Re: [PATCH v1] iTCO_wdt: ignore NMI_NOW bit on register comparison

[Guenter Roeck <linux@roeck-us.net>]

On 8/26/24 04:18, Mika Westerberg wrote:
> Hi,
> 
> On Mon, Aug 26, 2024 at 12:53:01AM -0700, Oleksandr Ocheretnyi wrote:
>> Commit da23b6faa8bf ("watchdog: iTCO: Add support for Cannon Lake
>> PCH iTCO") does not ignore NMI_NOW bit on write operation to TCO1_CNT
>> register what causes unexpected NMIs due to NMI_NOW bit inversion
>> during regular crashkernel's workflow with following logs:
>>
>> iTCO_vendor_support: vendor-support=0
>> iTCO_wdt iTCO_wdt: unable to reset NO_REBOOT flag, device
>>                                              disabled by hardware/BIOS
>>
>> This change clears NMI_NOW bit in the TCO1_CNT register to have no
>> effect on NMI_NOW bit inversion what can cause NMI immediately.
>>
>> Fixes: da23b6faa8bf ("watchdog: iTCO: Add support for Cannon Lake PCH iTCO")
>> Signed-off-by: Oleksandr Ocheretnyi <oocheret@cisco.com>
>> ---
>>   drivers/watchdog/iTCO_wdt.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/watchdog/iTCO_wdt.c b/drivers/watchdog/iTCO_wdt.c
>> index 264857d314da..679c115ef7d3 100644
>> --- a/drivers/watchdog/iTCO_wdt.c
>> +++ b/drivers/watchdog/iTCO_wdt.c
>> @@ -224,7 +224,7 @@ static int update_no_reboot_bit_cnt(void *priv, bool set)
>>   		val |= BIT(0);
>>   	else
>>   		val &= ~BIT(0);
>> -	outw(val, TCO1_CNT(p));
>> +	outw(val & ~BIT(8), TCO1_CNT(p));
> 
> I suggest adding some #define for the magical number 8 so that it is
> easier for anyone looking at this driver to figure out what it is doing.
> 
> Otherwise looks good to me, thanks!
> 

Not really; it appears to be hiding a change in code which is supposed to do
something different (it is supposed to set or clear the no_reboot bit),
with no explanation whatsoever. That warrants a comment in the code.
Also, I would prefer
	val = inw(TCO1_CNT(p)) & ~BIT(8);

Guenter

>>   	newval = inw(TCO1_CNT(p));
>>   
>>   	/* make sure the update is successful */
>> -- 
>> 2.39.3