[PATCH 6.6 603/638] net: dsa: mt7530: prevent possible incorrect XTAL frequency selection

Sasha Levin
2024-03-24T23:11:15Z

From: Arınç ÜNAL <arinc.unal@arinc9.com>

[ Upstream commit f490c492e946d8ffbe65ad4efc66de3c5ede30a4 ]

On MT7530, the HT_XTAL_FSEL field of the HWTRAP register stores a 2-bit
value that represents the frequency of the crystal oscillator connected to
the switch IC. The field is populated by the state of the ESW_P4_LED_0 and
ESW_P4_LED_0 pins, which is done right after reset is deasserted.

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

On MT7531, the XTAL25 bit of the STRAP register stores this. The LAN0LED0
pin is used to populate the bit. 25MHz when the pin is high, 40MHz when
it's low.

These pins are also used with LEDs, therefore, their state can be set to
something other than the bootstrapping configuration. For example, a link
may be established on port 3 before the DSA subdriver takes control of the
switch which would set ESW_P3_LED_0 to high.

Currently on mt7530_setup() and mt7531_setup(), 1000 - 1100 usec delay is
described between reset assertion and deassertion. Some switch ICs in real
life conditions cannot always have these pins set back to the bootstrapping
configuration before reset deassertion in this amount of delay. This causes
wrong crystal frequency to be selected which puts the switch in a
nonfunctional state after reset deassertion.

The tests below are conducted on an MT7530 with a 40MHz crystal oscillator
by Justin Swartz.

With a cable from an active peer connected to port 3 before reset, an
incorrect crystal frequency (0b11 = 25MHz) is selected:

[1] [3] [5]
: : :
_____________________________ __________________
ESW_P4_LED_0 |_______|
_____________________________
ESW_P3_LED_0 |__________________________

: : : :
: : [4]...:
: :
[2]................:

[1] Reset is asserted.
[2] Period of 1000 - 1100 usec.
[3] Reset is deasserted.
[4] Period of 315 usec. HWTRAP register is populated with incorrect
XTAL frequency.
[5] Signals reflect the bootstrapped configuration.

Increase the delay between reset_control_assert() and
reset_control_deassert(), and gpiod_set_value_cansleep(priv->reset, 0) and
gpiod_set_value_cansleep(priv->reset, 1) to 5000 - 5100 usec. This amount
ensures a higher possibility that the switch IC will have these pins back
to the bootstrapping configuration before reset deassertion.

With a cable from an active peer connected to port 3 before reset, the
correct crystal frequency (0b10 = 40MHz) is selected:

[1] [2-1] [3] [5]
: : : :
_____________________________ __________________
ESW_P4_LED_0 |_______|
___________________ _______
ESW_P3_LED_0 |_________| |__________________

: : : : :
: [2-2]...: [4]...:
[2]................:

[1] Reset is asserted.
[2] Period of 5000 - 5100 usec.
[2-1] ESW_P3_LED_0 goes low.
[2-2] Remaining period of 5000 - 5100 usec.
[3] Reset is deasserted.
[4] Period of 310 usec. HWTRAP register is populated with bootstrapped
XTAL frequency.
[5] Signals reflect the bootstrapped configuration.

ESW_P3_LED_0 low period before reset deassertion:

5000 usec
- 5100 usec
TEST RESET HOLD
# (usec)
---------------------
1 5410
2 5440
3 4375
4 5490
5 5475
6 4335
7 4370
8 5435
9 4205
10 4335
11 3750
12 3170
13 4395
14 4375
15 3515
16 4335
17 4220
18 4175
19 4175
20 4350

Min 3170
Max 5490

Median 4342.500
Avg 4466.500

Revert commit 2920dd92b980 ("net: dsa: mt7530: disable LEDs before reset").
Changing the state of pins via reset assertion is simpler and more
efficient than doing so by setting the LED controller off.

Fixes: b8f126a8d543 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Fixes: c288575f7810 ("net: dsa: mt7530: Add the support of MT7531 switch")
Co-developed-by: Justin Swartz <justin.swartz@risingedge.co.za>
Signed-off-by: Justin Swartz <justin.swartz@risingedge.co.za>
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/net/dsa/mt7530.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
index 4b6ac3f29f8f1..38fc968cd6d21 100644
--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -2244,11 +2244,11 @@ mt7530_setup(struct dsa_switch *ds)
*/
if (priv->mcm) {
reset_control_assert(priv->rstc);
- usleep_range(1000, 1100);
+ usleep_range(5000, 5100);
reset_control_deassert(priv->rstc);
} else {
gpiod_set_value_cansleep(priv->reset, 0);
- usleep_range(1000, 1100);
+ usleep_range(5000, 5100);
gpiod_set_value_cansleep(priv->reset, 1);
}

@@ -2450,11 +2450,11 @@ mt7531_setup(struct dsa_switch *ds)
*/
if (priv->mcm) {
reset_control_assert(priv->rstc);
- usleep_range(1000, 1100);
+ usleep_range(5000, 5100);
reset_control_deassert(priv->rstc);
} else {
gpiod_set_value_cansleep(priv->reset, 0);
- usleep_range(1000, 1100);
+ usleep_range(5000, 5100);
gpiod_set_value_cansleep(priv->reset, 1);
}

--
2.43.0