linux-kernel - Re: [PATCH v4] PCI/PM: Switch D3hot delay to use usleep

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Message-ID: <20220929193441.GA1918838@bhelgaas>
Date:   Thu, 29 Sep 2022 14:34:41 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Will McVicker <willmcvicker@...gle.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, kernel-team@...roid.com,
        Sajid Dalvi <sdalvi@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] PCI/PM: Switch D3hot delay to use usleep_range()

On Wed, Sep 21, 2022 at 09:27:35PM +0000, Will McVicker wrote:
> From: Sajid Dalvi <sdalvi@...gle.com>
> 
> The PCIe r6.0, sec 5.9 spec requires a 10ms D3hot delay (defined by
> PCI_PM_D3HOT_WAIT) for transitions to or from D3hot. So let's switch to
> usleep_range() for the delay time to improve the delay accuracy.
> 
> This patch is based off of a commit 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 usleep_range()
> therefore decreases the resume time on a Pixel 6 on average by 9ms.
> 
> Note: some ancient Intel chips do have a quirk that sets the delay to
> 120ms. Using usleep_delay() may add a few extra milliseconds for those
> chips.
> 
> [1] https://android.googlesource.com/kernel/gs/+/18a8cad68d8e6d50f339a716a18295e6d987cee3
> 
> Signed-off-by: Sajid Dalvi <sdalvi@...gle.com>
> Signed-off-by: Will McVicker <willmcvicker@...gle.com>
> Reviewed-by: Matthias Kaehlcke <mka@...omium.org>

Applied to pci/pm for v6.1, thanks, Will!

> ---
>  drivers/pci/pci.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> v4:
>  * Drop use of msleep() in favor of always using usleep_range().
>  * Update the commit message.
> 
> v3:
>  * Use DIV_ROUND_CLOSEST instead of bit manipulation.
>  * Minor refactor to use max() where relavant.
> 
> v2:
>  * Update to use 20-25% upper bound
>  * Update to use usleep_range() for <=20ms, else use msleep()
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 516bf0c2ca02..2127aba3550b 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -66,13 +66,15 @@ struct pci_pme_device {
>  
>  static void pci_dev_d3_sleep(struct pci_dev *dev)
>  {
> -	unsigned int delay = dev->d3hot_delay;
> -
> -	if (delay < pci_pm_d3hot_delay)
> -		delay = pci_pm_d3hot_delay;
> -
> -	if (delay)
> -		msleep(delay);
> +	unsigned int delay_ms = max(dev->d3hot_delay, pci_pm_d3hot_delay);
> +	unsigned int upper;
> +
> +	if (delay_ms) {
> +		/* Use a 20% upper bound, 1ms minimum */
> +		upper = max(DIV_ROUND_CLOSEST(delay_ms, 5), 1U);
> +		usleep_range(delay_ms * USEC_PER_MSEC,
> +			     (delay_ms + upper) * USEC_PER_MSEC);
> +	}
>  }
>  
>  bool pci_reset_supported(struct pci_dev *dev)
> 
> base-commit: fcf773ae8016c6bffe5d408d3eda50d981b946e6
> -- 
> 2.37.3.968.ga6b4b080e4-goog
>