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

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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: stable@vger.kernel.org
Link: https://patch.msgid.link/20260729075909.1219906-1-kwilczynski@kernel.org
This commit is contained in:
Krzysztof Wilczyński 2026-07-29 07:59:09 +00:00 committed by Bjorn Helgaas
parent dc59e4fea9
commit 4ff664a81d
1 changed files with 6 additions and 0 deletions
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6
drivers/pci/proc.c
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@ -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)