linux-kernel - Re: [PATCH v2 1/2] watchdog: dw_wdt: pat the watchdog before enabling it

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Date:	Mon, 26 Jan 2015 20:08:04 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Jisheng Zhang <jszhang@...vell.com>,
	Doug Anderson <dianders@...omium.org>
CC:	Wim Van Sebroeck <wim@...ana.be>, Heiko Stuebner <heiko@...ech.de>,
	Lunxue Dai <lunxue.dai@...k-chips.com>,
	Dinh Nguyen <dinguyen@...era.com>,
	"linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] watchdog: dw_wdt: pat the watchdog before enabling
 it

On 01/26/2015 07:49 PM, Jisheng Zhang wrote:
> Dear Doug,
>
> On Mon, 26 Jan 2015 15:27:15 -0800
> Doug Anderson <dianders@...omium.org> wrote:
>
>> On some dw_wdt implementations the "top" register may be initted to 0
>> at bootup.  In such a case, each "pat" of the watchdog will reset the
>> timer to 0xffff.  That's pretty short.
>>
>> The input clock of the wdt can be any of a wide range of values.  On
>> an rk3288 system, I've seen the wdt clock be 24.75 MHz.  That means
>> each tick is ~40ns and we'll count to 0xffff in ~2.6ms.
>>
>> Because of the above two facts, it's a really good idea to pat the
>> watchdog after initting the "top" register properly and before
>> enabling the watchdog.  If you don't then there's no way we'll get the
>> next heartbeat in time.
>>
>> Jisheng Zhang fixed this problem on some dw_mmc versions by using the
>
> s/dw_mmc/dw_wdt
>
>> TOP_INIT feature.  However, the dw_wdt on rk3288 doesn't have TOP_INIT
>> so it's a good idea to also pat the watchdog manually.
>
> Based on your register dumping, I see the following configurations on rk3288:
>
> WDT_DUAL_TOP is configured as false, so there's no TOP_INIT
>
> WDT_DFLT_TOP is configured as 0, so it will timeout soon.
>
>
> So an extra pat is a must on such platforms with similar configurations. And it
> doesn't hurt anything if we have an extra pat before enabling the WDT
>
> All in all, except the "dw_mmc" typo above, the patch looks good to me.
>

Jisheng,

it would be great if you can provide configuration information shown in
the (undocumented) registers.

Doug,

can you send another version with this information added as comment
to the code ? This will help others to understand what is going on
(and why) later on.

Thanks,
Guenter

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