[PATCH 0/8] [SRU] [X/raspi2] Fix led triggers on Rpi3b+

[Paolo Pisati]

BugLink: http://bugs.launchpad.net/bugs/1808366

Impact:

User are reporting that the green led (the one next to the red power led) is
not working on the RaspberryPi 3B+ board.

After debugging the issue, this is what i found:

1) during boot, all leds are initialized in a function loop, and if one of them
fails to setup, the loop tears down all initialized instances:

...
[ 2.299216] leds-gpio: probe of soc:leds failed with error -2
...

in this particular case, it's not the green action led that fails to
initialize, but it's red power led that fails and the loop tears down both of
them but since the red (by default) is connected to Vcc, it stays lit (see
point 3 below for more info on the wiring) so we have the impression that the
one to fail is the green one.

This can easily tested by adding debug to
drivers/leds/leds-gpio.c::gpio_leds_create() and
drivers/leds/leds-gpio.c::gpio_led_probe(), or commenting out the red pwr_led
block in arch/arm/boot/dts/bcm2710-rpi-3-b-plus.dts - the board will boot up,
the above leds-gpio error will disappear and the green action led will work
properly.

2) the pwr_led in the rpi3 b+ is not available direcly to the OS, instead only
the Broadcom firmware can access its gpio, thus a new mechanism (the exgpio
driver) that uses the bcm firmware as a middleman was developed: using a
mailbox mechanism, messages are sent from the Linux kernel to the Broadcom
firmware to query or set the status of the led, this way it's possible to plumb
the Linux led subsystem to this gpio

3) the biasing of the two leds (power and action) is "opposite" and can be
easily confirmed by looking at the board schematics[1]:

-D5 (the action led) uses the transistor Q5 in a switch configuration - when Q5
is not biased, it acts as an open switch and no current goes through the led -
so by default the led is off

-D6 (the power led) uses Q4 in a sink configuration - when Q4 is off, current
normally flows through the led, while when Q4 is biased, all current is sinked
to GND via Q4 - the led by default on

Fix:

The fix is composed of 8 patches:

1f50a81 UBUNTU: [Config] GPIO_BCM_EXP=y
53bc3cc UBUNTU: SAUCE: dts: rpi3: remove hpd-gpios overwrite
a1252a5 UBUNTU: SAUCE: dts: cm3: revert hpd-gpios to gpio
0d136d8 UBUNTU: SAUCE: make pwr_led GPIO_ACTIVE_LOW
e2d3ba2 UBUNTU: SAUCE: make it build on bcm2709
245fc18 Revert "UBUNTU: SAUCE: dts: use the virtgpio driver"
be25728 bcm2835-gpio-exp: Copy/paste error adding base twice
4ccdf22 bcm2835-gpio-exp: Driver for GPIO expander via mailbox service

>From the bottom:

-patch 0001 and 002 contain the "gpio via firmware" exgpio driver (and a fix
for it) - those are the original patches that are present in the Github's
RaspberryPi repository[2] rpi-4.9.y branch, and they were cherry-picked cleanly
from it (minus same mechanical changes in Kconfig and Makefile surrounding
context to make it apply).

-patch 0003 reverts a change that tried to pilot pwr led gpio using the default
virtgpio driver

-patch 004 fixes the arch name - in 4.9 where the driver originated, the
Raspberry arch was called "ARCH_BCM2835", while in Xenial 4.4 is
"ARCH_BCM2709"

-patch 005 fixes the pwr led biasing level

-patch 006 and 007 reduce the "regression surface": when patch 001 was
introduced in 4.9, the hdmi phy in the RaspberryPi3 and CM3 was moved to use
the new driver, but since our hdmi driver is significantly different from the
one in the rpy-4.9 tree, and since this bug deal only with leds and noone has
opened one against the hvmi video output, to reduce the regression surface, i
reverted these chunks in the new driver (using these two SAUCE patches) and
kept using the mechanism we used so far in Xenial.

-patch 008 enable the new driver

By apply these patches, the power led correctly initialize during boot, and as
a consequence the action led work too.

How to test:

Add this to config.txt:

dtparam=act_led_trigger=heartbeat
dtparam=pwr_led_trigger=heartbeat

and reboot - on a non-patched kernel, the power led (red) will be always-on,
while the action led (green) will be off.  On a patched kernel, both leds will
blink intermittently.

Regression:

It's new code, so there's always regression potential in it, but by reducing
the scope of the original patch, the impact is limited to the led code, and
only applies to the RaspberryPi3B+ board.

1: https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/rpi_SCH_3bplus_1p0_reduced.pdf
2: https://github.com/raspberrypi/linux

Dave Stevenson (2):
  bcm2835-gpio-exp: Driver for GPIO expander via mailbox service
  bcm2835-gpio-exp: Copy/paste error adding base twice

Paolo Pisati (6):
  Revert "UBUNTU: SAUCE: dts: use the virtgpio driver"
  UBUNTU: SAUCE: make it build on bcm2709
  UBUNTU: SAUCE: make pwr_led GPIO_ACTIVE_LOW
  UBUNTU: SAUCE: dts: cm3: revert hpd-gpios to gpio
  UBUNTU: SAUCE: dts: rpi3: remove hpd-gpios overwrite
  UBUNTU: [Config] GPIO_BCM_EXP=y

 arch/arm/boot/dts/bcm2710-rpi-3-b-plus.dts |   6 +-
 arch/arm/boot/dts/bcm2710-rpi-3-b.dts      |  18 ++
 arch/arm/configs/bcm2709_defconfig         |   1 +
 debian.raspi2/config/config.common.ubuntu  |   1 +
 drivers/gpio/Kconfig                       |   7 +
 drivers/gpio/Makefile                      |   1 +
 drivers/gpio/gpio-bcm-exp.c                | 254 +++++++++++++++++++++++++++++
 include/soc/bcm2835/raspberrypi-firmware.h |   4 +
 8 files changed, 289 insertions(+), 3 deletions(-)
 create mode 100644 drivers/gpio/gpio-bcm-exp.c

-- 
2.7.4