario Limonciello <superm1@...nel.org>
To: Aaron Erhardt <aer@...edocomputers.com>, David Airlie
 <airlied@...il.com>, Bjorn Helgaas <bhelgaas@...gle.com>
Cc: Alex Deucher <alexander.deucher@....com>,
 Christian König <christian.koenig@....com>,
 Simona Vetter <simona@...ll.ch>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
 open list <linux-kernel@...r.kernel.org>,
 "open list:PCI SUBSYSTEM" <linux-pci@...r.kernel.org>,
 Daniel Dadap <ddadap@...dia.com>
Subject: Re: [PATCH v10 3/4] fbcon: Use screen info to find primary device

On 10/28/25 5:16 AM, Aaron Erhardt wrote:
> 
> On Mon, Aug 11, 2025 at 11:26:05AM -0500, Mario Limonciello (AMD) wrote:
>> 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.
> 
> I have a question regarding this change. To me, the function name
> video_is_primary_device() implies that there is only one primary GPU.
> I would also expect that the 'boot_display' attribute added later in
> the patch series based on this function is only set for one GPU, but
> that is not necessarily the case. Since I'm working on a user-space
> program that reads the 'boot_display' attribute, I need to know what
> behavior is intended in order to do a correct implementation.
> 
>>
>> Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
>> Suggested-by: Thomas Zimmermann <tzimmermann@...e.de>
>> Suggested-by: Bjorn Helgaas <bhelgaas@...gle.com>
>> Signed-off-by: Mario Limonciello (AMD) <superm1@...nel.org>
>> ---
>> v10:
>>   * Rebase on 6.17-rc1
>>   * Squash 'fbcon: Stop using screen_info_pci_dev()'
>> ---
>>   arch/x86/video/video-common.c | 25 ++++++++++++++++++++++++-
>>   1 file changed, 24 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/video/video-common.c b/arch/x86/video/video-common.c
>> index 81fc97a2a837a..e0aeee99bc99e 100644
>> --- a/arch/x86/video/video-common.c
>> +++ b/arch/x86/video/video-common.c
>> @@ -9,6 +9,7 @@
>>   
>>   #include <linux/module.h>
>>   #include <linux/pci.h>
>> +#include <linux/screen_info.h>
>>   #include <linux/vgaarb.h>
>>   
>>   #include <asm/video.h>
>> @@ -27,6 +28,11 @@ EXPORT_SYMBOL(pgprot_framebuffer);
>>   
>>   bool video_is_primary_device(struct device *dev)
>>   {
>> +#ifdef CONFIG_SCREEN_INFO
>> +	struct screen_info *si = &screen_info;
>> +	struct resource res[SCREEN_INFO_MAX_RESOURCES];
>> +	ssize_t i, numres;
>> +#endif
>>   	struct pci_dev *pdev;
>>   
>>   	if (!dev_is_pci(dev))
>> @@ -34,7 +40,24 @@ 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;
> 
> This can mark a VGA device as primary GPU.
> 
>> +
>> +#ifdef CONFIG_SCREEN_INFO
>> +	numres = screen_info_resources(si, res, ARRAY_SIZE(res));
>> +	for (i = 0; i < numres; ++i) {
>> +		if (!(res[i].flags & IORESOURCE_MEM))
>> +			continue;
>> +
>> +		if (pci_find_resource(pdev, &res[i]))
>> +			return true;
>> +	}
>> +#endif
> 
> And then the new code can also choose a primary GPU.
> 
>> +
>> +	return false;
>>   }
>>   EXPORT_SYMBOL(video_is_primary_device);
>>   
> 
> In particular, I have hardware that has this exact configuration where
> two GPUs are marked as primary and have a 'boot_display' attribute: the
> first one through vga_default_device(), the second one through the new
> detection method.
> 
> Is this intended?
> 
> Kind regards
> Aaron
> 

I wouldn't have expected a case like this and I think it means there is 
a logic error.

Can you please file a kernel bugzilla with details about your system and 
CC me?

dmesg and lspci -vvnn please

Also; please clarify which GPU shows something when booting up.