Re: [RFC PATCH v2 1/2] PM: domains: Skip disabling unused domains if provider has sync_state

[Dmitry Baryshkov]

On 03/02/2023 03:20, Matthias Kaehlcke wrote:
> Hi Dmitry,
>
> On Thu, Feb 02, 2023 at 09:53:41PM +0200, Dmitry Baryshkov wrote:
> > On 02/02/2023 20:24, Matthias Kaehlcke wrote:
> > > Hi Abel,
> > >
> > > On Fri, Jan 27, 2023 at 12:40:53PM +0200, Abel Vesa wrote:
> > > > Currently, there are cases when a domain needs to remain enabled until
> > > > the consumer driver probes. Sometimes such consumer drivers may be built
> > > > as modules. Since the genpd_power_off_unused is called too early for
> > > > such consumer driver modules to get a chance to probe, the domain, since
> > > > it is unused, will get disabled. On the other hand, the best time for
> > > > an unused domain to be disabled is on the provider's sync_state
> > > > callback. So, if the provider has registered a sync_state callback,
> > > > assume the unused domains for that provider will be disabled on its
> > > > sync_state callback. Also provide a generic sync_state callback which
> > > > disables all the domains unused for the provider that registers it.
> > > >
> > > > Signed-off-by: Abel Vesa <abel.vesa@xxxxxxxxxx>
> > > > ---
> > > >
> > > > This approach has been applied for unused clocks as well.
> > > > With this patch merged in, all the providers that have sync_state
> > > > callback registered will leave the domains enabled unless the provider's
> > > > sync_state callback explicitly disables them. So those providers will
> > > > need to add the disabling part to their sync_state callback. On the
> > > > other hand, the platforms that have cases where domains need to remain
> > > > enabled (even if unused) until the consumer driver probes, will be able,
> > > > with this patch in, to run without the pd_ignore_unused kernel argument,
> > > > which seems to be the case for most Qualcomm platforms, at this moment.
> > >
> > > I recently encountered a related issue on a Qualcomm platform with a
> > > v6.2-rc kernel, which includes 3a39049f88e4 ("soc: qcom: rpmhpd: Use
> > > highest corner until sync_state"). The issue involves a DT node with a
> > > rpmhpd, the DT node is enabled, however the corresponding device driver
> > > is not enabled in the kernel. In such a scenario the sync_state callback
> > > is never called, because the genpd consumer never probes. As a result
> > > the Always-on subsystem (AOSS) of the SoC doesn't enter sleep mode during
> > > system suspend, which results in a substantially higher power consumption
> > > in S3.
> > >
> > > I wonder if genpd (and some other frameworks) needs something like
> > > regulator_init_complete(), which turns off unused regulators 30s after
> > > system boot. That's conceptually similar to the current
> > > genpd_power_off_unused(), but would provide time for modules being loaded.
> >
> > I think the overall goal is to move away from ad-hoc implementations like
> > clk_disable_unused/genpd_power_off_unused/regulator_init_complete towards
> > the sync_state.
>
> I generally agree with the goal of using common mechanisms whenever possible.
>
> > So inherently one either has to provide drivers for all devices in question
> > or disable unused devices in DT.
>
> I don't think that's a great solution, it essentially hands the issue down to
> the users or downstream maintainers of the kernel, who might not be aware that
> there is an issue, nor know about the specifics of genpd (or interconnects and
> clocks which have similar problems).

The goal is to move the control down to individual drivers. Previously 
we had issues with clk_disable_unused() disabling mdss/mdp clocks 
incorrectly, which frequently led to broken display output. Other 
clock/genpd/regulator drivers might have other internal dependencies. 
Thus it is not really possible to handle resource shutdown in the common 
 (framework) code.

> In general symptoms are probably subtle, like a (potentially substantially)
> increased power consumption during system suspend. The issue might have been
> introduced by an update to a newer kernel, which now includes a DT node for a
> new SoC feature which wasn't supported by the 'old' kernel. It's common
> practice to use the 'old' .config, at least as a starting point, which
> obviously doesn't enable the new driver. That happend to me with [1] when
> testing v6.1. It took me quite some time to track the 'culprit' commit down
> and then some debugging to understand what's going on. Shortly after that I
> ran into a related issue involving genpds when testing v6.2-rc, which again
> took a non-trivial amount of time to track down (and I'm familiar with the SoC
> platform and the general nature of the issue). I don't think it's reasonable
> to expect every user/downstream maintainer of an impacted system to go through
> this, one person at a time.

I think it would be nice to have some way of 'sync_pending' debug 
available (compare this to debugfs/devices_deferred).

Note, we are trying to make sure that all supported drivers are enabled 
at least as modules (if possible). If we fail, please send a patch 
fixing the defconfig.

> Maybe there could be a generic solution for drivers with a 'sync_state'
> callback, e.g. a the driver (or framework) could have a 'sync_state_timeout'
> callback (or similar), which is called by the driver framework if 'sync_state'
> wasn't called (for example) 30s after the device was probed. Then the provider
> can power off or throttle unclaimed resources.

I might be missing a point somewhere, but for me it looks like a logical 
solution. Please send a proposal.

--
With best wishes
Dmitry