From 4ff664a81d729b37f2eb65de80a670abfb61c9a0 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Krzysztof=20Wilczy=C5=84ski?= Date: Wed, 29 Jul 2026 07:59:09 +0000 Subject: [PATCH] PCI/proc: Avoid spurious runtime PM wakeup on config space accesses 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. 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. 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 [bhelgaas: order tags] Signed-off-by: Bjorn Helgaas Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260729075909.1219906-1-kwilczynski@kernel.org --- 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)