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

Ruslan Bilovol
2018-03-01T01:15:48Z

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.

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)
--
1.9.1