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SubjectRe: [PATCH] watchdog: at91sam9 wdt: various fixes
On Tue, 29 Oct 2013 14:27:38 -0700, Guenter Roeck <linux@roeck-us.net>
wrote:
> On Tue, Oct 29, 2013 at 06:22:47PM +0100, boris brezillon wrote:
>> On 29/10/2013 17:43, Guenter Roeck wrote:
>> >On Tue, Oct 29, 2013 at 05:22:50PM +0100, boris brezillon wrote:
>> >>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 <b.brezillon@overkiz.com>
>> >>>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.
>> >>
>> >But increasing anything in the watchdog driver itself won't help you there.
>> >You can not execute any kernel code before that kernel code is running.
>>
>> Of course, but you can at least try to minimize the time between the
>> watchdog driver init
>> and the first wathdog counter reset.
>>
> Sure.
>
>> >>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
>> >>
>> >Guess that is where I got lost. Do you mean the time from loading the driver
>> >to starting the watchdog application ? Because the init function is only
>> >executed after the driver is loaded, so nothing will help you if it takes
>> >too long for the driver to load.
>>
>> I think there is another bug here: the driver should not be compiled
>> as a module.
>>
>> Here is why:
>>
>> At POR the at91 SoC configure its watchdog with these default values:
>> - enabled
>> - min heartbeat = 0 ticks
>> - max heartbeat = 0xfff ticks <=> 16 secs
>> - some reset options
>> After a POR the watchdog can only be reconfigured once (and only once).
>> This configuration oftenly takes place in the the bootstrap (or bootloader),
>> but can eventually be done by the Linux kernel.
>>
>> If the watchdog is kept enabled by the bootstrap (or bootloader), this means
>> the linux kernel will have to reset the watchdog counter as soon as
>> possible to avoid
>> a possible watchdog reset.
>>
>> => If we authorize the at91 sam9 watchdog to be compiled as a
>> module, we're not sure
>> the module will be loaded soon enough to be able to reset the
>> watchdog counter.
>>
> Agreed, but that is only an issue _if_ the watchdog is enabled from ROMMON,
> which we don't know. This makes it a configuration issue: If the watchdog
> is enabled by ROMMON, the driver should not be built as module. On the other
> side, unless it is known for sure (say, from the HW architecture) that it
> is always enabled, we should not force everyone to build it into the kernel.
>

This is the case: the HW enable the watchdog with default timings
(min_heartbeat = 0 secs max_heartbeat = 16 secs) after a Reset or a
Power On
Reset.

The enable/disable bit is part of the config and thus can only be
configured once.

You then have these 2 use cases:

1) The ROMMON does not reconfigure the watchdog
=> The linux kernel must reset the watchdog counter within 16 secs.
2) The ROMMON reconfigure the watchdog
a) The ROMMON enable the watchdog with different timings
=> The linux kernel must reset the watchdog counter within X secs
(according
to the ROMMON config).
b) The ROMMON disable the watchdog
=> The watchdog is unusable and the linux watchdog driver is
useless


BTW, 16 seconds should be enough to
- load the kernel
- mount a rootfs
- load the at91 sam9 watchdog module



> Other drivers deal with that condition by only resetting and re-initializing
> the watchdog if it is already running. This driver is a bit of an exception,
> as it always enables the watchdog during initialization. Which is actually
> another reason to be able to build it as module: If the watchdog was not
> enabled by ROMMON, this ensures that it only starts running when the module
> is loaded.
>
> Thanks,
> Guenter
>
>> >
>> >You really have two times to deal with:
>> >- Time from ROMMON handoff to loading the driver
>> > Nothing you can do there. If the watchdog fires before the driver is loaded,
>> > you are lost. Only way t handle this situation is to increase the timeout
>> > in the ROMMON.
>> >- Time from loading driver to watchdog device open. This is really the time
>> > you are increasing with your change.
>>
>> This is where it gets a bit tricky.
>>
>> The heartbeat I'm talking about is not the "user space" heartbeat
>> (struct watchdog_device timeout field), it's the "hardware watchdog
>> counter reset"
>> heartbeat (struct at91wdt heartbeat field).
>>
>> Actually the at91 sam9 wdt driver does not provide a direct access
>> to the watchdog reset
>> counter functionnality.
>> Instead, it periodically reset the watchdog counter (based on the
>> timing config retrieved from
>> the hw registers), and eventually, if the user open a the watchdog
>> dev file and fails to reset
>> the watchdog (using the user space API), the drivers stops resetting
>> the hw counter, which leads
>> to a watchdog reset.
>>
>> I hope I'm clear enough, cause it's quite complicated to explain.
>>
>> Best Regards,
>>
>> Boris
>>
>> >Thanks,
>> >Guenter
>> >
>> >>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
>> >>>>
>> >>>>--
>> >>>>To unsubscribe from this list: send the line "unsubscribe linux-watchdog" in
>> >>>>the body of a message to majordomo@vger.kernel.org
>> >>>>More majordomo info at http://vger.kernel.org/majordomo-info.html
>> >>>>
>> >>--
>> >>To unsubscribe from this list: send the line "unsubscribe linux-watchdog" in
>> >>the body of a message to majordomo@vger.kernel.org
>> >>More majordomo info at http://vger.kernel.org/majordomo-info.html
>> >>
>>
>>



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