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Message-ID: <732aeb75-71e7-49e7-a5f2-2080ee94a273@suse.de>
Date: Mon, 30 Jun 2025 08:24:22 +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 v6 9/9] PCI: Add a new 'boot_display' attribute
Hi
Am 27.06.25 um 17:37 schrieb Mario Limonciello:
> On 6/27/2025 2:07 AM, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 27.06.25 um 06:31 schrieb Mario Limonciello:
>>> From: Mario Limonciello <mario.limonciello@....com>
>>>
>>> On systems with multiple GPUs there can be uncertainty which GPU is the
>>> primary one used to drive the display at bootup. In order to
>>> disambiguate
>>> this add a new sysfs attribute 'boot_display' that uses the output of
>>> video_is_primary_device() to populate whether a PCI device was used for
>>> driving the display.
>>>
>>> Signed-off-by: Mario Limonciello <mario.limonciello@....com>
>>> ---
>>> v6:
>>> * Only show for the device that is boot display
>>> * Only create after PCI device sysfs files are initialized to ensure
>>> that resources are ready.
>>> v4:
>>> * new patch
>>> ---
>>> Documentation/ABI/testing/sysfs-bus-pci | 8 +++++
>>> drivers/pci/pci-sysfs.c | 46
>>> +++++++++++++++++++++++++
>>
>> The code looks good. Just one more question: could this be added
>> independently from the PCI bus (at a reasonable cost)? There are
>> other busses that can host the boot display. Alternatively, we'd add
>> this attribute per bus as needed.
>
> It depends upon the underlying hardware implementation. On x86 it's
> always PCI and so I realized there is a requirement that PCI resources
> are setup before screen_info event works.
>
> That is the v5 version of this patch would have had a potential race
> condition with userspace where boot_display didn't always show '1' if
> userspace read it too quickly.
>
> Other architecture's hardware implementation might have similar problem.
>
> So in summary I think it would be better to do it per-bus. If we
> realize there is indeed code duplication we can always move this to a
> common helper at that point.
Ok, makes sense. With the kernel test robot's issues fixed:
Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
I guess that interface also needs some sort of OK from user-space devs?
Best regards
Thomas
>
>>
>> Best regards
>> Thomas
>>
>>> 2 files changed, 54 insertions(+)
>>>
>>> diff --git a/Documentation/ABI/testing/sysfs-bus-pci
>>> b/Documentation/ ABI/testing/sysfs-bus-pci
>>> index 69f952fffec72..8b455b1a58852 100644
>>> --- a/Documentation/ABI/testing/sysfs-bus-pci
>>> +++ b/Documentation/ABI/testing/sysfs-bus-pci
>>> @@ -612,3 +612,11 @@ Description:
>>> # ls doe_features
>>> 0001:01 0001:02 doe_discovery
>>> +
>>> +What: /sys/bus/pci/devices/.../boot_display
>>> +Date: October 2025
>>> +Contact: Linux PCI developers <linux-pci@...r.kernel.org>
>>> +Description:
>>> + This file indicates the device was used as a boot
>>> + display. If the device was used as the boot display, the file
>>> + will be present and contain "1".
>>> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
>>> index 268c69daa4d57..cc766461de1da 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_display_show(struct device *dev, struct
>>> device_attribute *attr,
>>> + char *buf)
>>> +{
>>> + return sysfs_emit(buf, "1\n");
>>> +}
>>> +static DEVICE_ATTR_RO(boot_display);
>>> +
>>> static ssize_t boot_vga_show(struct device *dev, struct
>>> device_attribute *attr,
>>> char *buf)
>>> {
>>> @@ -1246,6 +1254,37 @@ static int pci_create_attr(struct pci_dev
>>> *pdev, int num, int write_combine)
>>> return 0;
>>> }
>>> +/**
>>> + * pci_create_boot_display_file - create a file in sysfs for @dev
>>> + * @pdev: dev in question
>>> + *
>>> + * Creates a file `boot_display` in sysfs for the PCI device @pdev
>>> + * if it is the boot display device.
>>> + */
>>> +static int pci_create_boot_display_file(struct pci_dev *pdev)
>>> +{
>>> +#ifdef CONFIG_VIDEO
>>> + if (video_is_primary_device(&pdev->dev))
>>> + return sysfs_create_file(&pdev->dev.kobj,
>>> &dev_attr_boot_display.attr);
>>> +#endif
>>> + return 0;
>>> +}
>>> +
>>> +/**
>>> + * pci_remove_boot_display_file - remove the boot display file for
>>> @dev
>>> + * @pdev: dev in question
>>> + *
>>> + * Removes the file `boot_display` in sysfs for the PCI device @pdev
>>> + * if it is the boot display device.
>>> + */
>>> +static void pci_remove_boot_display_file(struct pci_dev *pdev)
>>> +{
>>> +#ifdef CONFIG_VIDEO
>>> + if (video_is_primary_device(&pdev->dev))
>>> + sysfs_remove_file(&pdev->dev.kobj,
>>> &dev_attr_boot_display.attr);
>>> +#endif
>>> +}
>>> +
>>> /**
>>> * pci_create_resource_files - create resource files in sysfs for
>>> @dev
>>> * @pdev: dev in question
>>> @@ -1654,9 +1693,15 @@ static const struct attribute_group
>>> pci_dev_resource_resize_group = {
>>> int __must_check pci_create_sysfs_dev_files(struct pci_dev *pdev)
>>> {
>>> + int retval;
>>> +
>>> if (!sysfs_initialized)
>>> return -EACCES;
>>> + retval = pci_create_boot_display_file(pdev);
>>> + if (retval)
>>> + return retval;
>>> +
>>> return pci_create_resource_files(pdev);
>>> }
>>> @@ -1671,6 +1716,7 @@ void pci_remove_sysfs_dev_files(struct pci_dev
>>> *pdev)
>>> if (!sysfs_initialized)
>>> return;
>>> + pci_remove_boot_display_file(pdev);
>>> pci_remove_resource_files(pdev);
>>> }
>>
>
--
--
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)
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