[U-Boot] [PATCH] watchdog: omap_wdt: improve watchdog reset path

Lukasz Majewski
2018-03-02T14:20:44Z

Hi Ruslan,

> Hi Lukasz,
>
> On Thu, Mar 1, 2018 at 12:53 PM, Lukasz Majewski <lukma@denx.de>
> wrote:
> > Hi Ruslan,
> >
> >> Remove busy looping during watchdog reset.
> >> Each polling of W_PEND_WTGR bit ("finish posted
> >> write") after watchdog reset takes 120-140us
> >> on BeagleBone Black board. Current U-Boot code
> >> has watchdog resets in random places and often
> >> there is situation when watchdog is reset
> >> few times in a row in nested functions.
> >> This adds extra delays and slows the whole system.
> >>
> >> Instead of polling W_PEND_WTGR bit, we skip
> >> watchdog reset if the bit is set. Anyway, watchdog
> >> is in the middle of reset *right now*, so we can
> >> just return.
> >
> > It seems like similar problem may be in the Linux kernel driver:
> > v4.16-rc1/source/drivers/watchdog/omap_wdt.c#L74
> >
> > Linux driver also waits for the write.
>
> Right, Linux driver has similar code but it doesn't affect
> performance. This is because of watchdog usage in Linux, it is
> enabled and reset by userspace. This is quite rare event.
> Moreover, since Linux has interrupts enabled and has scheduling,
> such busy loops in the omap driver are not very different to
> just mdelay() usage. The system can handle interrupts, and can
> even do preemption if PREEMPT is enabled.
> So use of such busy loops in that particular case shouldn't cause
> any noticeable performance issues.
>
> In U-Boot we have polling instead of interrupts and something like
> cooperative multitasking. Also watchdog resets much more often,
> and such busy pollings in the driver delay execution of other code.
>
> For example, in DFU we have indefinite loop, something like:
> --->
> | dfu_write()
> | watchdog_reset()
> | ...
> | handle_usb()
> | `----- watchdog_reset()
> | `---- ...
> `----'
>
> And each delay caused by watchdog reset adds significant
> overhead comparing to handling USB events or writing to the
> storage.
>
> So as bottom line, we don't need to do similar change in
> Linux driver because there is almost no impact on performance
> in that case. But in case of U-Boot which uses polling instead
> of interrupts, this patch causes such a big performance
> improvement.

Thank you for this work - it really improves speed considerably :-).

Reviewed-by: Lukasz Majewski <lukma@denx.de>

>
> Best regards,
> Ruslan
>
> >
> >>
> >> This noticeably increases performance of the
> >> system. Below are some measurements on BBB:
> >> - DFU upload over USB 15% faster
> >> - fastboot image upload 3x times faster
> >> - USB ep0 transfers with 4k packets 20% faster
> >>
> >> Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
> >> ---
> >> drivers/watchdog/omap_wdt.c | 21 +++++++++++++++------
> >> 1 file changed, 15 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/drivers/watchdog/omap_wdt.c
> >> b/drivers/watchdog/omap_wdt.c index 7b1f429..d724c96 100644
> >> --- a/drivers/watchdog/omap_wdt.c
> >> +++ b/drivers/watchdog/omap_wdt.c
> >> @@ -53,16 +53,25 @@ void hw_watchdog_reset(void)
> >> {
> >> struct wd_timer *wdt = (struct wd_timer *)WDT_BASE;
> >>
> >> - /* wait for posted write to complete */
> >> - while ((readl(&wdt->wdtwwps)) & WDT_WWPS_PEND_WTGR)
> >> - ;
> >> + /*
> >> + * Somebody just triggered watchdog reset and write to WTGR
> >> register
> >> + * is in progress. It is resetting right now, no need to
> >> trigger it
> >> + * again
> >> + */
> >> + if ((readl(&wdt->wdtwwps)) & WDT_WWPS_PEND_WTGR)
> >> + return;
> >>
> >> wdt_trgr_pattern = ~wdt_trgr_pattern;
> >> writel(wdt_trgr_pattern, &wdt->wdtwtgr);
> >>
> >> - /* wait for posted write to complete */
> >> - while ((readl(&wdt->wdtwwps) & WDT_WWPS_PEND_WTGR))
> >> - ;
> >> + /*
> >> + * Don't wait for posted write to complete, i.e. don't check
> >> + * WDT_WWPS_PEND_WTGR bit in WWPS register. There is no
> >> writes to
> >> + * WTGR register outside of this func, and if entering it
> >> + * we see WDT_WWPS_PEND_WTGR bit set, it means watchdog reset
> >> + * was just triggered. This prevents us from wasting time in
> >> busy
> >> + * polling of WDT_WWPS_PEND_WTGR bit.
> >> + */
> >> }
> >>
> >> static int omap_wdt_set_timeout(unsigned int timeout)
> >
> >
> >
> >
> > Best regards,
> >
> > Lukasz Majewski
> >
> > --
> >
> > DENX Software Engineering GmbH, Managing Director: Wolfgang
> > Denk HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell,
> > Germany Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email:
> > wd at denx.de

Best regards,

Lukasz Majewski

--

DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 488 bytes
Desc: OpenPGP digital signature
URL: <http://lists.denx.de/pipermail/u-boot/attachments/20180302/8ba39b3e/attachment.sig>