Re: [PATCH] PCI/PM: Avoid redundant delays on D3hot->D3cold

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Oct 03, 2025 at 03:40:09PM -0700, Brian Norris wrote:
> From: Brian Norris <briannorris@google.com>
> 
> When transitioning to D3cold, __pci_set_power_state() will first
> transition a device to D3hot. If the device was already in D3hot, this
> will add excess work:
> (a) read/modify/write PMCSR; and
> (b) excess delay (pci_dev_d3_sleep()).
> 
> For (b), we already performed the necessary delay on the previous D3hot
> entry; this was extra noticeable when evaluating runtime PM transition
> latency.
> 
> Check whether we're already in the target state before continuing.
> 
> Note that __pci_set_power_state() already does this same check for other
> state transitions, but D3cold is special because __pci_set_power_state()
> converts it to D3hot for the purposes of PMCSR.
> 
> This seems to be an oversight in commit 0aacdc957401 ("PCI/PM: Clean up
> pci_set_low_power_state()").
> 
> Fixes: 0aacdc957401 ("PCI/PM: Clean up pci_set_low_power_state()")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Brian Norris <briannorris@google.com>
> Signed-off-by: Brian Norris <briannorris@chromium.org>

Applied to pci/pm for v6.20, thanks!

I reversed the test to match similar tests in
pci_set_low_power_state(), __pci_set_power_state(), etc.

I dropped the stable tag because this is a performance improvement
that I can't quantify and doesn't seem to fit in the categories here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/stable-kernel-rules.rst?id=v6.18#n15

I'm not sure anybody pays attention to that list; lots of things I
don't expect get backported, so likely this will get backported as
well.  But as long as we have the documented list, I try to follow it.

Would be good to have Rafael's ack, but I don't think we need to wait
for it.

> ---
> 
>  drivers/pci/pci.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b0f4d98036cd..7517f1380201 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1539,6 +1539,9 @@ static int pci_set_low_power_state(struct pci_dev *dev, pci_power_t state, bool
>  	   || (state == PCI_D2 && !dev->d2_support))
>  		return -EIO;
>  
> +	if (state == dev->current_state)
> +		return 0;
> +
>  	pci_read_config_word(dev, dev->pm_cap + PCI_PM_CTRL, &pmcsr);
>  	if (PCI_POSSIBLE_ERROR(pmcsr)) {
>  		pci_err(dev, "Unable to change power state from %s to %s, device inaccessible\n",
> -- 
> 2.51.0.618.g983fd99d29-goog
>