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

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Message-ID: <b3462e88-e24a-43d9-8437-b6d378a3b5d3@kernel.org>
Date: Fri, 20 Jun 2025 10:56:53 -0500
From: Mario Limonciello <superm1@...nel.org>
To: Thomas Zimmermann <tzimmermann@...e.de>,
 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

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.

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

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)
>