Re: Leds-gpio discarding the entries in /sys/class/leds : Linux 5.4.38

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On 6/22/20 3:12 PM, Vishwanatha Subbanna wrote:
Hi Jacek,

On 22-Jun-2020, at 5:06 PM, Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx> wrote:

On 6/22/20 1:07 PM, Vishwanatha Subbanna wrote:
Hi Jacek,
On 22-Jun-2020, at 4:24 PM, Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx> wrote:

Hi Vishwanatha,

On 6/22/20 8:58 AM, Vishwanatha Subbanna wrote:
Thank you very much Jacek.
On 22-Jun-2020, at 3:12 AM, Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx> wrote:

Hi Vishwanatha,

On 6/20/20 7:25 PM, Vishwanatha Subbanna wrote:
Hi Jacek,
Thank you very much for the quick response. Greatly appreciate that.

You're welcome.

On 20-Jun-2020, at 3:27 AM, Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx> wrote:

Hi Vishwanatha,

Please refer to Documentation/devicetree/bindings/leds/leds-pca955x.txt.

At first glance I don't get why you have gpio-leds node, which is for
leds-gpio driver.
Not sure I understood it right.. But if you are asking me why I have "leds {"  and “gpio-leds” in there, then it is to get the entries in /sys/class/leds. The GPIOs from PCA9552 are connected to LED. Also, that is how we have had in the past, and that worked. Example: https://github.com/openbmc/linux/blob/dev-5.4/arch/arm/boot/dts/aspeed-bmc-opp-witherspoon.dts#L115

Thanks. Yeah, that looks OK, I had to take closer look at the driver.

The problem I am running into is for : https://github.com/openbmc/linux/blob/dev-5.4/arch/arm/boot/dts/aspeed-bmc-ibm-rainier.dts

On 6/19/20 3:34 PM, Vishwanatha Subbanna wrote:
Hello,
I am Vishwanath, working with IBM and looking for your help on one of the issues that I am running into. Would really appreciate help on this. I run Linux 5.4.38 I have 2 number of PCA9552 chips, one on the Planar and other on the card that is optionally pluggable. The optional card must be plugged prior to booting and is not hot pluggable. In my experiment, I am running *without* the optional card plugged in. In the device tree, I have a "leds {" section that looks like below for the PCA9552 that is on the planar and everything works fine and I can see /sys/class/leds/fan0
leds {
       compatible = "gpio-leds”;
       fan0 {
           retain-state-shutdown;
           default-state = "keep";
           gpios = <&pca0 0 GPIO_ACTIVE_LOW>;
       };
};
&i2c7 {
   status = "okay”;
   pca0: pca9552@61 {
      compatible = "nxp,pca9552";
       reg = <0x61>;
       #address-cells = <1>;
       #size-cells = <0>;
       gpio-controller;
       #gpio-cells = <2>;
       gpio@0 {
           reg = <0>;
           type = <PCA955X_TYPE_GPIO>;
};
   };
};
Similarly, if I update the device tree entry for PCA9552 for the card that is optionally pluggable, then I don’t see any leds entries in /sys/class/leds.

Please share your DT node after the update.

Pasting the GPIO_0 entry only here for brevity.
leds {
       compatible = "gpio-leds”;
       fan0 {
           retain-state-shutdown;
           default-state = "keep";
           gpios = <&pca0 0 GPIO_ACTIVE_LOW>;
       };
       nvmeslot0 {
           retain-state-shutdown;
           default-state = "keep";
           gpios = <&pca1 0 GPIO_ACTIVE_LOW>;
       };
};
&i2c7 {
   status = "okay”;
   pca0: pca9552@61 {
      compatible = "nxp,pca9552";
       reg = <0x61>;
       #address-cells = <1>;
       #size-cells = <0>;
       gpio-controller;
       #gpio-cells = <2>;
       gpio@0 {
           reg = <0>;
           type = <PCA955X_TYPE_GPIO>;
};
   };
};
&i2c13
{
   pca1: pca9552@60 {
      compatible = "nxp,pca9552";
      reg = <0x60>;
      #address-cells = <1>;
      #size-cells = <0>;
      gpio-controller;
      #gpio-cells = <2>;
      gpio@0 {
          reg = <0>;
          type = <PCA955X_TYPE_GPIO>;
      };
   };
};
Thanks
!! Vishwa !!
I don’t even see “fan0” that is on the PCA9552 on planar also. I was expecting that I should see “/sys/class/leds/fan0”.
However, I could see all the entries in “/proc/device-tree/leds”.
Data from the failure.
[    7.895757] leds-pca955x 7-0061: leds-pca955x: Using pca9552 16-bit LED driver at slave address 0x61
[    7.907659] leds-pca955x 7-0061: gpios 168...183

It is weird that you don't see "fan0" LED since this gpio seems to have
been properly registered according to this log.

This is exactly what I don’t understand. I would expect “fan0” to appear in /sys/class/leds. Is there any reason why this might not be appearing ?..

OK, now the reason is clear to me. If leds-gpio driver fails to register
any of the LEDs found in DT node it returns with an error from the
probe(), which results in unregistering any of the LEDs registered in
the previous iteration steps.

Look at the function gpio_leds_create() in
drivers/leds/leds-gpio.c.

Probably it is devm_fwnode_get_gpiod_from_child() that fails
while parsing nvmeslot0 node.
Is this how it is designed or a bug ?.. From a system standpoint, not having an optional card results in not seeing the ones that are present on the system. Would you think it is worthwhile to modify to not chuck off what is existing because something optional is not plugged in ?.. I believe the I2C driver handles this scenario by putting an error message but still consumes what is present.

Well, this code is in mainline for some time and we cannot guarantee
the someone does not rely on this behavior.


Should someone assume those behaviours ?.. Would it be okay to put an email to the gpio-leds community ?. Just in my opinion, I see a lot of value in modifying it. Also, is there an IRC where we can discuss this than the email ?

Changing this behavior could break someone's userspace, which is one of
the most vital no-nos in kernel development. Also, there is no such
entity like gpio-leds community - these are two separate kernel
subsystems, but leds-gpio driver is under LED maintainer's jurisdiction.

I don't think we need IRC discussion yet. Let's wait for the other
LED maintainer's opinion. Pavel?

You mentioned, that your card is not hot-pluggable so it is even more
justified to treat the two hardware setups as demanding a separate DT.


I mean, it is the same system that can either have a card on the slot or don’t have it. So, it’s not really a different system needing different DT.
Also, it has 3 slots and we can have multiple combinations :)

Otherwise you could probably employ DT overlays mechanism.


Hmm.. this looks interesting in a quick glance. However, I feel the current leds-gpio driver needs to be updated to not discard the valid ones. Besides someone may be counting on the existing behaviour, is there any reason why we want to maintain leds-gpio the way it is and not do what I2C driver does for example.

This is perfectly sufficient reason to not accept such a modification.

--
Best regards,
Jacek Anaszewski



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