[PATCH v2] pwm: atmel-tcb: Cache clock rates and mark chip as atomic

[Sangyun Kim]

atmel_tcb_pwm_apply() holds tcbpwmc->lock as a spinlock via
guard(spinlock)() and then calls atmel_tcb_pwm_config(), which calls
clk_get_rate() twice. clk_get_rate() acquires clk_prepare_lock (a
mutex), so this is a sleep-in-atomic-context violation.

On CONFIG_DEBUG_ATOMIC_SLEEP kernels every pwm_apply_state() that
enables or reconfigures the PWM triggers a "BUG: sleeping function
called from invalid context" warning.

Acquire exclusive control over the clock rates with
clk_rate_exclusive_get() at probe time and cache the rates in struct
atmel_tcb_pwm_chip, then read the cached rates from
atmel_tcb_pwm_config(). This keeps the spinlock-based mutual exclusion
introduced in commit 37f7707077f5 ("pwm: atmel-tcb: Fix race condition
and convert to guards") and removes the sleeping calls from the atomic
section.

With no sleeping calls left in .apply() and the regmap-mmio bus already
running with fast_io=true, also mark the chip as atomic so consumers
can use pwm_apply_atomic() from atomic context.

Fixes: 37f7707077f5 ("pwm: atmel-tcb: Fix race condition and convert to guards")
Signed-off-by: Sangyun Kim <sangyun.kim at snu.ac.kr>
---
Hi Uwe,

Thanks for the review! "Sangyun" is the right form to address me, no
worries.

Changes in v2:
 - Keep the spinlock instead of converting tcbpwmc->lock to a mutex.
 - Cache clk and slow_clk rates at probe via clk_rate_exclusive_get()
   so the .apply() path no longer calls clk_get_rate() under the
   spinlock.
 - Mark the chip as atomic now that .apply() has no sleeping calls.

Thanks,
Sangyun

 drivers/pwm/pwm-atmel-tcb.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/drivers/pwm/pwm-atmel-tcb.c b/drivers/pwm/pwm-atmel-tcb.c
index f9a9c12cbcdd..8d46ce28f736 100644
--- a/drivers/pwm/pwm-atmel-tcb.c
+++ b/drivers/pwm/pwm-atmel-tcb.c
@@ -50,6 +50,8 @@ struct atmel_tcb_pwm_chip {
 	spinlock_t lock;
 	u8 channel;
 	u8 width;
+	unsigned long rate;
+	unsigned long slow_rate;
 	struct regmap *regmap;
 	struct clk *clk;
 	struct clk *gclk;
@@ -266,7 +268,7 @@ static int atmel_tcb_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	int slowclk = 0;
 	unsigned period;
 	unsigned duty;
-	unsigned rate = clk_get_rate(tcbpwmc->clk);
+	unsigned long rate = tcbpwmc->rate;
 	unsigned long long min;
 	unsigned long long max;
 
@@ -294,7 +296,7 @@ static int atmel_tcb_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	 */
 	if (i == ARRAY_SIZE(atmel_tcb_divisors)) {
 		i = slowclk;
-		rate = clk_get_rate(tcbpwmc->slow_clk);
+		rate = tcbpwmc->slow_rate;
 		min = div_u64(NSEC_PER_SEC, rate);
 		max = min << tcbpwmc->width;
 
@@ -431,6 +433,7 @@ static int atmel_tcb_pwm_probe(struct platform_device *pdev)
 	}
 
 	chip->ops = &atmel_tcb_pwm_ops;
+	chip->atomic = true;
 	tcbpwmc->channel = channel;
 	tcbpwmc->width = config->counter_width;
 
@@ -438,16 +441,33 @@ static int atmel_tcb_pwm_probe(struct platform_device *pdev)
 	if (err)
 		goto err_gclk;
 
+	err = clk_rate_exclusive_get(tcbpwmc->clk);
+	if (err)
+		goto err_disable_clk;
+
+	err = clk_rate_exclusive_get(tcbpwmc->slow_clk);
+	if (err)
+		goto err_clk_unlock;
+
+	tcbpwmc->rate = clk_get_rate(tcbpwmc->clk);
+	tcbpwmc->slow_rate = clk_get_rate(tcbpwmc->slow_clk);
+
 	spin_lock_init(&tcbpwmc->lock);
 
 	err = pwmchip_add(chip);
 	if (err < 0)
-		goto err_disable_clk;
+		goto err_slow_clk_unlock;
 
 	platform_set_drvdata(pdev, chip);
 
 	return 0;
 
+err_slow_clk_unlock:
+	clk_rate_exclusive_put(tcbpwmc->slow_clk);
+
+err_clk_unlock:
+	clk_rate_exclusive_put(tcbpwmc->clk);
+
 err_disable_clk:
 	clk_disable_unprepare(tcbpwmc->slow_clk);
 
@@ -470,6 +490,8 @@ static void atmel_tcb_pwm_remove(struct platform_device *pdev)
 
 	pwmchip_remove(chip);
 
+	clk_rate_exclusive_put(tcbpwmc->slow_clk);
+	clk_rate_exclusive_put(tcbpwmc->clk);
 	clk_disable_unprepare(tcbpwmc->slow_clk);
 	clk_put(tcbpwmc->gclk);
 	clk_put(tcbpwmc->clk);
-- 
2.34.1