net: dsa: mt7530: disable LEDs before reset

net: dsa: mt7530: disable LEDs before reset

Disable LEDs just before resetting the MT7530 to avoid
situations where the ESW_P4_LED_0 and ESW_P3_LED_0 pin
states may cause an unintended external crystal frequency
to be selected.

The HT_XTAL_FSEL (External Crystal Frequency Selection)
field of HWTRAP (the Hardware Trap register) stores a
2-bit value that represents the state of the ESW_P4_LED_0
and ESW_P4_LED_0 pins (seemingly) sampled just after the
MT7530 has been reset, as:

    ESW_P4_LED_0    ESW_P3_LED_0    Frequency
    -----------------------------------------
    0               1               20MHz
    1               0               40MHz
    1               1               25MHz

The value of HT_XTAL_FSEL is bootstrapped by pulling
ESW_P4_LED_0 and ESW_P3_LED_0 up or down accordingly,
but:

  if a 40MHz crystal has been selected and
  the ESW_P3_LED_0 pin is high during reset,

  or a 20MHz crystal has been selected and
  the ESW_P4_LED_0 pin is high during reset,

  then the value of HT_XTAL_FSEL will indicate
  that a 25MHz crystal is present.

By default, the state of the LED pins is PHY controlled
to reflect the link state.

To illustrate, if a board has:

  5 ports with active low LED control,
  and HT_XTAL_FSEL bootstrapped for 40MHz.

When the MT7530 is powered up without any external
connection, only the LED associated with Port 3 is
illuminated as ESW_P3_LED_0 is low.

In this state, directly after mt7530_setup()'s reset
is performed, the HWTRAP register (0x7800) reflects
the intended HT_XTAL_FSEL (HWTRAP bits 10:9) of 40MHz:

  mt7530-mdio mdio-bus:1f: mt7530_read: 00007800 == 00007dcf

  >>> bin(0x7dcf >> 9 & 0b11)
  '0b10'

But if a cable is connected to Port 3 and the link
is active before mt7530_setup()'s reset takes place,
then HT_XTAL_FSEL seems to be set for 25MHz:

  mt7530-mdio mdio-bus:1f: mt7530_read: 00007800 == 00007fcf

  >>> bin(0x7fcf >> 9 & 0b11)
  '0b11'

Once HT_XTAL_FSEL reflects 25MHz, none of the ports
are functional until the MT7621 (or MT7530 itself)
is reset.

By disabling the LED pins just before reset, the chance
of an unintended HT_XTAL_FSEL value is reduced.

Signed-off-by: Justin Swartz <justin.swartz@risingedge.co.za>
Link: https://lore.kernel.org/r/20240305043952.21590-1-justin.swartz@risingedge.co.za
Signed-off-by: Jakub Kicinski <kuba@kernel.org>