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

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Message-ID: <817215d6-0a37-41a3-89a0-b7d2f7c67f1f@suse.de>
Date: Tue, 28 Oct 2025 17:50:43 +0100
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Mario Limonciello <superm1@...nel.org>,
 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>,
 "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

Hi

Am 28.10.25 um 14:15 schrieb Mario Limonciello:
> 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.

Is the value returned from vga_default_device() eq to NULL?

>>
>>> +
>>> +#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.

Maybe we should drop this block or move it to [1]? At [1] it would only 
run if the more sophisticated vgaarb has been disabled.

The vgaarb now selects CONFIG_SCREEN_INFO and already tests for 
overlapping resources. There's nothing here that vgaarb shouldn't 
already do. Yet, I don't understand how only one of the can be true at a 
time.

[1] 
https://elixir.bootlin.com/linux/v6.18-rc3/source/include/linux/vgaarb.h#L55

>>
>>> +
>>> +    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.

Agreed.

Best regards
Thomas


-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)