linux-kernel - Re: [PATCH] mfd: stm32-timers: avoid clearing auto reload register

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Message-ID: <YEAfpzWhfkfh0xTs@shinobu>
Date:   Thu, 4 Mar 2021 08:45:43 +0900
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Fabrice Gasnier <fabrice.gasnier@...s.st.com>
Cc:     lee.jones@...aro.org, alexandre.torgue@...s.st.com,
        mcoquelin.stm32@...il.com, olivier.moysan@...s.st.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH] mfd: stm32-timers: avoid clearing auto reload register

On Wed, Mar 03, 2021 at 06:51:35PM +0100, Fabrice Gasnier wrote:
> The ARR register is cleared unconditionally upon probing, after the maximum
> value has been read. This initial condition is rather not intuitive, when
> considering the counter child driver. It rather expects the maximum value
> by default:
> - The counter interface shows a zero value by default for 'ceiling'
>   attribute.
> - Enabling the counter without any prior configuration makes it doesn't
>   count.
> 
> The reset value of ARR register is the maximum. So Choice here
> is to backup it, and restore it then, instead of clearing its value.
> It also fixes the initial condition seen by the counter driver.
> 
> Fixes: d0f949e220fd ("mfd: Add STM32 Timers driver")
> 
> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@...s.st.com>

Acked-by: William Breathitt Gray <vilhelm.gray@...il.com>

> ---
>  drivers/mfd/stm32-timers.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/mfd/stm32-timers.c b/drivers/mfd/stm32-timers.c
> index add6033..44ed2fc 100644
> --- a/drivers/mfd/stm32-timers.c
> +++ b/drivers/mfd/stm32-timers.c
> @@ -158,13 +158,18 @@ static const struct regmap_config stm32_timers_regmap_cfg = {
>  
>  static void stm32_timers_get_arr_size(struct stm32_timers *ddata)
>  {
> +	u32 arr;
> +
> +	/* Backup ARR to restore it after getting the maximum value */
> +	regmap_read(ddata->regmap, TIM_ARR, &arr);
> +
>  	/*
>  	 * Only the available bits will be written so when readback
>  	 * we get the maximum value of auto reload register
>  	 */
>  	regmap_write(ddata->regmap, TIM_ARR, ~0L);
>  	regmap_read(ddata->regmap, TIM_ARR, &ddata->max_arr);
> -	regmap_write(ddata->regmap, TIM_ARR, 0x0);
> +	regmap_write(ddata->regmap, TIM_ARR, arr);
>  }
>  
>  static int stm32_timers_dma_probe(struct device *dev,
> -- 
> 2.7.4
> 

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