Message-ID: <CADyBb7sYPnvDVEJYVQm65B1JV=yvq10+ezs2x3f8F=TJCZ+OJA@mail.gmail.com>
Date:	Tue, 9 Jun 2015 11:59:32 +0800
From:	Fu Wei <fu.wei@...aro.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Timur Tabi <timur@...eaurora.org>,
	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
	Linaro ACPI Mailman List <linaro-acpi@...ts.linaro.org>,
	linux-watchdog@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	Wei Fu <tekkamanninja@...il.com>,
	G Gregory <graeme.gregory@...aro.org>,
	Al Stone <al.stone@...aro.org>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	Arnd Bergmann <arnd@...db.de>, vgandhi@...eaurora.org,
	wim@...ana.be, Jon Masters <jcm@...hat.com>,
	Leo Duran <leo.duran@....com>, Jon Corbet <corbet@....net>,
	Mark Rutland <mark.rutland@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>, rjw@...ysocki.net
Subject: Re: [PATCH v4 5/7] Watchdog: introduce ARM SBSA watchdog driver

Hi Guenter,


On 9 June 2015 at 02:26, Guenter Roeck <linux@...ck-us.net> wrote:
> On 06/08/2015 09:05 AM, Fu Wei wrote:
>>
>> Hi Gurnter
>>
>> On 3 June 2015 at 01:07, Guenter Roeck <linux@...ck-us.net> wrote:
>>>
>>> On 06/02/2015 09:55 AM, Fu Wei wrote:
>>>>
>>>>
>>>> Hi Timur,
>>>>
>>>> Thanks , feedback inline
>>>>
>>>> On 2 June 2015 at 23:32, Timur Tabi <timur@...eaurora.org> wrote:
>>>>>
>>>>>
>>>>> On 06/01/2015 11:05 PM, fu.wei@...aro.org wrote:
>>>>>
>>>>>> +/*
>>>>>> + * help functions for accessing 32bit registers of SBSA Generic
>>>>>> Watchdog
>>>>>> + */
>>>>>> +static void sbsa_gwdt_cf_write(unsigned int reg, u32 val,
>>>>>> +                              struct watchdog_device *wdd)
>>>>>> +{
>>>>>> +       struct sbsa_gwdt *gwdt = to_sbsa_gwdt(wdd);
>>>>>> +
>>>>>> +       writel_relaxed(val, gwdt->control_base + reg);
>>>>>> +}
>>>>>> +
>>>>>> +static void sbsa_gwdt_rf_write(unsigned int reg, u32 val,
>>>>>> +                              struct watchdog_device *wdd)
>>>>>> +{
>>>>>> +       struct sbsa_gwdt *gwdt = to_sbsa_gwdt(wdd);
>>>>>> +
>>>>>> +       writel_relaxed(val, gwdt->refresh_base + reg);
>>>>>> +}
>>>>>> +
>>>>>> +static u32 sbsa_gwdt_cf_read(unsigned int reg, struct watchdog_device
>>>>>> *wdd)
>>>>>> +{
>>>>>> +       struct sbsa_gwdt *gwdt = to_sbsa_gwdt(wdd);
>>>>>> +
>>>>>> +       return readl_relaxed(gwdt->control_base + reg);
>>>>>> +}
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> I still think you should get rid of these functions and just call
>>>>> readl_relaxed() and writel_relaxed() every time, but I won't complain
>>>>> again
>>>>> if you keep them.
>>>>
>>>>
>>>>
>>>> yes, that make sense, and will reduce the size of code, and I think
>>>> the code's readability will be OK too.
>>>> will try in my next patch,
>>>>
>>>>>
>>>>>> +static irqreturn_t sbsa_gwdt_interrupt(int irq, void *dev_id)
>>>>>> +{
>>>>>> +       struct sbsa_gwdt *gwdt = (struct sbsa_gwdt *)dev_id;
>>>>>> +       struct watchdog_device *wdd = &gwdt->wdd;
>>>>>> +
>>>>>> +       if (wdd->pretimeout)
>>>>>> +               /* The pretimeout is valid, go panic */
>>>>>> +               panic("SBSA Watchdog pre-timeout");
>>>>>> +       else
>>>>>> +               /* We don't use pretimeout, trigger WS1 now*/
>>>>>> +               sbsa_gwdt_set_wcv(wdd, 0);
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> I don't like this.
>>>>
>>>>
>>>>
>>>> If so, what is your idea ,if pretimeout == 0?
>>>>
>>>> the reason of using WCV as (timout - pretimeout): it can provide the
>>>> longer timeout period,
>>>> (1)If we use WOR, it can only provide 10s @ 400MHz(max).
>>>> as Guenter said earlier, the default timer out for most watchdog will
>>>> be 30s, so I think 10s limit will be a little short
>>>> (2)we can always program WCV just like ping.
>>>> (3)if a timeout arrives, WOR will be use, so use it as pretimeout, but
>>>> we still can make this pretimeout longer by programming WCV(I don't
>>>> think it's necessary)
>>>>
>>>>
>>>>> The triggering of the hardware reset should never depend
>>>>> on an interrupt being handled properly.
>>>>
>>>>
>>>>
>>>> if this fail, system reset in 1S, because WOR == (1s)
>>>>
>>> So ?
>>
>>
>> Even the interrupt routine isn't triggered,  (WOR + system counter) -->
>> WCV,
>> then, sy system reset in 1S.
>>
>> the hardware reset doesn't depend on an interrupt.
>>
>>
>>>
>>>>> You should always program WCV
>>>>> correctly in advance.  This is especially true since pre-timeout will
>>>>> probably rarely be used.
>>>>
>>>>
>>>>
>>>> always programming WCV is doable.  But I absolutely can not agree "
>>>> pre-timeout will probably rarely be used"
>>>> If so, SBSA watchdog is just a normal watchdog,  This use case just
>>>> makes this HW useless.
>>>> If so, go to use SP805.
>>>> you still don't see the importance of this warning and pretimeout to a
>>>> real server.
>>>>
>>>
>>> If pretimeout isn't used, why not just set WCV = timeout, WOR = 0 ?
>>
>>
>> Because if WOR = 0 , according to SBSA,  once you want to enable watchdog,
>> (0 + system counter) --> WCV , then , WS0 and WS1 will be triggered
>> immediately.
>> we have not a chance(a time slot) to update WCV.
>>
>
> I would have thought that this is exactly what we want if pretimeout is not
> used.

Although pretimeout == 0 is not good for a server, but If
administrator set up pretimeout == 0. *I thinks we should trigger WS1
ASAP* . Because  WS1 maybe a interrupter or a reboot signal, that is
why we can not reboot system directly.

This driver is SBSA watchdog driver, that means we need to follow SBSA spec:
(1) SBSA watchdog has two stage timeouts --> timeout and pretimeout is
the best solution in watchdog framework, at least for now.
(2) The watchdog has the following output signals:
    Watchdog Signal 0 (WS0)---> "The initial signal is typically wired
to an interrupt and alerts the system."(original word from spec), I
thinks the key work should be "interrupt" and "alerts". So in WS0
interrupt routine, reset is absolutely a wrong operation. Although I
think we should make this "alerts" more useful. But for the first
version of driver, I thinks panic is useful enough.
    Watchdog Signal 1 (WS1). ---> "The signal is fed to a higher agent
as an interrupt or reset for it to take executive action." (original
word from spec) . The key work should be "interrupt", "or" and "reset"
. So  WS1 maybe a interrupt.
so even in the WS0 interrupt routine, if  pretimeout == 0 , we need to
trigger WS1(that is what my patch is doing now, set WCV to 0, so WCV
is always less than SystemCounter, and in this situation(WS0 = TRUE),
WS1 will be trigger immediately), but definitely not a reset too.

But in worst case,  if the WS0 is triggered, but the interrupt routine
doesn't work(can not set up WCV), it doesn't matter, we just need to
wait for a WOR(1s in my driver) timeout, then WS1 will be triggered.
That is hardware mechanism, once we config SBSA watchdog correctly,
that should work.  If it doesn't, I think the chip design doesn't
follow the SBSA spec.

Make a summary here, for SBSA watchdog driver,  it need to support two
stage timeouts and need to trigger WS0/1 when timeouts occur(can not
simply reset system in interrupt routines).
If a driver doesn't do these above, the driver can not be called SBSA
watchdog driver.

But according to SBSA, even pretimeout == 0, we can not setup WOR = 0.
if we make WOR = 0 , once we set up WCS(enabling watchdog), that cause
a explicit watchdog refresh, then WCV = (0 + system counter), so WS0
and WS1 will be triggered serially and immediately(in theory, the
"delay" also depend on implementation). So in my patchset , if
pretimeout == 0, WOR will be 1s at least to make sure we have time to
setup WCV. I have made comments in the patch for explaining this.

Maybe some people want to ask: if we can not set up WOR = 0, but
pretimeout can be 0 and timeout can not, why I still want to use WOR
for pretimeout and using WCV as (timout - pretimeout) ??
For this:
(1)WCV can provide the longer timeout period, If we use WOR, it can
only provide 10s @ 400MHz(max). The default timer out for most
watchdog will be 30s, so I think 10s limit will be a little short.
(2)we can always program(write) WCV just like ping.
(3)if the first timeout occurs, WOR will be loaded to WCV(WOR + system
counter)  automatically , so why not just use WOR as pretimeout?
Although we still can make this pretimeout longer by programming WCV,
I don't think it's necessary for now.


> What am I missing here ?
>
> Guenter
>



-- 
Best regards,

Fu Wei
Software Engineer
Red Hat Software (Beijing) Co.,Ltd.Shanghai Branch
Ph: +86 21 61221326(direct)
Ph: +86 186 2020 4684 (mobile)
Room 1512, Regus One Corporate Avenue,Level 15,
One Corporate Avenue,222 Hubin Road,Huangpu District,
Shanghai,China 200021
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