[PATCH v3 2/2] net: dsa: mt7530: Use GPIO polarity to generate correct reset sequence

[Chen Minqiang <ptpt52@gmail.com>]

The MT7530/MT7531 reset pin is active-low in hardware, but the driver
historically hardcoded a high-active reset sequence by toggling the GPIO
as 0 → 1. This only worked because several DTS files incorrectly marked
the reset GPIO as active-high, making both DTS and driver wrong in the
same way.

This patch changes the driver to respect the GPIO polarity using
gpiod_is_active_low(), and generates the reset sequence as:

    assert   = drive logical active level
    deassert = drive logical inactive level

As a result, both cases now correctly produce the required
high → low → high transition on the actual reset pin.

Compatibility
-------------

This change makes the driver fully backward-compatible with older,
incorrect DTS files that marked the reset line as GPIO_ACTIVE_HIGH:

 * Old DTS marked active-high:
       is_active_low = 0
       driver drives 0 → 1
       actual levels: high → low → high  (correct)

 * New DTS marked active-low:
       is_active_low = 1
       driver drives 1 → 0
       actual levels: high → low → high  (correct)

Therefore, regardless of whether a DTS is old or new, correct or
incorrect, the driver now generates the correct electrical reset pulse.

Going forward, DTS files should use GPIO_ACTIVE_LOW to match the
hardware, but no regressions will occur with older DTS blobs.

Signed-off-by: Chen Minqiang <ptpt52@gmail.com>
---
 drivers/net/dsa/mt7530.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
index 548b85befbf4..615e9a5709ca 100644
--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -2405,9 +2405,10 @@ mt7530_setup(struct dsa_switch *ds)
 		usleep_range(5000, 5100);
 		reset_control_deassert(priv->rstc);
 	} else {
-		gpiod_set_value_cansleep(priv->reset, 0);
+		int is_active_low = !!gpiod_is_active_low(priv->reset);
+		gpiod_set_value_cansleep(priv->reset, is_active_low);
 		usleep_range(5000, 5100);
-		gpiod_set_value_cansleep(priv->reset, 1);
+		gpiod_set_value_cansleep(priv->reset, !is_active_low);
 	}
 
 	/* Waiting for MT7530 got to stable */
@@ -2643,9 +2644,10 @@ mt7531_setup(struct dsa_switch *ds)
 		usleep_range(5000, 5100);
 		reset_control_deassert(priv->rstc);
 	} else {
-		gpiod_set_value_cansleep(priv->reset, 0);
+		int is_active_low = !!gpiod_is_active_low(priv->reset);
+		gpiod_set_value_cansleep(priv->reset, is_active_low);
 		usleep_range(5000, 5100);
-		gpiod_set_value_cansleep(priv->reset, 1);
+		gpiod_set_value_cansleep(priv->reset, !is_active_low);
 	}
 
 	/* Waiting for MT7530 got to stable */
-- 
2.17.1