Re: [PATCH] pmdomain: mediatek: fix race condition in power on/power off sequences

AngeloGioacchino Del Regno
2023-11-29T12:52:52Z

Il 29/11/23 12:31, Eugen Hristev ha scritto:
> It can happen that during the power off sequence for a power domain
> another power on sequence is started, and it can lead to powering on and
> off in the same time for the similar power domain.
> This can happen if parallel probing occurs: one device starts probing, and
> one power domain is probe deferred, this leads to all power domains being
> rolled back and powered off, while in the same time another device starts
> probing and requests powering on the same power domains or similar.
>
> This was encountered on MT8186, when the sequence is :
> Power on SSUSB
> Power on SSUSB_P1
> Power on DIS
> -> probe deferred
> Power off DIS
> Power off SSUSB_P1
> Power off SSUSB
>
> During the sequence of powering off SSUSB, some new similar sequence starts,
> and during the power on of SSUSB, clocks are enabled.
> In this case, powering off SSUSB fails from the first sequence, because
> power off ACK bit check times out (as clocks are powered back on by the second
> sequence). In consequence, powering it on also times out, and it leads to
> the whole power domain in a bad state.
>
> To solve this issue, added a mutex that locks the whole power off/power on
> sequence such that it would never happen that multiple sequences try to
> enable or disable the same power domain in parallel.
>
> Fixes: 59b644b01cf4 ("soc: mediatek: Add MediaTek SCPSYS power domains")
> Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>

I don't think that it's a race between genpd_power_on() and genpd_power_off() calls
at all, because genpd *does* have locking after all... at least for probe and for
parents of a power domain (and more anyway).

As far as I remember, what happens when you start .probe()'ing a device is:
platform_probe() -> dev_pm_domain_attach() -> genpd_dev_pm_attach()

There, you end up with

if (power_on) {
genpd_lock(pd);
ret = genpd_power_on(pd, 0);
genpd_unlock(pd);
}

...but when you fail probing, you go with genpd_dev_pm_detach(), which then calls

/* Check if PM domain can be powered off after removing this device. */
genpd_queue_power_off_work(pd);

but even then, you end up being in a worker doing

genpd_lock(genpd);
genpd_power_off(genpd, false, 0);
genpd_unlock(genpd);

...so I don't understand why this mutex can resolve the situation here (also: are
you really sure that the race is solved like that?)

I'd say that this probably needs more justification and a trace of the actual
situation here.

Besides, if this really resolves the issue, I would prefer seeing variants of
scpsys_power_{on,off}() functions, because we anyway don't need to lock mutexes
during this driver's probe (add_subdomain calls scpsys_power_on()).
In that case, `scpsys_power_on_unlocked()` would be an idea... but still, please
analyze why your solution works, if it does, because I'm not convinced.

Cheers,
Angelo

> ---
> drivers/pmdomain/mediatek/mtk-pm-domains.c | 24 +++++++++++++++++-----
> 1 file changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/pmdomain/mediatek/mtk-pm-domains.c b/drivers/pmdomain/mediatek/mtk-pm-domains.c
> index d5f0ee05c794..4f136b47e539 100644
> --- a/drivers/pmdomain/mediatek/mtk-pm-domains.c
> +++ b/drivers/pmdomain/mediatek/mtk-pm-domains.c
> @@ -9,6 +9,7 @@
> #include <linux/io.h>
> #include <linux/iopoll.h>
> #include <linux/mfd/syscon.h>
> +#include <linux/mutex.h>
> #include <linux/of.h>
> #include <linux/of_clk.h>
> #include <linux/platform_device.h>
> @@ -56,6 +57,7 @@ struct scpsys {
> struct device *dev;
> struct regmap *base;
> const struct scpsys_soc_data *soc_data;
> + struct mutex mutex;
> struct genpd_onecell_data pd_data;
> struct generic_pm_domain *domains[];
> };
> @@ -238,9 +240,13 @@ static int scpsys_power_on(struct generic_pm_domain *genpd)
> bool tmp;
> int ret;
>
> + mutex_lock(&scpsys->mutex);
> +
> ret = scpsys_regulator_enable(pd->supply);
> - if (ret)
> + if (ret) {
> + mutex_unlock(&scpsys->mutex);
> return ret;
> + }
>
> ret = clk_bulk_prepare_enable(pd->num_clks, pd->clks);
> if (ret)
> @@ -291,6 +297,7 @@ static int scpsys_power_on(struct generic_pm_domain *genpd)
> goto err_enable_bus_protect;
> }
>
> + mutex_unlock(&scpsys->mutex);
> return 0;
>
> err_enable_bus_protect:
> @@ -305,6 +312,7 @@ static int scpsys_power_on(struct generic_pm_domain *genpd)
> clk_bulk_disable_unprepare(pd->num_clks, pd->clks);
> err_reg:
> scpsys_regulator_disable(pd->supply);
> + mutex_unlock(&scpsys->mutex);
> return ret;
> }
>
> @@ -315,13 +323,15 @@ static int scpsys_power_off(struct generic_pm_domain *genpd)
> bool tmp;
> int ret;
>
> + mutex_lock(&scpsys->mutex);
> +
> ret = scpsys_bus_protect_enable(pd);
> if (ret < 0)
> - return ret;
> + goto err_mutex_unlock;
>
> ret = scpsys_sram_disable(pd);
> if (ret < 0)
> - return ret;
> + goto err_mutex_unlock;
>
> if (pd->data->ext_buck_iso_offs && MTK_SCPD_CAPS(pd, MTK_SCPD_EXT_BUCK_ISO))
> regmap_set_bits(scpsys->base, pd->data->ext_buck_iso_offs,
> @@ -340,13 +350,15 @@ static int scpsys_power_off(struct generic_pm_domain *genpd)
> ret = readx_poll_timeout(scpsys_domain_is_on, pd, tmp, !tmp, MTK_POLL_DELAY_US,
> MTK_POLL_TIMEOUT);
> if (ret < 0)
> - return ret;
> + goto err_mutex_unlock;
>
> clk_bulk_disable_unprepare(pd->num_clks, pd->clks);
>
> scpsys_regulator_disable(pd->supply);
>
> - return 0;
> +err_mutex_unlock:
> + mutex_unlock(&scpsys->mutex);
> + return ret;
> }
>
> static struct
> @@ -700,6 +712,8 @@ static int scpsys_probe(struct platform_device *pdev)
> return PTR_ERR(scpsys->base);
> }
>
> + mutex_init(&scpsys->mutex);
> +
> ret = -ENODEV;
> for_each_available_child_of_node(np, node) {
> struct generic_pm_domain *domain;