5.15-stable review patch. If anyone has any objections, please let me know.
------------------
From: Krzysztof Wilczyński <kwilczynski@kernel.org>
commit 4ff664a81d729b37f2eb65de80a670abfb61c9a0 upstream.
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 <kwilczynski@kernel.org>
[bhelgaas: order tags]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: <redacted>
Link: https://patch.msgid.link/20260729075909.1219906-1-kwilczynski@kernel.org (local)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/pci/proc.c | 6 ++++++
1 file changed, 6 insertions(+)
--- a/drivers/pci/proc.c
+++ b/drivers/pci/proc.c
@@ -45,6 +45,9 @@ static ssize_t proc_bus_pci_read(struct
else
size = 64;
+ if (!nbytes)
+ return 0;
+
if (pos >= size)
return 0;
if (nbytes >= size)
@@ -123,6 +126,9 @@ static ssize_t proc_bus_pci_write(struct
if (ret)
return ret;
+ if (!nbytes)
+ return 0;
+
if (pos >= size)
return 0;
if (nbytes >= size)