Subject: [PATCH 1/2] watchdog: sama5d4: Cache MR instead of a partial config

From: [email-masked] (Guenter Roeck)
Date: Mon, 30 Jan 2017 09:58:13 -0800

On Mon, Jan 30, 2017 at 06:18:47PM +0100, Alexandre Belloni wrote:
> .config is used to cache a part of WDT_MR at probe time and is not used
> afterwards. Instead of doing that, actually cache MR and avoid reading it
> every time it is modified.
> 
The semantic change here is that the old code leaves "other" bits in the
mr register alone. I assume you have verified that clearing those bits
does not have negative impact.

Also, I am not really sure if replacing a read from a register is more
costly than a read from memory. Is that change really worth it ?
I mean, sure, the way the 'config' variable is used is a bit odd,
but I don't really see the improvement in its replacement either.

> Signed-off-by: Alexandre Belloni <[email-masked]>
> ---
>  drivers/watchdog/sama5d4_wdt.c | 45 ++++++++++++++++++------------------------
>  1 file changed, 19 insertions(+), 26 deletions(-)
> 
> diff --git a/drivers/watchdog/sama5d4_wdt.c b/drivers/watchdog/sama5d4_wdt.c
> index a49634cdc1cc..6dd07bef515a 100644
> --- a/drivers/watchdog/sama5d4_wdt.c
> +++ b/drivers/watchdog/sama5d4_wdt.c
> @@ -28,7 +28,7 @@
>  struct sama5d4_wdt {
>  	struct watchdog_device	wdd;
>  	void __iomem		*reg_base;
> -	u32	config;
> +	u32			mr;
>  };
>  
>  static int wdt_timeout = WDT_DEFAULT_TIMEOUT;
> @@ -53,11 +53,9 @@ MODULE_PARM_DESC(nowayout,
>  static int sama5d4_wdt_start(struct watchdog_device *wdd)
>  {
>  	struct sama5d4_wdt *wdt = watchdog_get_drvdata(wdd);
> -	u32 reg;
>  
> -	reg = wdt_read(wdt, AT91_WDT_MR);
> -	reg &= ~AT91_WDT_WDDIS;
> -	wdt_write(wdt, AT91_WDT_MR, reg);
> +	wdt->mr &= ~AT91_WDT_WDDIS;
> +	wdt_write(wdt, AT91_WDT_MR, wdt->mr);
>  
>  	return 0;
>  }
> @@ -65,11 +63,9 @@ static int sama5d4_wdt_start(struct watchdog_device *wdd)
>  static int sama5d4_wdt_stop(struct watchdog_device *wdd)
>  {
>  	struct sama5d4_wdt *wdt = watchdog_get_drvdata(wdd);
> -	u32 reg;
>  
> -	reg = wdt_read(wdt, AT91_WDT_MR);
> -	reg |= AT91_WDT_WDDIS;
> -	wdt_write(wdt, AT91_WDT_MR, reg);
> +	wdt->mr |= AT91_WDT_WDDIS;
> +	wdt_write(wdt, AT91_WDT_MR, wdt->mr);
>  
>  	return 0;
>  }
> @@ -88,14 +84,12 @@ static int sama5d4_wdt_set_timeout(struct watchdog_device *wdd,
>  {
>  	struct sama5d4_wdt *wdt = watchdog_get_drvdata(wdd);
>  	u32 value = WDT_SEC2TICKS(timeout);
> -	u32 reg;
>  
> -	reg = wdt_read(wdt, AT91_WDT_MR);
> -	reg &= ~AT91_WDT_WDV;
> -	reg &= ~AT91_WDT_WDD;
> -	reg |= AT91_WDT_SET_WDV(value);
> -	reg |= AT91_WDT_SET_WDD(value);
> -	wdt_write(wdt, AT91_WDT_MR, reg);
> +	wdt->mr &= ~AT91_WDT_WDV;
> +	wdt->mr &= ~AT91_WDT_WDD;
> +	wdt->mr |= AT91_WDT_SET_WDV(value);
> +	wdt->mr |= AT91_WDT_SET_WDD(value);
> +	wdt_write(wdt, AT91_WDT_MR, wdt->mr);
>  
>  	wdd->timeout = timeout;
>  
> @@ -132,19 +126,19 @@ static int of_sama5d4_wdt_init(struct device_node *np, struct sama5d4_wdt *wdt)
>  {
>  	const char *tmp;
>  
> -	wdt->config = AT91_WDT_WDDIS;
> +	wdt->mr = AT91_WDT_WDDIS;
>  
>  	if (!of_property_read_string(np, "atmel,watchdog-type", &tmp) &&
>  	    !strcmp(tmp, "software"))
> -		wdt->config |= AT91_WDT_WDFIEN;
> +		wdt->mr |= AT91_WDT_WDFIEN;
>  	else
> -		wdt->config |= AT91_WDT_WDRSTEN;
> +		wdt->mr |= AT91_WDT_WDRSTEN;
>  
>  	if (of_property_read_bool(np, "atmel,idle-halt"))
> -		wdt->config |= AT91_WDT_WDIDLEHLT;
> +		wdt->mr |= AT91_WDT_WDIDLEHLT;
>  
>  	if (of_property_read_bool(np, "atmel,dbg-halt"))
> -		wdt->config |= AT91_WDT_WDDBGHLT;
> +		wdt->mr |= AT91_WDT_WDDBGHLT;
>  
>  	return 0;
>  }
> @@ -163,11 +157,10 @@ static int sama5d4_wdt_init(struct sama5d4_wdt *wdt)
>  	reg &= ~AT91_WDT_WDDIS;
>  	wdt_write(wdt, AT91_WDT_MR, reg);
>  
There is (at least) one read of the mr register left above this code.
It is not entirely obvious to me why it would make sense to retain
this single read instead of just clearing the AT91_WDT_WDDIS bit
from the cached value and writing the rest. Maybe this is to make
sure that WDV or WDD isn't changed with the bit set, but ....
the explanation associated with clearing the bit is a bit odd either
case. It states that AT91_WDT_WDDIS must not be set (meaning the watchdog
must be running ? Shouldn't it be the opposite ?) when changing WDV or WDD,
then violates this rule when updating the timeout (which can happen with
the watchdog running or not).

> -	reg = wdt->config;
> -	reg |= AT91_WDT_SET_WDD(value);
> -	reg |= AT91_WDT_SET_WDV(value);
> +	wdt->mr |= AT91_WDT_SET_WDD(value);
> +	wdt->mr |= AT91_WDT_SET_WDV(value);
>  
> -	wdt_write(wdt, AT91_WDT_MR, reg);
> +	wdt_write(wdt, AT91_WDT_MR, wdt->mr);
>  
>  	return 0;
>  }
> @@ -211,7 +204,7 @@ static int sama5d4_wdt_probe(struct platform_device *pdev)
>  			return ret;
>  	}
>  
> -	if ((wdt->config & AT91_WDT_WDFIEN) && irq) {
> +	if ((wdt->mr & AT91_WDT_WDFIEN) && irq) {
>  		ret = devm_request_irq(&pdev->dev, irq, sama5d4_wdt_irq_handler,
>  				       IRQF_SHARED | IRQF_IRQPOLL |
>  				       IRQF_NO_SUSPEND, pdev->name, pdev);
> -- 
> 2.11.0
> 
> --
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