On Thu, Apr 29, 2021 at 10:37 PM Martin Blumenstingl
[off-list ref] wrote:

Stefan reports that rebooting his ODROID-N2+ (using a G12B SoC) results
in the board hanging. His kernel config uses:
  CONFIG_MESON_EE_PM_DOMAINS=y
  CONFIG_DRM_MESON=m

He reports that his kernel config results in the DRM driver's .shutdown
callback to be executed after the power domain driver's .shutdown
callback. That's problematic because meson_ee_pwrc_shutdown disables the
clock which are used by the VPU IP. This causes the board to hang.

Further he reports that changing from CONFIG_DRM_MESON=m to
CONFIG_DRM_MESON=y reverses the order in which the DRM and power domain
driver's shutdown functions are executed, making the reboot successful.

The reason why we use meson_ee_pwrc_shutdown is because of the VPU power
domain (which is causing the problem described above). It can be left
enabled by u-boot. According to the original TOFIX comment in
meson_ee_pwrc_init_domain we need to be careful because disabling the
power domain could "cause system errors". As a workaround the clocks
are manually enabled in meson_ee_pwrc_init_domain and the power domain
is marked as GENPD_FLAG_ALWAYS_ON (so it can never be turned off).

Experimenting has shown that the power domain itself can be disabled as
long as we keep the clocks enabled if u-boot enabled the power domain
but we don't have any driver enabled for the VPU (CONFIG_DRM_MESON=n).

Keeping the clocks enabled is the responsibility of the CCF drivers, not
the power domain driver. Even better: this is already covered as all
gates in the VPU and VAPB tree on GX an G12 SoCs have the
CLK_IGNORE_UNUSED flag set, meaning: if the bootloader has previously
enabled the clock we're not touching it until a driver explicitly asks
to enable (and then disable) it. In case of CONFIG_DRM_MESON=n we're
never calling meson_ee_pwrc_on, meaning that we always keep the state of
the clocks as set by u-boot.

The original TOFIX comment also mentioned that we need to make sure not
to mess up the clock's prepare/enable ref-counters. This is the only
requirement that's left for the meson-ee-pwrc driver that needs to be
managed for the VPU power domain.

Three steps can improve this situation:
- Don't prepare and enable the clocks (just to fix the ref-counting) in
  meson_ee_pwrc_init_domain if u-boot left that power domain enabled.
  Instead, remember if the clocks are enabled in meson_ee_pwrc_{on,off}
  and only disable them if we have previously turned them on ourselves.
- Drop GENPD_FLAG_ALWAYS_ON as we can always manage the state of the VPU
  power domain if both the power domain controller and DRM driver are
  enabled (=m or =y). If the power domain driver is enabled but the DRM
  driver is disabled we can still use meson_ee_pwrc_off because it's not
  trying to disable the clocks anymore
- Drop meson_ee_pwrc_shutdown as it's the responsibility of the genpd
  framework to call meson_ee_pwrc_off when needed (either when a power
  domain is being disabled - regardless of whether it's was used by a
  driver before or not). Now there's also no more shutdown callback
  ordering dependency between the power domain driver and other drivers
  anymore.

Fixes: eef3c2ba0a42a6 ("soc: amlogic: Add support for Everything-Else power domains controller")
Reported-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>

unfortunately I need to add:
Nacked-by: myself

it turns out that the genpd framework does not call .power_on when the
power domain is already powered on during initialization
(pm_genpd_init)

I wonder if we need to extend the genpd to handle this use-case.
CCF (common clock framework) for example has CLK_IGNORE_UNUSED and a
.disable_unused callback which can be used to specifically manage the
"unused" use-case

my idea #1:
- add a GENPD_FLAG_IGNORE_UNUSED flag
- set it for the VPU power domain
- skip "unused" power domains which have the GENPD_FLAG_IGNORE_UNUSED flag set
- drop the GENPD_FLAG_ALWAYS_ON flag from the VPU power domain

my idea #2:
- a power_off_unused callback (with the same arguments as power_off)
could be introduced
- in pm_genpd_init we check if that callback is initialized, if not we
assign the power_off callback
- instead of using the power_off callback when disabling an unused
power domain the new power_off_unused callback is used
- for the meson-ee-pwrc we implement a special meson_ee_pwrc_power_off
function that is a no-op


Best regards,
Martin