linux-kernel - Re: [PATCH v1] iTCO_wdt: ignore NMI

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Message-ID: <124070dc-77ed-4854-9322-9cc9a4c54b57@roeck-us.net>
Date: Mon, 26 Aug 2024 08:15:54 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Mika Westerberg <mika.westerberg@...ux.intel.com>,
 Oleksandr Ocheretnyi <oocheret@...co.com>
Cc: Wim Van Sebroeck <wim@...ux-watchdog.org>, Jean Delvare
 <jdelvare@...e.de>, Wolfram Sang <wsa@...nel.org>,
 linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org,
 xe-linux-external@...co.com
Subject: Re: [PATCH v1] iTCO_wdt: ignore NMI_NOW bit on register comparison

On 8/26/24 08:12, Guenter Roeck wrote:
> 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@...co.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);
> 

On top of that, I fail to understand "on register comparison" in the subject.
What register comparison ?

Guenter