[PATCH] clk: qcom: ipq-cmn-pll: keep the CMN block bus clocks enabled

[Stanislaw Pal <kuncy7@gmail.com>]

The probe function takes a runtime PM reference to enable the GCC AHB &
SYS clocks of the CMN PLL block, registers the clocks, and then drops
the reference. Once the autosuspend kicks in, pm_clk gates both clocks
a few milliseconds after probe has returned.

That is wrong on two counts:

  - The clock ops access the CMN PLL registers without taking a runtime
    PM reference of their own, so any later clk_set_rate() or rate
    recalculation touches the register file with its bus clock gated.

  - On IPQ5018 the consequences are not contained to this device:
    gating the CMN block bus clocks makes the SoC hang on a subsequent
    bus access. Measured on a TP-Link Archer AX55 v1 (IPQ5018), the
    machine dies silently within a few milliseconds of the CMN PLL
    probe returning - mid-character on the UART - and the victim is
    whichever device happens to probe next. Whether a given boot
    survives the window is a micro-timing lottery: different binary
    layouts of the same kernel ranged from occasional failures to a
    100% reproducible boot loop.

Keep the runtime PM usage count elevated on the probe success path, so
the bus clocks stay enabled. With this one change the same board went
from boot-looping to surviving every boot attempt (verified across
three previously-failing kernel binaries, plus repeated soft and hard
resets on the final one).

Fixes: f81715a4c87c ("clk: qcom: Add CMN PLL clock controller driver for IPQ SoC")
Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Pal <kuncy7@gmail.com>
---
One more observation from the same debugging session, for the people
who know this silicon: on IPQ5018 the CMN_PLL_LOCKED bit (offset 0x64,
bit 8) never asserts. Every clk_cmn_pll_set_rate() call runs its
regmap_read_poll_timeout() to the full 100 ms timeout - including under
the bootloader-programmed configuration the hardware demonstrably runs
fine with - and nobody notices, because clk_change_rate() ignores the
.set_rate return value, so the -ETIMEDOUT is swallowed silently. Is the
lock status readable at a different offset on this SoC, or is the bit
simply not functional there? Happy to send a follow-up (surfacing the
error, or skipping the poll where it cannot work) once someone can say
what the intended behavior is.

 drivers/clk/qcom/ipq-cmn-pll.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

--- a/drivers/clk/qcom/ipq-cmn-pll.c
+++ b/drivers/clk/qcom/ipq-cmn-pll.c
@@ -454,11 +454,19 @@
 
 	/* Register CMN PLL clock and fixed rate output clocks. */
 	ret = ipq_cmn_pll_register_clks(pdev);
-	pm_runtime_put(dev);
-	if (ret)
+	if (ret) {
+		pm_runtime_put(dev);
 		return dev_err_probe(dev, ret,
 				     "Failed to register CMN PLL clocks\n");
+	}
 
+	/*
+	 * Keep the runtime PM usage count elevated on the success path, so
+	 * the CMN block AHB & SYS clocks stay enabled: the clock ops access
+	 * the CMN PLL registers without taking a runtime PM reference, and
+	 * on IPQ5018 gating these clocks after probe hangs the SoC on a
+	 * subsequent bus access.
+	 */
 	return 0;
 }