linux-kernel - Re: [PATCH v1] PCI/PM: Switch D3Hot delay to use usleep

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Date:   Tue, 16 Aug 2022 14:06:06 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Will McVicker <willmcvicker@...gle.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, kernel-team@...roid.com,
        Sajid Dalvi <sdalvi@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] PCI/PM: Switch D3Hot delay to use usleep_range

On Thu, Aug 11, 2022 at 06:40:01PM +0000, Will McVicker wrote:
> From: Sajid Dalvi <sdalvi@...gle.com>
> 
> Since the PCI spec requires a 10ms D3Hot delay (defined by
> PCI_PM_D3HOT_WAIT) and a few of the PCI quirks update the D3Hot delay up
> to 20ms, let's switch from msleep to usleep_range to improve the delay
> accuracy.
> 
> This patch came from Sajid Dalvi <sdalvi@...gle.com> in the Pixel 6
> kernel tree [1]. Testing on a Pixel 6, found that the 10ms delay for
> the Exynos PCIe device was on average delaying for 19ms when the spec
> requires 10ms. Switching from msleep to uslseep_delay therefore
> decreases the resume time on a Pixel 6 on average by 9ms.
> 
> [1] https://android.googlesource.com/kernel/gs/+/18a8cad68d8e6d50f339a716a18295e6d987cee3
> 
> Signed-off-by: Sajid Dalvi <sdalvi@...gle.com>
> Signed-off-by: Will McVicker <willmcvicker@...gle.com>
> ---
>  drivers/pci/pci.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 95bc329e74c0..5ae5b3c4dc9b 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -72,7 +72,8 @@ static void pci_dev_d3_sleep(struct pci_dev *dev)
>  		delay = pci_pm_d3hot_delay;
>  
>  	if (delay)
> -		msleep(delay);
> +		usleep_range(delay * USEC_PER_MSEC,
> +			     (delay + 2) * USEC_PER_MSEC);

You could also use fsleep(), which ends up calling usleep_range()
for (usecs > 10 && usecs <= 20000).