counter: stm32-timer-cnt: fix ceiling miss-alignment with reload register

Ceiling value may be miss-aligned with what's actually configured into the
ARR register. This is seen after probe as currently the ARR value is zero,
whereas ceiling value is set to the maximum. So:
- reading ceiling reports zero
- in case the counter gets enabled without any prior configuration,
  it won't count.
- in case the function gets set by the user 1st, (priv->ceiling) is used.

Fix it by getting rid of the cached "priv->ceiling" variable. Rather use
the ARR register value directly by using regmap read or write when needed.
There should be no drawback on performance as priv->ceiling isn't used in
performance critical path.
There's also no point in writing ARR while setting function (sms), so
it can be safely removed.

Fixes: ad29937 ("counter: Add STM32 Timer quadrature encoder")
Suggested-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/1614793789-10346-1-git-send-email-fabrice.gasnier@foss.st.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: Fabrice Gasnier

drivers/counter/stm32-timer-cnt.c

@@ -31,7 +31,6 @@ struct stm32_timer_cnt {
 	struct counter_device counter;
 	struct regmap *regmap;
 	struct clk *clk;
-	u32 ceiling;
 	u32 max_arr;
 	bool enabled;
 	struct stm32_timer_regs bak;
@@ -75,8 +74,10 @@ static int stm32_count_write(struct counter_device *counter,
 			     const unsigned long val)
 {
 	struct stm32_timer_cnt *const priv = counter->priv;
+	u32 ceiling;
 
-	if (val > priv->ceiling)
+	regmap_read(priv->regmap, TIM_ARR, &ceiling);
+	if (val > ceiling)
 		return -EINVAL;
 
 	return regmap_write(priv->regmap, TIM_CNT, val);
@@ -138,10 +139,6 @@ static int stm32_count_function_set(struct counter_device *counter,
 
 	regmap_update_bits(priv->regmap, TIM_CR1, TIM_CR1_CEN, 0);
 
-	/* TIMx_ARR register shouldn't be buffered (ARPE=0) */
-	regmap_update_bits(priv->regmap, TIM_CR1, TIM_CR1_ARPE, 0);
-	regmap_write(priv->regmap, TIM_ARR, priv->ceiling);
-
 	regmap_update_bits(priv->regmap, TIM_SMCR, TIM_SMCR_SMS, sms);
 
 	/* Make sure that registers are updated */
@@ -199,7 +196,6 @@ static ssize_t stm32_count_ceiling_write(struct counter_device *counter,
 	regmap_update_bits(priv->regmap, TIM_CR1, TIM_CR1_ARPE, 0);
 	regmap_write(priv->regmap, TIM_ARR, ceiling);
 
-	priv->ceiling = ceiling;
 	return len;
 }
 
@@ -374,7 +370,6 @@ static int stm32_timer_cnt_probe(struct platform_device *pdev)
 
 	priv->regmap = ddata->regmap;
 	priv->clk = ddata->clk;
-	priv->ceiling = ddata->max_arr;
 	priv->max_arr = ddata->max_arr;
 
 	priv->counter.name = dev_name(dev);