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

[Matthias Kaehlcke <mka@chromium.org>]

On Fri, Feb 03, 2023 at 10:00:27PM +0200, Dmitry Baryshkov wrote:
> 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@linaro.org>
> > > > > ---
> > > > > 
> > > > > 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).

Most folks are probably not even aware that they have a 'sync_state' issue and
wouldn't look in debugfs, so I think this would have to be something proactive,
like a warning log that is enabled by default (possibly with the option to
disable it). Something in debugfs could be a nice complement.

> 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.

That's great, however not everybody uses the defconfig, it's just a default.

> > 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.

I started working on a patch, I'll probably send it out next week if I don't
encounter any evident major issues.