linux-kernel - Re: [PATCH 1/2] watchdog: dw: RMW the control register

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Date:   Fri, 9 Mar 2018 19:28:48 -0800
From:   Brian Norris <briannorris@...omium.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        linux-kernel@...r.kernel.org, linux-watchdog@...r.kernel.org,
        Doug Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: Re: [PATCH 1/2] watchdog: dw: RMW the control register

Hi,

On Fri, Mar 09, 2018 at 07:20:38PM -0800, Guenter Roeck wrote:
> On 03/09/2018 06:44 PM, Brian Norris wrote:
> > RK3399 has rst_pulse_length in CONTROL_REG[4:2], determining the length
> > of pulse to issue for system reset. We shouldn't clobber this value,
> > because that might make the system reset ineffective. On RK3399, we're
> > seeing that a value of 000b (meaning 2 cycles) yields an unreliable
> > (partial?) reset, and so we only fully reset after the watchdog fires a
> > second time. If we retain the system default (010b, or 8 clock cycles),
> > then the watchdog reset is much more reliable.
> > 
> > Read-modify-write retains the system value and improves reset
> > reliability.
> > 
> > Signed-off-by: Brian Norris <briannorris@...omium.org>
> > ---
> >   drivers/watchdog/dw_wdt.c | 10 ++++++++--
> >   1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/watchdog/dw_wdt.c b/drivers/watchdog/dw_wdt.c
> > index c2f4ff516230..6925d3e6c6b3 100644
> > --- a/drivers/watchdog/dw_wdt.c
> > +++ b/drivers/watchdog/dw_wdt.c
> > @@ -34,6 +34,7 @@
> >   #define WDOG_CONTROL_REG_OFFSET		    0x00
> >   #define WDOG_CONTROL_REG_WDT_EN_MASK	    0x01
> > +#define WDOG_CONTROL_REG_RESP_MODE_MASK	    0x02
> >   #define WDOG_TIMEOUT_RANGE_REG_OFFSET	    0x04
> >   #define WDOG_TIMEOUT_RANGE_TOPINIT_SHIFT    4
> >   #define WDOG_CURRENT_COUNT_REG_OFFSET	    0x08
> > @@ -124,11 +125,16 @@ static int dw_wdt_set_timeout(struct watchdog_device *wdd, unsigned int top_s)
> >   static int dw_wdt_start(struct watchdog_device *wdd)
> >   {
> >   	struct dw_wdt *dw_wdt = to_dw_wdt(wdd);
> > +	u32 val;
> >   	dw_wdt_set_timeout(wdd, wdd->timeout);
> > -	writel(WDOG_CONTROL_REG_WDT_EN_MASK,
> > -	       dw_wdt->regs + WDOG_CONTROL_REG_OFFSET);
> > +	val = readl(dw_wdt->regs + WDOG_CONTROL_REG_OFFSET);
> > +	/* Disable interrupt mode; always perform system reset. */
> > +	val &= ~WDOG_CONTROL_REG_RESP_MODE_MASK;
> 
> You don't talk about this change in the description.

I guess I could mention it. I was assuming that was an intended behavior
of the existing driver: that we set resp_mode=0 (via clobber), so we
always get a system reset (we don't try to handle any interrupt in this
driver).

I'll include something along those lines in the commit message.

> > +	/* Enable watchdog. */
> > +	val |= WDOG_CONTROL_REG_WDT_EN_MASK;
> > +	writel(val, dw_wdt->regs + WDOG_CONTROL_REG_OFFSET);
> >   	return 0;
> >   }
> > 
> 
> Similar code is in dw_wdt_restart(), where it may be equally or even
> more important. Granted, only if the watchdog isn't running, but still...

Oh, I misread that code. It looked like an read/modify/write already,
but it was actually just a read/check/write. I should fix that, since
otherwise the restart will clobber the very thing I'm trying to fix
here, which might actually make the intended machine restart quite
ineffective.

Thanks,
Brian