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Subject[PATCH RFC] soc: amlogic: meson-ee-pwrc: Drop the .shutdown callback from the driver
Date
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>
---
Neil, I would like you to specifically review (and re-test) the original
TOFIX part. I *believe* that I understand the original problem and hope
that my approach also works in that scenario, but I am not 100% sure.

Kevin, as you're my go-to genpd expert I am also asking you to review
this to point out any issues you see.


drivers/soc/amlogic/meson-ee-pwrc.c | 76 ++++++++++++-----------------
1 file changed, 31 insertions(+), 45 deletions(-)

diff --git a/drivers/soc/amlogic/meson-ee-pwrc.c b/drivers/soc/amlogic/meson-ee-pwrc.c
index 50bf5d2b828b..534c33ba31ce 100644
--- a/drivers/soc/amlogic/meson-ee-pwrc.c
+++ b/drivers/soc/amlogic/meson-ee-pwrc.c
@@ -353,10 +353,26 @@ static int meson_ee_pwrc_off(struct generic_pm_domain *domain)

if (pwrc_domain->num_clks) {
msleep(20);
- clk_bulk_disable_unprepare(pwrc_domain->num_clks,
- pwrc_domain->clks);
+
+ /*
+ * We are only allowed to turn off the clocks here if we
+ * have previously enabled them ourselves. In other words:
+ * for an "unused" power domain (which is not used by any
+ * power domain consumer) we have not enabled the clocks
+ * previously so we keep them in the state they are.
+ * This is relevant for the VPU power domain which may
+ * have been enabled by u-boot. If the display driver in
+ * Linux is disabled then we need to keep the clocks in
+ * the state as u-boot set them, otherwise the board will
+ * hang.
+ */
+ if (pwrc_domain->enabled)
+ clk_bulk_disable_unprepare(pwrc_domain->num_clks,
+ pwrc_domain->clks);
}

+ pwrc_domain->enabled = false;
+
return 0;
}

@@ -392,8 +408,14 @@ static int meson_ee_pwrc_on(struct generic_pm_domain *domain)
if (ret)
return ret;

- return clk_bulk_prepare_enable(pwrc_domain->num_clks,
- pwrc_domain->clks);
+ ret = clk_bulk_prepare_enable(pwrc_domain->num_clks,
+ pwrc_domain->clks);
+ if (ret)
+ return ret;
+
+ pwrc_domain->enabled = true;
+
+ return 0;
}

static int meson_ee_pwrc_init_domain(struct platform_device *pdev,
@@ -434,33 +456,11 @@ static int meson_ee_pwrc_init_domain(struct platform_device *pdev,
dom->base.power_on = meson_ee_pwrc_on;
dom->base.power_off = meson_ee_pwrc_off;

- /*
- * TOFIX: This is a special case for the VPU power domain, which can
- * be enabled previously by the bootloader. In this case the VPU
- * pipeline may be functional but no driver maybe never attach
- * to this power domain, and if the domain is disabled it could
- * cause system errors. This is why the pm_domain_always_on_gov
- * is used here.
- * For the same reason, the clocks should be enabled in case
- * we need to power the domain off, otherwise the internal clocks
- * prepare/enable counters won't be in sync.
- */
- if (dom->num_clks && dom->desc.get_power && !dom->desc.get_power(dom)) {
- ret = clk_bulk_prepare_enable(dom->num_clks, dom->clks);
- if (ret)
- return ret;
-
- dom->base.flags = GENPD_FLAG_ALWAYS_ON;
- ret = pm_genpd_init(&dom->base, NULL, false);
- if (ret)
- return ret;
- } else {
- ret = pm_genpd_init(&dom->base, NULL,
- (dom->desc.get_power ?
- dom->desc.get_power(dom) : true));
- if (ret)
- return ret;
- }
+ ret = pm_genpd_init(&dom->base, NULL,
+ (dom->desc.get_power ?
+ dom->desc.get_power(dom) : true));
+ if (ret)
+ return ret;

return 0;
}
@@ -528,19 +528,6 @@ static int meson_ee_pwrc_probe(struct platform_device *pdev)
return of_genpd_add_provider_onecell(pdev->dev.of_node, &pwrc->xlate);
}

-static void meson_ee_pwrc_shutdown(struct platform_device *pdev)
-{
- struct meson_ee_pwrc *pwrc = platform_get_drvdata(pdev);
- int i;
-
- for (i = 0 ; i < pwrc->xlate.num_domains ; ++i) {
- struct meson_ee_pwrc_domain *dom = &pwrc->domains[i];
-
- if (dom->desc.get_power && !dom->desc.get_power(dom))
- meson_ee_pwrc_off(&dom->base);
- }
-}
-
static struct meson_ee_pwrc_domain_data meson_ee_g12a_pwrc_data = {
.count = ARRAY_SIZE(g12a_pwrc_domains),
.domains = g12a_pwrc_domains,
@@ -606,7 +593,6 @@ MODULE_DEVICE_TABLE(of, meson_ee_pwrc_match_table);

static struct platform_driver meson_ee_pwrc_driver = {
.probe = meson_ee_pwrc_probe,
- .shutdown = meson_ee_pwrc_shutdown,
.driver = {
.name = "meson_ee_pwrc",
.of_match_table = meson_ee_pwrc_match_table,
--
2.31.1
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