Re: [PATCH v2 5/7] Watchdog: introduce "pretimeout" into framework

[Timo Kokkonen]

On 22.05.2015 11:23, Fu Wei wrote:
> Hi Timo,
>
>
> On 22 May 2015 at 14:30, Timo Kokkonen <timo.kokkonen@xxxxxxxxxx> wrote:
> > On 21.05.2015 11:32, fu.wei@xxxxxxxxxx wrote:
> > > From: Fu Wei <fu.wei@xxxxxxxxxx>
> > >
> > > Also update Documentation/watchdog/watchdog-kernel-api.txt to
> > > introduce:
> > > (1)the new elements in the watchdog_device and watchdog_ops struct;
> > > (2)the new API "watchdog_init_timeouts".
> > >
> > > Reasons:
> > > (1)kernel already has two watchdog drivers are using "pretimeout":
> > >          drivers/char/ipmi/ipmi_watchdog.c
> > >          drivers/watchdog/kempld_wdt.c(but the definition is different)
> > > (2)some other dirvers are going to use this: ARM SBSA Generic Watchdog
> >
> > Hi,
> >
> > As I was proposing some other API changes with my early-timeout-sec work, I
> > can see my work is going to collide with your API change proposal a bit. So
> > maybe I should ask your opinion as well..
>
> Thank you for reminding me of that, I saw "early-timeout-sec", but
> because I don't get it in kernel API or watchdog_core.c, so I did not
> pay attention to it.
> Sorry.
>
> > Could this pretimeout feature be something that other drivers could benefit
> > too?
>
> yes , as you may know, I am making a patch for pretimeout support in
> watchdog framework
>
> > I can see that it does not do anything else except call panic() before
> > letting the watchdog expire. This is something that could be emulated easily
> > by the watchdog core for drivers that don't know anything about pretimeouts
> > at all.
>
> For SBSA watchdog, there are two stage timeout, and according to kernel doc:
> ----------------------
> Pretimeouts:
>
> Some watchdog timers can be set to have a trigger go off before the
> actual time they will reset the system.  This can be done with an NMI,
> interrupt, or other mechanism.  This allows Linux to record useful
> information (like panic information and kernel coredumps) before it
> resets.
> ----------------------
>
> I think panic() is the right way to do now.  but people may also need
> to config :
> PANIC_TIMEOUT [!=0]
> KEXEC
> KDUMP
> for debug reason

Yes, so basically if we hit pretimeout, we probably have already 
crashed. The only difference is that we now have some seconds time to 
dump out useful data and then either reboot or let the actual watchdog 
reset take care of resetting the device. panic() sounds like a good 
thing to do. Maybe you could also dump registers on other CPUs too or 
try to get out some other useful information about the kernel in case it 
has crashed or something. But I'm just guessing.

> > The way I was planning the API change there would need to be a small change
> > with each watchdog driver in order to let the watchdog core take over
> > generic behaviour on behalf of the driver. My goal was to make the change so
> > that each driver that gets converted to the new API extensions gets a
> > support for early-timeout-sec for free, without needing to enable support
> > for it any way. If the API was designed properly, also pretimeouts could be
> > handled easily and maybe even so that other drivers could have that feature
> > even though their hardware does not explicitly give any support for it.
>
> could you please point out your patch , then I can learn your idea :-)
> For now , I am working on a common "Pretimeouts" following the concept
> in kernel doc.
>
> > Any thoughts?
>
> my thoughts is in  my pretimeout patch , would you provide some suggestion ?

Here is an archive link to my patch set:

http://www.spinics.net/lists/linux-watchdog/msg06473.html

Your patch set is adding a new call to the core for adjusting the 
pretimoeut, which is probably a good thing in case the driver needs 
special handling for it anyway. But if the core had capability to 
emulate pretimeout for drivers that can't support it in hardware, it 
would be good if there was a way for the core to support it even though 
the driver had zero code for it. The core also has 
watchdog_init_timeout() right now but even that is not called from that 
many drivers. I would like to fix that too so that drivers would not 
need to bother so much about it but let core take care of it more. This 
is why I proposed the watchdog_init_params() call that could dig all the 
generic parameters from device tree on behalf of the driver. This is 
where I though timeout and early-timeout-sec parameters would be parsed 
from device tree, but it could also parse pretimeout parameter as well. 
Apparently Guenter did not like my approach so we might need some other 
way to do it.

I don't get to say how this should be done, I'm just throwing ideas 
here. But I think the watchdog core would be a lot better place for 
implementing the generic features than drivers. That way a lot of 
drivers could enable support the new features for free.

Thanks,
-Timo
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