Re: [PATCH RFC] soc: amlogic: meson-ee-pwrc: Drop the .shutdown callback from the driver

[Martin Blumenstingl]

On Thu, Apr 29, 2021 at 10:37 PM Martin Blumenstingl
<martin.blumenstingl@xxxxxxxxxxxxxx> 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@xxxxxxxx>
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@xxxxxxxxxxxxxx>
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