Re: [PATCH] net: dsa: mt7530: disable LEDs before reset

Justin Swartz
2024-03-12T12:02:03Z

Arınç

On 2024-03-12 04:41, Arınç ÜNAL wrote:
> On 12.03.2024 03:07, Justin Swartz wrote:
>> I took a similar approach, with calls to dev_info()
>> throughout mt7530_setup() plus mt7530_write(), mt7530_read()
>> and mt7530_rmw() to get an idea of the flow of execution and
>> which registers were being manipulated.
>>
>> Calls to dev_info() affected timing, so I switched to using
>> trace_printk() and then read the output from the debugfs's
>> tracing/trace file instead of from the console.
>>
>> I attached a logic analyzer to ESW_P4_LED_0 and ESW_P3_LED_0,
>> and manually triggered sampling as soon as execution of the
>> kernel was reported on UART1.
>>
>>
>> -- Test #1 -----------------------------------------------------------
>>
>> For the sake of maximal reproducability, I removed the delay
>> between the assertion and deassertion of reset and added
>> HWTRAP value tracing:
>>
>> ---%---
>> --- a/drivers/net/dsa/mt7530.c
>> +++ b/drivers/net/dsa/mt7530.c
>> @@ -2243,7 +2243,6 @@ mt7530_setup(struct dsa_switch *ds)
>>      */
>>     if (priv->mcm) {
>>         reset_control_assert(priv->rstc);
>> -        usleep_range(1000, 1100);
>>         reset_control_deassert(priv->rstc);
>>     } else {
>>         gpiod_set_value_cansleep(priv->reset, 0);
>> @@ -2260,6 +2259,10 @@ mt7530_setup(struct dsa_switch *ds)
>>         return ret;
>>     }
>>
>> +    static char ht_xtal_fsel[4][6] = { "?????", "20MHz", "40MHz",
>> "25MHz" };
>> +    trace_printk("HWTRAP = %x, HT_XTAL_FSEL = %s\n",
>> +        val, ht_xtal_fsel[(val & HWTRAP_XTAL_MASK) >> 9]);
>> +
>>         id = mt7530_read(priv, MT7530_CREV);
>>         id >>= CHIP_NAME_SHIFT;
>>         if (id != MT7530_ID) {
>> ---%---
>>
>> (a) Without a cable connected to Port 3 (lan4) before reset, the
>> correct external crystal frequency is selected:
>>
>>               [3]      [4]     [6]
>>               :        :       :
>>               _________
>> ______________________________________
>> ESW_P4_LED_0           |_______|
>>                         _______
>> ESW_P3_LED_0  _________|       |______________________________________
>>
>>                         :     :
>>                         [5]...:
>>
>> [3] Port 4 LED Off. Port 3 LED On.
>> [4] Signals inverted. (reset_control_assert; reset_control_deassert)
>> [5] Period of 310 usec.
>> [6] Signals reflect the bootstrapped configuration.
>>
>>
>> ~ # sed -n -e '$s/^.*\. //p' /sys/kernel/debug/tracing/trace
>>     2.432646: mt7530_setup: HWTRAP = 7dcf, HT_XTAL_FSEL = 40MHz
>>
>>
>> (b) When a cable attached to an active peer is connected to
>> Port 3 (lan4) before reset, the incorrect crystal frequency
>> selection (0b11 = 25MHz) always occurs:
>>
>>               [7]      [8]     [10]    [12]
>>               :        :       :       :
>>               _________
>> ______________________________________
>> ESW_P4_LED_0           |_______|
>>               _________         _______
>> ESW_P3_LED_0           |_______|       |______________________________
>>
>>                         :     : :     :
>>                         [9]...: [11]..:
>>
>>  [7] Port 4 LED Off. Port 3 LED Off.
>>  [8] Signals inverted. (reset_control_assert; reset_control_deassert)
>>  [9] Period of 320 usec.
>> [10] Signals inverted.
>> [11] Period of 300 usec.
>> [12] Signals reflect the bootstrapped configuration.
>>
>> ~ # sed -n -e '$s/^.*\. //p' /sys/kernel/debug/tracing/trace
>>     2.432884: mt7530_setup: HWTRAP = 7fcf, HT_XTAL_FSEL = 25MHz
>>
>>
>> -- Test #2 -----------------------------------------------------------
>>
>> To attempt to determine which transitions are associated
>> with asserting and deasserting reset, I performed another
>> test with a delay of what I intended to be a 1 sec period
>> where the MT7530 is held in reset:
>>
>> ---%---
>> --- a/drivers/net/dsa/mt7530.c
>> +++ b/drivers/net/dsa/mt7530.c
>> @@ -2243,7 +2243,7 @@ mt7530_setup(struct dsa_switch *ds)
>>      */
>>     if (priv->mcm) {
>>         reset_control_assert(priv->rstc);
>> -        usleep_range(1000, 1100);
>> +        usleep_range(1000000, 1000000);
>>         reset_control_deassert(priv->rstc);
>>     } else {
>>         gpiod_set_value_cansleep(priv->reset, 0);
>> @@ -2260,6 +2260,10 @@ mt7530_setup(struct dsa_switch *ds)
>>         return ret;
>>     }
>>
>> +    static char ht_xtal_fsel[4][6] = { "?????", "20MHz", "40MHz",
>> "25MHz" };
>> +    trace_printk("HWTRAP = %x, HT_XTAL_FSEL = %s\n",
>> +        val, ht_xtal_fsel[(val & HWTRAP_XTAL_MASK) >> 9]);
>> +
>>     id = mt7530_read(priv, MT7530_CREV);
>>     id >>= CHIP_NAME_SHIFT;
>>     if (id != MT7530_ID) {
>> ---%---
>>
>> (a) Without a cable connected to Port 3 (lan4) before reset, the
>> correct external crystal frequency is again selected:
>>
>>               [13]     [14]    [16]
>>               :        :       :
>>               _________
>> ______________________________________
>> ESW_P4_LED_0           |_______|
>>                         _______
>> ESW_P3_LED_0  _________|       |______________________________________
>>
>>                         :     :
>>                         [15]..:
>>
>> [13] Port 4 LED Off. Port 3 LED On.
>> [14] Signals inverted. (reset_control_deassert?)
>> [15] Period of 310 usec.
>> [16] Signals reflect the bootstrapped configuration.
>>
>>
>> ~ # sed -n -e '$s/^.*\. //p' /sys/kernel/debug/tracing/trace
>>     3.437461: mt7530_setup: HWTRAP = 7dcf, HT_XTAL_FSEL = 40MHz
>>
>>
>> No difference is apparent in the timing diagram, compared
>> to the result from Test #1a, but it is obvious that the code
>> which follows the reset was executed about 1 second later.
>>
>>
>> (b) With a cable from an active peer connected to Port 3
>> (lan4) before reset, the correct crystal frequency
>> (0b10 = 40MHz) is selected:
>>
>>               [17]     [18]                    [20]   [22]
>>               :        :                       :      :
>>               ______________________ \ \ ______
>> _______________
>> ESW_P4_LED_0                         / /       |______|
>>               _________              \ \        ______
>> ESW_P3_LED_0           |____________ / / ______|      |_______________
>>                                      \ \
>>                         :                     : :    :
>>                         [19]..................: [21].:
>>
>> [17] Port 4 LED Off. Port 3 LED Off.
>> [18] ESW_P3_LED_0 set low. (reset_control_assert?)
>> [19] Period of 1000.325 msec.
>> [20] Signals inverted. (reset_control_deassert?)
>> [21] Period of 310 usec.
>> [22] Signals reflect the bootstrapped configuration.
>>
>>
>> ~ # sed -n -e '$s/^.*\. //p' /sys/kernel/debug/tracing/trace
>>     3.433235: mt7530_setup: HWTRAP = 7dcf, HT_XTAL_FSEL = 40MHz
>>
>>
>> So it appears that when reset is deasserted, the ESW_P4_LED_0
>> and ESW_P3_LED_0 levels are inverted for a period of about
>> 300 - 310 usec in Test #1a, #1b, #2a, and #2b.
>>
>> Co-incidentally, these inverted levels are the active low
>> representation of what ends up in HT_XTAL_FSEL. And in all
>> four examples, have been the inversion of what immediately
>> preceded reset_control_deassert().
>>
>>
>> -- Test #3 -----------------------------------------------------------
>>
>> Now it seems that there is a signature that can be used
>> to identify when reset_control_deassert() is executed,
>> the time of reset_control_assert()'s execution should be
>> between (at most) 1100 and (at least) 1000 usec prior
>> based on the unmodified reset logic.
>>
>> So this patch only includes the HT_XTAL_FSEL trace.
>>
>> ---%---
>> --- a/drivers/net/dsa/mt7530.c
>> +++ b/drivers/net/dsa/mt7530.c
>> @@ -2260,6 +2260,10 @@ mt7530_setup(struct dsa_switch *ds)
>>                 return ret;
>>         }
>>
>> +       static char ht_xtal_fsel[4][6] = { "?????", "20MHz", "40MHz",
>> "25MHz" };
>> +       trace_printk("HWTRAP = %x, HT_XTAL_FSEL = %s\n",
>> +               val, ht_xtal_fsel[(val & HWTRAP_XTAL_MASK) >> 9]);
>> +
>>         id = mt7530_read(priv, MT7530_CREV);
>>         id >>= CHIP_NAME_SHIFT;
>>         if (id != MT7530_ID) {
>> ---%---
>>
>> The purpose of the following examples are to show the
>> variance in how long it takes for ESW_P3_LED_0 to go low.
>>
>> (a) With a cable from an active peer connected to Port 3
>> (lan4) before reset, the correct crystal frequency
>> (0b10 = 40MHz) is selected.
>>
>>               [23]    [24]       [26]      [28]    [30]
>>               :       :          :         :       :
>>               _____________________________
>> __________________
>> ESW_P4_LED_0                               |_______|
>>               ___________________           _______
>> ESW_P3_LED_0                     |_________|       |__________________
>>
>>                        :          :       : :     :
>>                        :          [27]....: [29]..:
>>                        [25]...............:
>>
>> [23] Port 4 LED Off. Port 3 LED Off.
>> [24] Approximate point of reset_control_assert?
>> [25] Period of approximately 1000 - 1100 usec.
>> [26] ESW_P3_LED_0 finally goes low.
>> [27] Period of 415 usec.
>> [28] Signals inverted. (reset_control_deassert?)
>> [29] Period of 310 usec.
>> [30] Signals reflect the bootstrapped configuration.
>>
>>
>> (b) With a cable from an active peer connected to Port 3
>> (lan4) before reset, the correct crystal frequency
>> (0b10 = 40MHz) is selected.
>>
>>               [31]    [32]            [34] [36]    [38]
>>               :       :                  : :       :
>>               _____________________________
>> __________________
>> ESW_P4_LED_0                               |_______|
>>               ___________________________   _______
>> ESW_P3_LED_0                             |_|       |__________________
>>
>>                        :                  : :     :
>>                        :               [35] [37]..:
>>                        :                  :
>>                        [33]...............:
>>
>> [31] Port 4 LED Off. Port 3 LED Off.
>> [32] Approximate point of reset_control_assert?
>> [33] Period of approximately 1000 - 1100 usec.
>> [34] ESW_P3_LED_0 finally goes low.
>> [35] Period of 50 usec.
>> [36] Signals inverted. (reset_control_deassert?)
>> [37] Period of 310 usec.
>> [38] Signals reflect the bootstrapped configuration.
>>
>>
>> (c) With a cable from an active peer connected to Port 3
>> (lan4) before reset, an incorrect crystal frequency
>> (0b11 = 25MHz) is selected.
>>
>>               [45]    [46]                 [48]    [50]
>>               :       :                    :       :
>>               _____________________________
>> __________________
>> ESW_P4_LED_0                               |_______|
>>               _____________________________
>> ESW_P3_LED_0                               |__________________________
>>
>>                        :                  : :     :
>>                        :                  : [49]..:
>>                        :                  :
>>                        [47]...............:
>>
>> [45] Port 4 LED Off. Port 3 LED Off.
>> [46] Approximate point of reset_control_assert?
>> [47] Period of approximately 1000 - 1100 usec.
>> [48] Signals inverted. (reset_control_deassert?)
>> [49] Period of 315 usec.
>> [50] Signals reflect the bootstrapped configuration.
>>
>> ~ # sed -n -e '$s/^.*\. //p' /sys/kernel/debug/tracing/trace
>>     2.617819: mt7530_setup: HWTRAP = 7fcf, HT_XTAL_FSEL = 25MHz
>>
>> ~ # cat /proc/cmdline
>> console=ttyS0,57600 loglevel=7 printk.time=1 root=/dev/ram0 debug
>> rdinit=/linuxrc
>>
>>
>> -- End of Tests ------------------------------------------------------
>>
>> It seems that the incorrect crystal frequency is selected more
>> often when debugging messages are present (such as printk()
>> based approaches) and especially when loglevel=7 and printk.time=1
>> are specified on the command line.
>>
>> For the sake of reference: I disabled ethernet support in my build
>> of (mainline) U-boot, and my kernel configuration is a very lean
>> selection of options suited for IP routing and a few peripherals
>> on the I2C and SPI buses.
>>
>> My userland is confined to an initramfs built around busybox.
>>
>> I also disable gmac1 because I need a few of the rgmii2 pins for
>> modem control signalling.
>>
>> Regards
>> Justin
>>
>> PS: Yes, I only have access to MT7621AT SoCs.
>
> Thanks for the detailed presentation. I've simplified it to this, from
> what
> I understood.
>
> --- Currently
> -------------------------------------------------------------
>
> With a cable from an active peer connected to Port 3 (lan4) before
> reset:
>
> Test 1, 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_control_assert.
> [2] Period of 1000 - 1100 usec.
> [2-1] ESW_P3_LED_0 goes low.
> [2-2] Period of 415 usec.
> [3] reset_control_deassert.
> [4] Period of 310 usec. HWTRAP register is populated with bootstrapped
> XTAL frequency.
> [5] Signals reflect the bootstrapped configuration.
>
> Test 2, an incorrect crystal frequency (0b11 = 25MHz) is selected.
>
> [2] [4] [6]
> : : :
> _____________________________ __________________
> ESW_P4_LED_0 |_______|
> _____________________________
> ESW_P3_LED_0 |__________________________
>
> : : : :
> : : [5]...:
> : :
> [3]................:
>
> [1] reset_control_assert.
> [2] Period of 1000 - 1100 usec.
> [3] reset_control_deassert.
> [5] Period of 315 usec. HWTRAP register is populated with incorrect
> XTAL frequency.
> [6] Signals reflect the bootstrapped configuration.
>
> --- 1 second delay between assert and deassert
> ----------------------------
>
> With a cable from an active peer connected to Port 3 (lan4) 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_control_assert.
> [3] Period of 1000.325 msec.
> [2-1] ESW_P3_LED_0 goes low.
> [2-2] Remaining period of 1000.325 msec.
> [3] reset_control_deassert.
> [4] Period of 310 usec. HWTRAP register is populated with bootstrapped
> XTAL frequency.
> [5] Signals reflect the bootstrapped configuration.
>
> ---
>
> Wouldn't it be a better idea to 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) instead of disabling the LED
> controller and delaying before reset?

I've done some additional tests to see what the difference would be
between increasing the reset hold period vs. disabling the LEDs then
sleeping before reset.

I wanted to know:

When an active link is present on Port 3 at boot, what are the
minimum, maximum and average periods between ESW_P3_LED_0
going low and the signal inversion that seems to co-incide with
reset_control_deassert() for each approach?

I created two patches:

WITH INCREASED RESET DELAY

As I submitted a patch that added an intended 1000 - 1100 usec delay
before reset, I thought it would be fair to increase the reset hold
period by the same value.

---%---
--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -2243,11 +2243,11 @@ mt7530_setup(struct dsa_switch *ds)
*/
if (priv->mcm) {
reset_control_assert(priv->rstc);
- usleep_range(1000, 1100);
+ usleep_range(2000, 2200);
reset_control_deassert(priv->rstc);
} else {
gpiod_set_value_cansleep(priv->reset, 0);
- usleep_range(1000, 1100);
+ usleep_range(2000, 2200);
gpiod_set_value_cansleep(priv->reset, 1);
}
---%---

DISABLE LEDS BEFORE RESET

Reset hold period unchanged from the intended 1000 - 1100 usec.

---%---
--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -2238,6 +2238,12 @@ mt7530_setup(struct dsa_switch *ds)
}
}

+ /* Disable LEDs before reset to prevent the MT7530 sampling a
+ * potentially incorrect HT_XTAL_FSEL value.
+ */
+ mt7530_write(priv, MT7530_LED_EN, 0);
+ usleep_range(1000, 1100);
+
/* Reset whole chip through gpio pin or memory-mapped registers
for
* different type of hardware
*/
---%---

I ran 20 tests per patch, applied exclusively. 40 tests in total.

<-- ESW_P3_LED_0 Low Period before Reset Deassertion -->

TEST WITH INCREASED RESET DELAY DISABLE LEDS BEFORE RESET
# (usec) (usec)
-------------------------------------------------------------------
1 182 4790
2 370 3470
3 240 4635
4 1475 4850
5 70 4775
6 2730 4575
7 3180 4565
8 265 5650
9 270 4690
10 1525 4450
11 3210 4735
12 120 4690
13 185 4625
14 305 7020
15 2975 4720
16 245 4675
17 350 4740
18 80 17920
19 150 17665
20 100 4620

Min 70 3470
Max 3210 17920

Mean 270 4720
Avg 923.421 6161.579

Every test resulted in a 40MHz HT_XTAL_FSEL, but after seeing 70 usec
and 80 usec periods I wondered how many more tests it may take before
an 25MHz HT_XTAL_FSEL appears.

I was also surprised by the 17920 usec and 17665 usec periods listed
under the DISABLED LEDS BEFORE RESET column. Nothing unusual seemed
to be happening, at least as far as the kernel message output was
concerned.

What do you make of these results?

> One MT7531 pin used for an LED is also used to decide the crystal
> frequency
> between 40MHz and 25Mhz. We should implement this there as well.
>
> Arınç