[PATCH v3 4/7] pwm: rockchip: Eliminate potential race condition when probing

Simon South simon at simonsouth.net
Wed Dec 23 11:01:06 EST 2020

Commit 48cf973cae33 ("pwm: rockchip: Avoid glitches on already running
PWMs") introduced a potential race condition in rockchip_pwm_probe(): A
consumer could enable an inactive PWM, or disable a running one, between
rockchip_pwm_probe() registering the device via pwmchip_add() and checking
whether it is enabled (to determine whether it was started by a
bootloader). This could result in a device's PWM clock being either enabled
once more than necessary, potentially causing it to continue running when
no longer needed, or disabled once more than necessary, producing a warning
from the kernel.

Eliminate these possibilities by modifying rockchip_pwm_probe() so it
checks whether a device is enabled before registering it rather than after.

Also update the code that handles errors from pwmchip_add() to account for
the fact a device's PWM clock may now be disabled and that its APB clock
certainly is, and eliminate the "err_pclk" goto target as it is no longer
of use.

Fixes: 48cf973cae33 ("pwm: rockchip: Avoid glitches on already running PWMs")
Reported-by: Trent Piepho <tpiepho at gmail.com>
Signed-off-by: Simon South <simon at simonsouth.net>
 drivers/pwm/pwm-rockchip.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/drivers/pwm/pwm-rockchip.c b/drivers/pwm/pwm-rockchip.c
index 3b1aa5daafff..d904a5d24885 100644
--- a/drivers/pwm/pwm-rockchip.c
+++ b/drivers/pwm/pwm-rockchip.c
@@ -289,6 +289,7 @@ static int rockchip_pwm_probe(struct platform_device *pdev)
 	struct rockchip_pwm_chip *pc;
 	struct resource *r;
 	u32 enable_conf, ctrl;
+	bool enabled;
 	int ret, count;
 	id = of_match_device(rockchip_pwm_dt_ids, &pdev->dev);
@@ -351,24 +352,27 @@ static int rockchip_pwm_probe(struct platform_device *pdev)
 		pc->chip.of_pwm_n_cells = 3;
-	ret = pwmchip_add(&pc->chip);
-	if (ret < 0) {
-		dev_err(&pdev->dev, "pwmchip_add() failed: %d\n", ret);
-		goto err_pclk;
-	}
 	/* Keep the PWM clk enabled if the PWM appears to be up and running. */
 	enable_conf = pc->data->enable_conf;
 	ctrl = readl_relaxed(pc->base + pc->data->regs.ctrl);
-	if ((ctrl & enable_conf) != enable_conf)
+	enabled = ((ctrl & enable_conf) == enable_conf);
+	if (!enabled)
+	ret = pwmchip_add(&pc->chip);
+	if (ret < 0) {
+		dev_err(&pdev->dev, "pwmchip_add() failed: %d\n", ret);
+		if (enabled)
+			clk_disable(pc->clk);
+		clk_unprepare(pc->clk);
+		clk_unprepare(pc->pclk);
+		return ret;
+	}
 	return 0;
-	clk_disable_unprepare(pc->pclk);

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