linux-kernel - Re: [PATCH v3 7/7] fbcon: Make a symlink to the device selected as primary

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Message-ID: <f9bd6fb1-ca21-4a23-9548-8d9b42f3c9b6@suse.de>
Date: Mon, 23 Jun 2025 12:32:33 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Mario Limonciello <superm1@...nel.org>,
 Bjorn Helgaas <bhelgaas@...gle.com>
Cc: Alex Deucher <alexander.deucher@....com>,
 Christian König <christian.koenig@....com>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Lukas Wunner <lukas@...ner.de>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, 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 v3 7/7] fbcon: Make a symlink to the device selected as
 primary

Hi

Am 20.06.25 um 17:56 schrieb Mario Limonciello:
> On 6/20/25 3:47 AM, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 20.06.25 um 04:49 schrieb Mario Limonciello:
>>> From: Mario Limonciello <mario.limonciello@....com>
>>>
>>> Knowing which device is the primary device can be useful for userspace
>>> to make decisions on which device to start a display server.
>>>
>>> Create a link to that device called 'primary_device'.
>>>
>>> Signed-off-by: Mario Limonciello <mario.limonciello@....com>
>>> ---
>>>   drivers/video/fbdev/core/fbcon.c | 10 +++++++++-
>>>   1 file changed, 9 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/ 
>>> core/fbcon.c
>>> index 2df48037688d1..46f21570723e5 100644
>>> --- a/drivers/video/fbdev/core/fbcon.c
>>> +++ b/drivers/video/fbdev/core/fbcon.c
>>
>> You cannot rely on this, as fbcon might be disabled entirely.
>
> So the other idea I had was to have a new file boot_console.

'console' already has a meaning, so I'd prefer boot_display. Apart from 
naming, this is a good idea.

>
> How would you feel about this instead (or even in addition to the 
> symlink)?

We likely won't need the symlink then.

Best regards
Thomas

>
> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index 268c69daa4d5..8535950b4c0f 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -30,6 +30,7 @@
>  #include <linux/msi.h>
>  #include <linux/of.h>
>  #include <linux/aperture.h>
> +#include <asm/video.h>
>  #include "pci.h"
>
>  #ifndef ARCH_PCI_DEV_GROUPS
> @@ -679,6 +680,13 @@ const struct attribute_group *pcibus_groups[] = {
>         NULL,
>  };
>
> +static ssize_t boot_console_show(struct device *dev, struct 
> device_attribute *attr,
> +                                char *buf)
> +{
> +       return sysfs_emit(buf, "%u\n", video_is_primary_device(dev));
> +}
> +static DEVICE_ATTR_RO(boot_console);
> +
>  static ssize_t boot_vga_show(struct device *dev, struct 
> device_attribute *attr,
>                              char *buf)
>  {
> @@ -1698,6 +1706,7 @@ late_initcall(pci_sysfs_init);
>
>  static struct attribute *pci_dev_dev_attrs[] = {
>         &dev_attr_boot_vga.attr,
> +       &dev_attr_boot_console.attr,
>         NULL,
>  };
>
> @@ -1710,6 +1719,9 @@ static umode_t pci_dev_attrs_are_visible(struct 
> kobject *kobj,
>         if (a == &dev_attr_boot_vga.attr && pci_is_vga(pdev))
>                 return a->mode;
>
> +       if (a == &dev_attr_boot_console.attr && pci_is_display(pdev))
> +               return a->mode;
> +
>         return 0;
>  }
>
>
>>
>> Best regards
>> Thomas
>>
>>> @@ -2934,7 +2934,7 @@ static void fbcon_select_primary(struct 
>>> fb_info *info)
>>>   {
>>>       if (!map_override && primary_device == -1 &&
>>>           video_is_primary_device(info->device)) {
>>> -        int i;
>>> +        int i, r;
>>>           printk(KERN_INFO "fbcon: %s (fb%i) is primary device\n",
>>>                  info->fix.id, info->node);
>>> @@ -2949,6 +2949,10 @@ static void fbcon_select_primary(struct 
>>> fb_info *info)
>>>                      first_fb_vc + 1, last_fb_vc + 1);
>>>               info_idx = primary_device;
>>>           }
>>> +        r = sysfs_create_link(&fbcon_device->kobj, 
>>> &info->device->kobj,
>>> +                      "primary_device");
>>> +        if (r)
>>> +            pr_err("fbcon: Failed to link to primary device: %d\n", 
>>> r);
>>>       }
>>>   }
>>> @@ -3376,6 +3380,10 @@ void __init fb_console_init(void)
>>>   void __exit fb_console_exit(void)
>>>   {
>>> +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DETECT_PRIMARY
>>> +    if (primary_device != -1)
>>> +        sysfs_remove_link(&fbcon_device->kobj, "primary_device");
>>> +#endif
>>>   #ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
>>>       console_lock();
>>>       if (deferred_takeover)
>>
>

-- 
--
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)