From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E9C2A469D; Tue, 22 Jul 2025 01:59:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753149580; cv=none; b=MsG2tHqZ4pEHvTCV8VizobZJ8NfLaw9Fu87xQtLik2ez3xt/4cD5Tb227ABUiT4EbNbef5bjsosHwTo98p5Cy6hbBvr0m8Ym9Iig4vktHAPWiJQMfliOO/K5gMUMrUcN4ATx2INj5GUfN3S5kl+d7OCALRn+RPs5GCRRcGKEUHM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753149580; c=relaxed/simple; bh=bm0m4nJCJwr818tAwn/pFGu42E+zoi1w1iM6fehYaY4=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition:In-Reply-To; b=DYaOQtddDmMUkfOYfdOesNhD9pSxqE4hC0CSZ6M+1f2PkKAWwifEidv7/txoQgeMYyek2QcSIg8O4IuiCaB3Bb/c02gIpkNMd7B/cZdsZLC+l046/liNIDQyUuWFAtAFdXs1Gje70YZqcSiGRbwEz2F5G2yK4vERx2nm63nA0ok= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EWC0dAlh; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="EWC0dAlh" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 31C6EC4CEF1; Tue, 22 Jul 2025 01:59:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1753149576; bh=bm0m4nJCJwr818tAwn/pFGu42E+zoi1w1iM6fehYaY4=; h=Date:From:To:Cc:Subject:In-Reply-To:From; b=EWC0dAlhj3RGOrRbeKwXNZEQj7dwLpmNlpJMJOGQFJE85GZ+moiEa1Lo5iMos3Qjr P9nKYIeDLQ4eZy0j7EiIWFsHjBTwjNESgh3OSUPqB7N7PW7quWVlVHqlONZtK0g8lF KJ1oidnQYnfWv7Q8vIVp65CV6DUkdL+QnxJfOZXNcw3OSkjTFxFXT1rr0R8rsDpY2K ucJCsndqhKZW8W29Y/taeY0dH/RYfm1yodHowS3LRR9UKf92hKqZPyPhytsK3OFjLD v1k5RcwtfMZ6LCbEpQGU4fldzhk5SDEE8kKB4AqQrAUPuZEs1TypBGhJ9m7qreBZG5 Xs6Eq5MSZ11og== Date: Mon, 21 Jul 2025 20:59:34 -0500 From: Bjorn Helgaas To: Mario Limonciello Cc: David Airlie , Bjorn Helgaas , Alex Deucher , Christian =?utf-8?B?S8O2bmln?= , Simona Vetter , Lukas Wunner , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Woodhouse , Lu Baolu , Joerg Roedel , Will Deacon , Robin Murphy , Alex Williamson , Jaroslav Kysela , Takashi Iwai , "open list:DRM DRIVERS" , open list , "open list:INTEL IOMMU (VT-d)" , "open list:PCI SUBSYSTEM" , "open list:VFIO DRIVER" , "open list:SOUND" , Daniel Dadap , Mario Limonciello Subject: Re: [PATCH v9 9/9] PCI: Add a new 'boot_display' attribute Message-ID: <20250722015934.GA2763711@bhelgaas> Precedence: bulk X-Mailing-List: linux-pci@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <860bcc59-f0d0-4c8d-865c-89127c213cdf@kernel.org> On Mon, Jul 21, 2025 at 07:28:07PM -0500, Mario Limonciello wrote: > On 7/21/25 6:00 PM, Bjorn Helgaas wrote: > > On Fri, Jul 18, 2025 at 12:44:11PM -0500, Mario Limonciello wrote: > > > On 7/18/2025 12:36 PM, Bjorn Helgaas wrote: > > > > On Fri, Jul 18, 2025 at 12:29:05PM -0500, Mario Limonciello wrote: > > > > > On 7/18/2025 12:25 PM, Bjorn Helgaas wrote: > > > > > > On Thu, Jul 17, 2025 at 12:38:12PM -0500, Mario Limonciello wrote: > > > > > > > From: Mario Limonciello > > > > > > > > > > > > > > On systems with multiple GPUs there can be uncertainty which GPU is the > > > > > > > primary one used to drive the display at bootup. In some desktop > > > > > > > environments this can lead to increased power consumption because > > > > > > > secondary GPUs may be used for rendering and never go to a low power > > > > > > > state. 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. > > > > > > > > > > > > > +What: /sys/bus/pci/devices/.../boot_display > > > > > > > +Date: October 2025 > > > > > > > +Contact: Linux PCI developers > > > > > > > +Description: > > > > > > > + This file indicates that displays connected to the device were > > > > > > > + used to display the boot sequence. If a display connected to > > > > > > > + the device was used to display the boot sequence the file will > > > > > > > + be present and contain "1". > > > > > > > > > > > > > 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); > > > > > > > > > > > > In addition to Mani's question about whether /sys/bus/pci/ is > > > > > > the right place for this (which is a very good question), it's > > > > > > also been pointed out to me that we've been trying to get rid > > > > > > of pci_create_sysfs_dev_files() for years. > > > > > > > > > > > > If it's possible to make this a static attribute that would be > > > > > > much, much cleaner. > > > > > > > > > > Right - I tried to do this, but the problem is at the time the > > > > > PCI device is created the information needed to make the > > > > > judgement isn't ready. The options end up being: > > > > > * a sysfs file for every display device with 0/1 > > > > > * a sysfs file that is not accurate until later in the boot > > > > > > > > What's missing? The specifics might be helpful if someone has > > > > another crack at getting rid of pci_create_sysfs_dev_files() in > > > > the future. > > > > > > The underlying SCREEN_INFO code tries to walk through all the PCI > > > devices in a loop, but at the time all the devices are walked the > > > memory regions associated with the device weren't populated. > > > > Which loop are you referring to that walks through all the PCI > > devices? I see this: > > > > efifb_set_system > > for_each_pci_dev(dev) > > > > but that only looks at VGA devices and IIUC you also want to look at > > non-VGA GPUs. [I assume the loop is the "while (pdev = pci_get_base_class(PCI_BASE_CLASS_DISPLAY))" in __screen_info_pci_dev(), which indeed walks through all known PCI devices] > > I don't see a loop in *this* series, where the screen_info path looks > > like this: > > > > pci_create_boot_display_file > > video_is_primary_device > > screen_info_pci_dev # added by "fbcon: Use screen info to find primary device" > > screen_info_resources > > __screen_info_pci_dev > > > > and we're basically matching the screen_info base/address with BAR > > values. > > > > The usual problem is that BARs may not have been assigned by the > > time pci_device_add() -> device_add() creates the static > > attributes. > > > > So we call pci_assign_unassigned_root_bus_resources() to assign > > all the BARs. Then we call pci_create_sysfs_dev_files(), where > > pci_create_resource_files() creates a "resource%d" file for each > > BAR. > > > > But since we're trying to find the GPU that was used by BIOS, I > > assume its BARs were programmed by BIOS and we shouldn't have to > > wait until after pci_assign_unassigned_root_bus_resources(). > > Yes it was screen_info_pci_dev() and __screen_info_pci_dev(). The > resources weren't ready on the first call into > __screen_info_pci_dev(). > > That's why the attribute needed to be created later. I don't understand this. IIUC, screen_info contains addresses programmed by BIOS. If we want to use that to match with a PCI device, we have to compare with the BAR contents *before* Linux does any assignments of its own. So the only thing this should depend on is the BAR value at BIOS -> Linux handoff, which we know at the time of device_add(), and we should be able to do something like this: bool pci_video_is_primary_device(struct pci_dev *pdev) { struct screen_info *si = &screen_info; struct resource res[SCREEN_INFO_MAX_RESOURCES]; ssize_t i, numres; numres = screen_info_resources(si, res, ARRAY_SIZE(res)); ... for (i = 0; i < numres; ++i) { if (pci_find_resource(pdev, &res[i])) return true; } return false; } static umode_t pci_dev_boot_display_is_visible(...) { struct pci_dev *pdev = to_pci_dev(kobj_to_dev(kobj)); if (pci_video_is_primary_device(pdev)) return a->mode; return 0; } We should be able to check each BAR of each device in this path, with no loop through the devices at all: pci_device_add device_add device_add_attrs device_add_groups ... create_files grp->is_visible() pci_dev_boot_display_is_visible Bjorn