[PATCH RFC] pmdomain: ti-sci: Set PD on/off state according to the HW state

[Ulf Hansson]

On Tue, 22 Oct 2024 at 08:41, Tomi Valkeinen
<tomi.valkeinen at ideasonboard.com> wrote:
>
> At the moment the driver sets the power state of all the PDs it creates
> to off, regardless of the actual HW state. This has two drawbacks:
>
> 1) The kernel cannot disable unused PDs automatically for power saving,
>    as it thinks they are off already
>
> 2) A more specific case (but perhaps applicable to other scenarios
>    also): bootloader enabled splash-screen cannot be kept on the screen.
>
> The issue in 2) is that the driver framework automatically enables the
> device's PD before calling probe() and disables it after the probe().
> This means that when the display subsystem (DSS) driver probes, but e.g.
> fails due to deferred probing, the DSS PD gets turned off and the driver
> cannot do anything to affect that.
>
> Solving the 2) requires more changes to actually keep the PD on during
> the boot, but a prerequisite for it is to have the correct power state
> for the PD.
>
> The downside with this patch is that it takes time to call the 'is_on'
> op, and we need to call it for each PD. In my tests with AM62 SK, using
> defconfig, I see an increase from ~3.5ms to ~7ms. However, the added
> feature is valuable, so in my opinion it's worth it.
>
> The performance could probably be improved with a new firmware API which
> returns the power states of all the PDs.
>
> There's also a related HW issue at play here: if the DSS IP is enabled
> and active, and its PD is turned off without first disabling the DSS
> display outputs, the DSS IP will hang and causes the kernel to halt if
> and when the DSS driver accesses the DSS registers the next time.
>
> With the current upstream kernel, with this patch applied, this means
> that if the bootloader enables the display, and the DSS driver is
> compiled as a module, the kernel will at some point disable unused PDs,
> including the DSS PD. When the DSS module is later loaded, it will hang
> the kernel.
>
> The same issue is already there, even without this patch, as the DSS
> driver may hit deferred probing, which causes the PD to be turned off,
> and leading to kernel halt when the DSS driver is probed again. This
> issue has been made quite rare with some arrangements in the DSS
> driver's probe, but it's still there.
>
> So, because of the DSS hang issues, I think this patch is still an RFC.
> Hopefully we can sort out all the issues, but this patch (or similar)
> will be part of the solution so I'd like to get some acks/nacks/comments
> for this. Also, this change might have side effects to other devices
> too, if the drivers expect the PD to be on, so testing is needed.
>
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen at ideasonboard.com>

This patch seems reasonable to me, however I am deferring to apply it
until yours and other confirmations about tests.

In regards to the "disable unused genpd" problem, I am working on it
in-between all the other stuff. I do understand that it probably
starts being annoying waiting for me, so I will try to get this
prioritized. We really need to get this solved.

Kind regards
Uffe

> ---
>  drivers/pmdomain/ti/ti_sci_pm_domains.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pmdomain/ti/ti_sci_pm_domains.c b/drivers/pmdomain/ti/ti_sci_pm_domains.c
> index 1510d5ddae3d..14c51a395d7e 100644
> --- a/drivers/pmdomain/ti/ti_sci_pm_domains.c
> +++ b/drivers/pmdomain/ti/ti_sci_pm_domains.c
> @@ -126,6 +126,23 @@ static bool ti_sci_pm_idx_exists(struct ti_sci_genpd_provider *pd_provider, u32
>         return false;
>  }
>
> +static bool ti_sci_pm_pd_is_on(struct ti_sci_genpd_provider *pd_provider,
> +                              int pd_idx)
> +{
> +       bool is_on;
> +       int ret;
> +
> +       if (!pd_provider->ti_sci->ops.dev_ops.is_on)
> +               return false;
> +
> +       ret = pd_provider->ti_sci->ops.dev_ops.is_on(pd_provider->ti_sci,
> +                                                    pd_idx, NULL, &is_on);
> +       if (ret)
> +               return false;
> +
> +       return is_on;
> +}
> +
>  static int ti_sci_pm_domain_probe(struct platform_device *pdev)
>  {
>         struct device *dev = &pdev->dev;
> @@ -161,6 +178,8 @@ static int ti_sci_pm_domain_probe(struct platform_device *pdev)
>                                 break;
>
>                         if (args.args_count >= 1 && args.np == dev->of_node) {
> +                               bool is_on;
> +
>                                 if (args.args[0] > max_id) {
>                                         max_id = args.args[0];
>                                 } else {
> @@ -189,7 +208,10 @@ static int ti_sci_pm_domain_probe(struct platform_device *pdev)
>                                 pd->idx = args.args[0];
>                                 pd->parent = pd_provider;
>
> -                               pm_genpd_init(&pd->pd, NULL, true);
> +                               is_on = ti_sci_pm_pd_is_on(pd_provider,
> +                                                          pd->idx);
> +
> +                               pm_genpd_init(&pd->pd, NULL, !is_on);
>
>                                 list_add(&pd->node, &pd_provider->pd_list);
>                         }
>
> ---
> base-commit: 98f7e32f20d28ec452afb208f9cffc08448a2652
> change-id: 20241022-tisci-pd-boot-state-33cf02efd378
>
> Best regards,
> --
> Tomi Valkeinen <tomi.valkeinen at ideasonboard.com>
>