[PATCH net-next] net: dsa: microchip: Adjust reset release timing to match reference reset circuit

KSZ8794CNX datasheet section 8.0 RESET CIRCUIT describes recommended
circuit for interfacing with CPU/FPGA reset consisting of 10k pullup
resistor and 10uF capacitor to ground. This circuit takes ~100 mS to
rise enough to release the reset.

For maximum supply voltage VDDIO=3.3V VIH=2.0V R=10kR C=10uF that is
                    VDDIO - VIH
  t = R * C * -ln( ------------- ) = 10000*0.00001*-(-0.93)=0.093 S
                       VDDIO
so we need ~95 mS for the reset to really de-assert, and then the
original 100uS for the switch itself to come out of reset. Simply
msleep() for 100 mS which fits the constraint with a bit of extra
space.

Fixes: 5b797980908a ("net: dsa: microchip: Implement recommended reset timing")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Michael Grzeschik <redacted>
Cc: Paul Barker <redacted>
---
 drivers/net/dsa/microchip/ksz_common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/dsa/microchip/ksz_common.c b/drivers/net/dsa/microchip/ksz_common.c
index 489963664443..389abfd27770 100644
--- a/drivers/net/dsa/microchip/ksz_common.c
+++ b/drivers/net/dsa/microchip/ksz_common.c

@@ -400,7 +400,7 @@ int ksz_switch_register(struct ksz_device *dev,
 		gpiod_set_value_cansleep(dev->reset_gpio, 1);
 		usleep_range(10000, 12000);
 		gpiod_set_value_cansleep(dev->reset_gpio, 0);
-		usleep_range(100, 1000);
+		msleep(100);
 	}
 
 	mutex_init(&dev->dev_mutex);

-- 
2.29.2

On 1/19/2021 6:01 PM, Marek Vasut wrote:

KSZ8794CNX datasheet section 8.0 RESET CIRCUIT describes recommended
circuit for interfacing with CPU/FPGA reset consisting of 10k pullup
resistor and 10uF capacitor to ground. This circuit takes ~100 mS to
rise enough to release the reset.

For maximum supply voltage VDDIO=3.3V VIH=2.0V R=10kR C=10uF that is
                    VDDIO - VIH
  t = R * C * -ln( ------------- ) = 10000*0.00001*-(-0.93)=0.093 S
                       VDDIO
so we need ~95 mS for the reset to really de-assert, and then the
original 100uS for the switch itself to come out of reset. Simply
msleep() for 100 mS which fits the constraint with a bit of extra
space.

This is nitpicking but the unit symbol for seconds is 's', not 'S' which
is for Siemens.

With that fixed:

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
-- 
Florian