phy: qcom-snps-femto-v2: keep cfg_ahb_clk enabled during runtime suspend

In the dwc3 core, both system and runtime suspend end up calling
dwc3_suspend_common(). From there, what happens for the PHYs depends on
the USB mode and whether the controller is entering system or runtime
suspend.

HOST mode:
  (1) system suspend on a non-wakeup-capable controller

  The [1] if branch is taken. dwc3_core_exit() is called, which ends up
  calling phy_power_off() and phy_exit(). Those two functions decrease the
  PM runtime count at some point, so they will trigger the PHY runtime
  sleep (assuming the count is right).

  (2) runtime suspend / system suspend on a wakeup-capable controller

  The [1] branch is not taken. dwc3_suspend_common() calls
  phy_pm_runtime_put_sync(). Assuming the ref count is right, the PHY
  runtime suspend op is called.

DEVICE mode:
  dwc3_core_exit() is called on both runtime and system sleep
  unless the controller is already runtime suspended.

OTG mode:
  (1) system suspend : dwc3_core_exit() is called

  (2) runtime suspend : do nothing

In host mode, the code seems to make a distinction between 1) runtime
sleep / system sleep for wakeup-capable controller, and 2) system sleep
for non-wakeup-capable controller, where phy_power_off() and phy_exit()
are only called for the latter. This suggests the PHY is not supposed to
be in a fully powered-off state for runtime sleep and system sleep for
wakeup-capable controller.

Moreover, downstream, cfg_ahb_clk only gets disabled for system suspend.
The clocks are disabled by phy->set_suspend() [2] which is only called
in the system sleep path through dwc3_core_exit() [3].

With that in mind, don't disable the clocks during the femto PHY runtime
suspend callback. The clocks will only be disabled during system suspend
for non-wakeup-capable controllers, through dwc3_core_exit().

[1] https://elixir.bootlin.com/linux/v6.4/source/drivers/usb/dwc3/core.c#L1988
[2] https://git.codelinaro.org/clo/la/kernel/msm-5.4/-/blob/LV.AU.1.2.1.r2-05300-gen3meta.0/drivers/usb/phy/phy-msm-snps-hs.c#L524
[3] https://git.codelinaro.org/clo/la/kernel/msm-5.4/-/blob/LV.AU.1.2.1.r2-05300-gen3meta.0/drivers/usb/dwc3/core.c#L1915

Signed-off-by: Adrien Thierry <athierry@redhat.com>
Link: https://lore.kernel.org/r/20230629144542.14906-2-athierry@redhat.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/phy/qualcomm/phy-qcom-snps-femto-v2.c

@@ -165,22 +165,13 @@ static int qcom_snps_hsphy_suspend(struct qcom_snps_hsphy *hsphy)
 					   0, USB2_AUTO_RESUME);
 	}
 
-	clk_disable_unprepare(hsphy->cfg_ahb_clk);
 	return 0;
 }
 
 static int qcom_snps_hsphy_resume(struct qcom_snps_hsphy *hsphy)
 {
-	int ret;
-
 	dev_dbg(&hsphy->phy->dev, "Resume QCOM SNPS PHY, mode\n");
 
-	ret = clk_prepare_enable(hsphy->cfg_ahb_clk);
-	if (ret) {
-		dev_err(&hsphy->phy->dev, "failed to enable cfg ahb clock\n");
-		return ret;
-	}
-
 	return 0;
 }