Subject: [PATCH] watchdog: at91sam9_wdt: various fixes

From: [email-masked] (boris brezillon)
Date: Tue, 29 Oct 2013 17:22:50 +0100

On 29/10/2013 16:45, Guenter Roeck wrote:
> On Tue, Oct 29, 2013 at 11:37:33AM +0100, Boris BREZILLON wrote:
>> Fix the secs_to_ticks macro in case 0 is passed as an argument.
>>
>> Rework the heartbeat calculation to increase the security margin of the
>> watchdog reset timer.
>>
>> Use the min_heartbeat value instead of the calculated heartbeat value for
>> the first watchdog reset.
>>
>> Signed-off-by: Boris BREZILLON <[email-masked]>
> Hi Boris,
>
> can you possibly split the three changes into separate patches ?

Sure. My first idea was to split this in 3 patches, but, as the buggy 
at91 watchdog series was
already applied to linux-watchdog-next, I thought it would be faster to 
only provide one
patch to fix all the issues at once.

>
> The first is a no-brainer. Gives my opinion of my code review capabilities
> a slight damper ;-).
>
> For the other two problems, it might make sense to describe
> the problems you are trying to solve.
>
> Couple of comments inline.
>
> Thanks,
> Guenter
>
>
>> ---
>>   drivers/watchdog/at91sam9_wdt.c |   35 ++++++++++++++++++++++++-----------
>>   1 file changed, 24 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/watchdog/at91sam9_wdt.c b/drivers/watchdog/at91sam9_wdt.c
>> index 9bd089e..f1b59f1 100644
>> --- a/drivers/watchdog/at91sam9_wdt.c
>> +++ b/drivers/watchdog/at91sam9_wdt.c
>> @@ -51,7 +51,7 @@
>>   #define ticks_to_hz_rounddown(t)	((((t) + 1) * HZ) >> 8)
>>   #define ticks_to_hz_roundup(t)		(((((t) + 1) * HZ) + 255) >> 8)
>>   #define ticks_to_secs(t)		(((t) + 1) >> 8)
>> -#define secs_to_ticks(s)		(((s) << 8) - 1)
>> +#define secs_to_ticks(s)		(s ? (((s) << 8) - 1) : 0)
>>   
> 	(s)
>
>>   #define WDT_MR_RESET	0x3FFF2FFF
>>   
>> @@ -61,6 +61,11 @@
>>   /* Watchdog max delta/value in secs */
>>   #define WDT_COUNTER_MAX_SECS	ticks_to_secs(WDT_COUNTER_MAX_TICKS)
>>   
>> +/* Watchdog heartbeat shift used for security margin:
>> + * we'll try to rshift the heartbeat value with this value to secure
>> + * the watchdog reset. */
>> +#define WDT_HEARTBEAT_SHIFT	2
>> +
>>   /* Hardware timeout in seconds */
>>   #define WDT_HW_TIMEOUT 2
>>   
>> @@ -158,7 +163,9 @@ static int at91_wdt_init(struct platform_device *pdev, struct at91wdt *wdt)
>>   	int err;
>>   	u32 mask = wdt->mr_mask;
>>   	unsigned long min_heartbeat = 1;
>> +	unsigned long max_heartbeat;
>>   	struct device *dev = &pdev->dev;
>> +	int shift;
>>   
>>   	tmp = wdt_read(wdt, AT91_WDT_MR);
>>   	if ((tmp & mask) != (wdt->mr & mask)) {
>> @@ -181,23 +188,27 @@ static int at91_wdt_init(struct platform_device *pdev, struct at91wdt *wdt)
>>   	if (delta < value)
>>   		min_heartbeat = ticks_to_hz_roundup(value - delta);
>>   
>> -	wdt->heartbeat = ticks_to_hz_rounddown(value);
>> -	if (!wdt->heartbeat) {
>> +	max_heartbeat = ticks_to_hz_rounddown(value);
>> +	if (!max_heartbeat) {
>>   		dev_err(dev,
>>   			"heartbeat is too small for the system to handle it correctly\n");
>>   		return -EINVAL;
>>   	}
>>   
>> -	if (wdt->heartbeat < min_heartbeat + 4) {
>> +	for (shift = WDT_HEARTBEAT_SHIFT; shift > 0; shift--) {
>> +		if ((max_heartbeat >> shift) < min_heartbeat)
>> +			continue;
>> +
>> +		wdt->heartbeat = max_heartbeat >> shift;
>> +		break;
>> +	}
>> +
>> +	if (!shift)
>>   		wdt->heartbeat = min_heartbeat;
> Correct me if I am wrong, but it seems to me that
>
> 	if ((max_heartbeat >> 2) >= min_heartbeat)
> 		 wdt->heartbeat = max_heartbeat >> 2;
> 	else if ((max_heartbeat >> 1) >= min_heartbeat)
> 		wdt->heartbeat = max_heartbeat >> 1;
> 	else
> 		wdt->heartbeat = min_heartbeat;
>
> would accomplish the same and be easier to understand.

This is exactly what I'm trying to accomplish.
I used the for loop in case we ever want to change the 
WDT_HEARTBEAT_SHIFT value
(which is unlikely to happen).

I'll move to your proposition.

>
> However, given that, I wonder if it is really necessary to bail out above if
> max_heartbeat is 0. After all, you set heartbeat to min_heartbeat anyway
> in this case.

Yes it is necessary. The max_heartbeat is a configuration that cannot be 
changed once configured.
We have to inform the user that his max_heartbeat value is too small to 
be handled correctly by the Linux kernel.

If we simply use the min_heartbeat value as heartbeat and ignore the 
wrong max_heartbeat value,
the watchdog will reset the SoC before we can ever reset the watchdog 
counter.

>
>> +
>> +	if (max_heartbeat < min_heartbeat + 4)
>>   		dev_warn(dev,
>>   			 "min heartbeat and max heartbeat might be too close for the system to handle it correctly\n");
>> -		if (wdt->heartbeat < 4)
>> -			dev_warn(dev,
>> -				 "heartbeat might be too small for the system to handle it correctly\n");
>> -	} else {
>> -		wdt->heartbeat -= 4;
>> -	}
>>   
>>   	if ((tmp & AT91_WDT_WDFIEN) && wdt->irq) {
>>   		err = request_irq(wdt->irq, wdt_interrupt,
>> @@ -213,7 +224,9 @@ static int at91_wdt_init(struct platform_device *pdev, struct at91wdt *wdt)
>>   			 tmp & wdt->mr_mask, wdt->mr & wdt->mr_mask);
>>   
>>   	setup_timer(&wdt->timer, at91_ping, (unsigned long)wdt);
>> -	mod_timer(&wdt->timer, jiffies + wdt->heartbeat);
>> +	/* Use min_heartbeat the first time because the watchdog timer might
>> +	 * be running for a long time when we reach this init function. */
> 	/*
> 	 * Multi-line comment style
> 	 *
> 	 * Not really sure I understand what this accomplishes. What problem
> 	 * are you trying to solve here ?
> 	 */

Sure, I'll change the comment style.

What, I'm trying to explain, is that the watchdog might (or should) have 
been resetted
before loading the linux kernel. But loading the kernel takes some time 
(uncompressing,
low level init, ...), and if we take the heartbeat (or max_heartbeat / 4 
in the common case) value as
the next trigger to reset the watchdog counter, the watchdog timer might 
have already expired.

Here is an example:
  - max_heartbeat configured to 8 seconds
  - min_heartbeat configured to 1 second
  => heartbeat = 2s
  - kernel boot time (before at91 watchdog is loaded) = 6 secs

If I use the heartbeat value when configuring the first expiration of 
the timer,
it might be too late to reset the watchdog counter.

I'll try to find a proper to explain this use case :-).

>> +	mod_timer(&wdt->timer, jiffies + min_heartbeat);
>>   
>>   	/* Try to set timeout from device tree first */
>>   	if (watchdog_init_timeout(&wdt->wdd, 0, dev))
>> -- 
>> 1.7.9.5
>>
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