linux-kernel - Re: [PATCH v2] iTCO_wdt: mask NMI_NOW bit for update_no_reboot

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Message-ID: <1ac5e79b-f313-44aa-a19c-dca05574ba91@roeck-us.net>
Date: Thu, 12 Sep 2024 14:15:27 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Oleksandr Ocheretnyi <oocheret@...co.com>
Cc: jdelvare@...e.de, linux-kernel@...r.kernel.org,
 linux-watchdog@...r.kernel.org, mika.westerberg@...ux.intel.com,
 wim@...ux-watchdog.org, wsa@...nel.org, xe-linux-external@...co.com
Subject: Re: [PATCH v2] iTCO_wdt: mask NMI_NOW bit for update_no_reboot_bit()
 call

On 9/12/24 07:19, Oleksandr Ocheretnyi wrote:
> Commit da23b6faa8bf ("watchdog: iTCO: Add support for Cannon Lake
> PCH iTCO") does not mask NMI_NOW bit during TCO1_CNT register's
> value comparison for update_no_reboot_bit() call causing following
> failure:
> 
>     ...
>     iTCO_vendor_support: vendor-support=0
>     iTCO_wdt iTCO_wdt: unable to reset NO_REBOOT flag, device
>                                      disabled by hardware/BIOS
>     ...
> 
> and this can lead to unexpected NMIs later during regular
> crashkernel's workflow because of watchdog probe call failures.
> 
> This change masks NMI_NOW bit for TCO1_CNT register values to
> avoid unexpected NMI_NOW bit inversions.
> 
> Fixes: da23b6faa8bf ("watchdog: iTCO: Add support for Cannon Lake PCH iTCO")
> Signed-off-by: Oleksandr Ocheretnyi <oocheret@...co.com>
> ---
>   drivers/watchdog/iTCO_wdt.c | 23 +++++++++++++++++++----
>   1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/watchdog/iTCO_wdt.c b/drivers/watchdog/iTCO_wdt.c
> index 264857d314da..46c09359588f 100644
> --- a/drivers/watchdog/iTCO_wdt.c
> +++ b/drivers/watchdog/iTCO_wdt.c
> @@ -82,6 +82,12 @@
>   #define TCO2_CNT(p)	(TCOBASE(p) + 0x0a) /* TCO2 Control Register	*/
>   #define TCOv2_TMR(p)	(TCOBASE(p) + 0x12) /* TCOv2 Timer Initial Value*/
>   
> +/* NMI_NOW is bit 8 of TCO1_CNT register
> + * Read/Write
> + * This bit is implemented as RW but has no effect on HW.
> + */
> +#define NMI_NOW		BIT(8)
> +
>   /* internal variables */
>   struct iTCO_wdt_private {
>   	struct watchdog_device wddev;
> @@ -217,15 +223,24 @@ static int update_no_reboot_bit_mem(void *priv, bool set)
>   static int update_no_reboot_bit_cnt(void *priv, bool set)
>   {
>   	struct iTCO_wdt_private *p = priv;
> -	u16 val, newval;
> -
> -	val = inw(TCO1_CNT(p));
> +	u16 val, newval, mask = (~NMI_NOW);
> +
Unnecessary (). Either case, please just mask against ~NMI_NOW directly.
The mask variable is not necessary.

> +	/* writing back 1b1 to NMI_NOW of TCO1_CNT register

Standard multi-line comments, please.

Thanks,
Guenter

> +	 * causes NMI_NOW bit inversion what consequently does
> +	 * not allow to perform the register's value comparison
> +	 * properly.
> +	 *
> +	 * NMI_NOW bit masking for TCO1_CNT register values
> +	 * helps to avoid possible NMI_NOW bit inversions on
> +	 * following write operation.
> +	 */
> +	val = inw(TCO1_CNT(p)) & mask;
>   	if (set)
>   		val |= BIT(0);
>   	else
>   		val &= ~BIT(0);
>   	outw(val, TCO1_CNT(p));
> -	newval = inw(TCO1_CNT(p));
> +	newval = inw(TCO1_CNT(p)) & mask;
>   
>   	/* make sure the update is successful */
>   	return val != newval ? -EIO : 0;