[PATCH v2 4/4] watchdog: cadence_wdt: Support all available prescaler values

[Melin Tomas <tomas.melin@vaisala.com>]

Cadence watchdog HW supports prescaler values of
8, 64, 512 and 4096.

Add support to select prescaler values of 8 and 64 for lower
input clock frequencies.

Prescaler value is selected to keep timeout resolution of 1 second.
For clock frequencies below 32kHz, 1 second resolution does
no longer hold, thereby returning an error.

Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>
---
 drivers/watchdog/cadence_wdt.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/watchdog/cadence_wdt.c b/drivers/watchdog/cadence_wdt.c
index 4657800d9d8e..39109b5721c1 100644
--- a/drivers/watchdog/cadence_wdt.c
+++ b/drivers/watchdog/cadence_wdt.c
@@ -33,16 +33,17 @@
 #define CDNS_WDT_COUNTER_VALUE_DIVISOR 0x1000
 
 /* Clock prescaler value and selection */
+#define CDNS_WDT_PRESCALE_8	8
 #define CDNS_WDT_PRESCALE_64	64
 #define CDNS_WDT_PRESCALE_512	512
 #define CDNS_WDT_PRESCALE_4096	4096
+#define CDNS_WDT_PRESCALE_SELECT_8	0
 #define CDNS_WDT_PRESCALE_SELECT_64	1
 #define CDNS_WDT_PRESCALE_SELECT_512	2
 #define CDNS_WDT_PRESCALE_SELECT_4096	3
 
-/* Input clock frequency */
-#define CDNS_WDT_CLK_10MHZ	10000000
-#define CDNS_WDT_CLK_75MHZ	75000000
+/* Base input clock frequency */
+#define CDNS_WDT_CLK_32KHZ 32768
 
 /* Counter maximum value */
 #define CDNS_WDT_COUNTER_MAX 0xFFF
@@ -304,7 +305,16 @@ static int cdns_wdt_probe(struct platform_device *pdev)
 	}
 
 	clock_f = clk_get_rate(wdt->clk);
-	if (clock_f <= CDNS_WDT_CLK_75MHZ) {
+	if (clock_f < CDNS_WDT_CLK_32KHZ) {
+		dev_err(&pdev->dev, "cannot find suitable clock prescaler\n");
+		return -ERANGE;
+	} else if (clock_f <= CDNS_WDT_CLK_32KHZ * CDNS_WDT_PRESCALE_8) {
+		wdt->prescaler = CDNS_WDT_PRESCALE_8;
+		wdt->ctrl_clksel = CDNS_WDT_PRESCALE_SELECT_8;
+	} else if (clock_f <= CDNS_WDT_CLK_32KHZ * CDNS_WDT_PRESCALE_64) {
+		wdt->prescaler = CDNS_WDT_PRESCALE_64;
+		wdt->ctrl_clksel = CDNS_WDT_PRESCALE_SELECT_64;
+	} else if (clock_f <= CDNS_WDT_CLK_32KHZ * CDNS_WDT_PRESCALE_512) {
 		wdt->prescaler = CDNS_WDT_PRESCALE_512;
 		wdt->ctrl_clksel = CDNS_WDT_PRESCALE_SELECT_512;
 	} else {
-- 
2.17.2