From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 4D9FA428820 for ; Wed, 29 Jul 2026 07:59:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785311952; cv=none; b=ZdeFD1QcaJ9VndHtX3rydPfcfsN/ROTjbDtVzEfamYd8gUffIBFME52BprfuqRNm8RTstzohfrGiV9z5mLI/K6gebNsTSQHtjcjD/WyW214GWK8DgLvzW6wl6STb5quBEs2tH9JXpxRCFjbkg7JVlBGVAT8O4T9aASZDa+BbcGE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785311952; c=relaxed/simple; bh=WIh5o7xISrSFH33w+NqHaNQ3V48+Gj01Af3ZGhrnNf0=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version:Content-Type; b=mdYrseqI2pwwJM8Y9cmK+glF7v9xlYcMQU8HzWhik7LeubaUIr/Gv7aFWCNZ8hBuVT7xPkiofsj5TwdzvxBmE9i6o11nGl6Xw7SpHzQBo8SGMxWBdoO4QVpG2euL/kRrlRSbEkR0JuZOVbH6/ngB4U4quKg82wOu7cJks9tKmcE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TV4ogEz7; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="TV4ogEz7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4D2101F00A3A; Wed, 29 Jul 2026 07:59:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785311951; bh=FQmbpYQiwsfl9H+AZx0J03pUC7H7nO/Yl065kplxRs4=; h=From:To:Cc:Subject:Date; b=TV4ogEz7YcYs6DdErNLoEb9US4Dj12LhEWn2DkEgNDDsMSla178gc6dIQXeytXOck G8pBDJ8g2fbG9qH9bbRTsQKb4Kx/nwvYUKjO7Yczg54h75K43iAldWwbbEyB5IgfXm fi+AsAsEB5ltlH9AcR+S304uhIJXDIx46SOHgOG0AzZaTqBKv83rFO12GT70K+mwjR +EZGXmb71cSBvPQLBVG+EW0yjW5dCdMv31GeMP9FEt1rWzb0xlA2k/nCGDUISFX4Kq Qi7ZDBboRYl8QYV8xG88mJLyKr6n8qLrrQ5zIPg/IE8Zjp+07FGPSWpm4rcOtC49i6 fhW1n94Q02kGw== From: =?UTF-8?q?Krzysztof=20Wilczy=C5=84ski?= To: Bjorn Helgaas Cc: Bjorn Helgaas , Manivannan Sadhasivam , Lorenzo Pieralisi , linux-pci@vger.kernel.org Subject: [PATCH] PCI/proc: Avoid spurious runtime PM wakeup on config space accesses Date: Wed, 29 Jul 2026 07:59:09 +0000 Message-ID: <20260729075909.1219906-1-kwilczynski@kernel.org> X-Mailer: git-send-email 2.55.0 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=UTF-8 Content-Transfer-Encoding: 8bit Currently, proc_bus_pci_read() and proc_bus_pci_write() do not return early for zero-length configuration space accesses at valid offsets. Thus, such an access invokes pci_config_pm_runtime_get() and pci_config_pm_runtime_put() around transfer blocks that do nothing. This is a problem because pci_config_pm_runtime_get() synchronously resumes the upstream bridge through pm_runtime_get_sync(), and resumes the device itself through pm_runtime_resume() when it is in D3cold, only for the handler to return zero immediately afterwards. Such a spurious wakeup wastes power and adds needless resume latency. The sysfs core already returns early for in-range zero-length binary attribute accesses before pci_read_config() or pci_write_config() is invoked. In contrast, the VFS forwards zero-length requests to the procfs callbacks, where they continue into runtime PM handling. Therefore, return early from proc_bus_pci_read() and proc_bus_pci_write() when nbytes is zero, before any runtime PM involvement. The value returned to userspace at these offsets remains zero, so the change is not visible to userspace. Signed-off-by: Krzysztof Wilczyński --- drivers/pci/proc.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/drivers/pci/proc.c b/drivers/pci/proc.c index 71ad289fcb8e..fcbd75d53ac5 100644 --- a/drivers/pci/proc.c +++ b/drivers/pci/proc.c @@ -46,6 +46,9 @@ static ssize_t proc_bus_pci_read(struct file *file, char __user *buf, else size = 64; + if (!nbytes) + return 0; + if (pos >= size) return 0; if (nbytes >= size) @@ -122,6 +125,9 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf, if (ret) return ret; + if (!nbytes) + return 0; + if (pos >= size) return 0; if (nbytes >= size) -- 2.55.0