[PATCH] PCI/proc: Avoid spurious runtime PM wakeup on config space accesses

["Krzysztof Wilczyński" <kwilczynski@kernel.org>]

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 <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)
-- 
2.55.0