[PATCH v4] PCI/PM: Switch D3hot delay to use usleep_range()

[Will McVicker <willmcvicker@google.com>]

From: Sajid Dalvi <sdalvi@google.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@google.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@google.com>
Signed-off-by: Will McVicker <willmcvicker@google.com>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
---
 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