RE: [RFC] platform: detach from PM domains on shutdown

[Peng Fan <peng.fan@nxp.com>]

> -----Original Message-----
> From: Rafael J. Wysocki [mailto:rjw@rjwysocki.net]
> Sent: 2018年5月18日 15:55
> To: Peng Fan <peng.fan@nxp.com>
> Cc: Rafael J. Wysocki <rafael@kernel.org>; Ulf Hansson
> <ulf.hansson@linaro.org>; Rafael J. Wysocki <rafael.j.wysocki@intel.com>;
> Fabio Estevam <fabio.estevam@nxp.com>; Greg Kroah-Hartman
> <gregkh@linuxfoundation.org>; Linux Kernel Mailing List
> <linux-kernel@vger.kernel.org>; Linux PM <linux-pm@vger.kernel.org>;
> dl-linux-imx <linux-imx@nxp.com>
> Subject: Re: [RFC] platform: detach from PM domains on shutdown
> 
> On Thursday, May 17, 2018 2:37:31 PM CEST Peng Fan wrote:
> >
> > > -----Original Message-----
> > > From: rjwysocki@gmail.com [mailto:rjwysocki@gmail.com] On Behalf Of
> > > Rafael J. Wysocki
> > > Sent: 2018年5月17日 16:01
> > > To: Peng Fan <peng.fan@nxp.com>
> > > Cc: Rafael J. Wysocki <rafael@kernel.org>; Ulf Hansson
> > > <ulf.hansson@linaro.org>; Rafael J. Wysocki
> > > <rafael.j.wysocki@intel.com>; Fabio Estevam <fabio.estevam@nxp.com>;
> > > Greg Kroah-Hartman <gregkh@linuxfoundation.org>; Linux Kernel
> > > Mailing List <linux-kernel@vger.kernel.org>; Linux PM
> > > <linux-pm@vger.kernel.org>; dl-linux-imx <linux-imx@nxp.com>
> > > Subject: Re: [RFC] platform: detach from PM domains on shutdown
> > >
> > > On Thu, May 17, 2018 at 4:33 AM, Peng Fan <peng.fan@nxp.com> wrote:
> > > >
> > > >
> > > >> -----Original Message-----
> > > >> From: rjwysocki@gmail.com [mailto:rjwysocki@gmail.com] On Behalf
> > > >> Of Rafael J. Wysocki
> > > >> Sent: 2018年5月17日 5:35
> > > >> To: Ulf Hansson <ulf.hansson@linaro.org>
> > > >> Cc: Peng Fan <peng.fan@nxp.com>; Rafael J. Wysocki
> > > >> <rafael.j.wysocki@intel.com>; Fabio Estevam
> > > >> <fabio.estevam@nxp.com>; Greg Kroah-Hartman
> > > >> <gregkh@linuxfoundation.org>; Linux Kernel Mailing List
> > > >> <linux-kernel@vger.kernel.org>; Linux PM
> > > >> <linux-pm@vger.kernel.org>; dl-linux-imx <linux-imx@nxp.com>
> > > >> Subject: Re: [RFC] platform: detach from PM domains on shutdown
> > > >>
> > > >> On Wed, May 16, 2018 at 11:52 AM, Ulf Hansson
> > > >> <ulf.hansson@linaro.org>
> > > >> wrote:
> > > >> > On 15 May 2018 at 11:01, Peng Fan <peng.fan@nxp.com> wrote:
> > > >> >> When reboot Linux, the PM domains attached to a device are not
> > > >> >> shutdown. To SoCs which relys on reset the whole SoC, there is
> > > >> >> no need to shutdown PM domains, but to Linux running in a
> > > >> >> virtual machine with devices pass-through, we could not reset the
> whole SoC.
> > > >> >> Currently we need Linux to shutdown its PM domains when reboot.
> > > >> >
> > > >> > I am not sure I understand exactly why the PM domain needs to
> > > >> > be shutdown for these cases, could you please elaborate a bit on that.
> > > >> >
> > > >> > BTW, what platform are you running on and also what PM domains
> > > >> > are being
> > > >> used?
> > > >> >
> > > >> > Anyway, it seems like there may be need for certain cases, but
> > > >> > certainly not all - especially since it may slow down the
> > > >> > shutdown process, when not needed.
> > > >> >
> > > >> > Can we make this runtime configurable, via sysfs or whatever
> > > >> > that makes
> > > >> sense!?
> > > >> >
> > > >> >>
> > > >> >> commit 2d30bb0b3889 ("platform: Do not detach from PM domains
> > > >> >> on shutdown"), removes what this patch tries to add, because of a
> warning.
> > > >> >> commit e79aee49bcf9 ("PM: Avoid false-positive warnings in
> > > >> >> dev_pm_domain_set()") already fixes the false alarm warning.
> > > >> >> So let's detach the power domain to shutdown PM domains after
> > > >> >> driver
> > > shutdown.
> > > >> >>
> > > >> >> Signed-off-by: Peng Fan <peng.fan@nxp.com>
> > > >> >> ---
> > > >> >>
> > > >> >> I do not find a better place to shutdown power domain when
> > > >> >> reboot Linux, so add back the line that commit 2d30bb0b3889
> > > >> >> removes, because it is a false alarm warning as commit e79aee49bcf9
> describes.
> > > >> >>
> > > >> >>  drivers/base/platform.c | 1 +
> > > >> >>  1 file changed, 1 insertion(+)
> > > >> >>
> > > >> >> diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> > > >> >> index 8075ddc70a17..a5929f24dc3c 100644
> > > >> >> --- a/drivers/base/platform.c
> > > >> >> +++ b/drivers/base/platform.c
> > > >> >> @@ -616,6 +616,7 @@ static void platform_drv_shutdown(struct
> > > >> >> device
> > > >> >> *_dev)
> > > >> >>
> > > >> >>         if (drv->shutdown)
> > > >> >>                 drv->shutdown(dev);
> > > >> >> +       dev_pm_domain_detach(_dev, true);
> > > >> >
> > > >> > This would somewhat work, but only for platform devices. To
> > > >> > make this fully work, we need to call dev_pm_domain_detach()
> > > >> > from amba, spi, etc as well.
> > > >> >
> > > >> > Perhaps another option to manage this more generally, an
> > > >> > without having detach devices, could be to extend the struct
> > > >> > dev_pm_domain with a new callback, "->shutdown()" and then make
> > > >> > the driver core call it from device_shutdown().
> > > >>
> > > >> I'm sensing a possible ordering slippery slope with this (it will
> > > >> only work if all of the drivers/bus types etc do the right thing
> > > >> in their
> > > >> ->shutdown callbacks so nothing depends on the domain going forward).
> > > >>
> > > >> > Typically, for genpd, I would probably count the number of
> > > >> > calls being made to ->shutdown() per PM domain, then when it
> > > >> > reaches the number of attached devices to it, allow to power off it.
> > > >> >
> > > >> > Let's see what Rafael thinks about it.
> > > >>
> > > >> I'm not sure about the use case.  The hypervisor should be able
> > > >> to take care of turning power domains off on the client OS reboot
> > > >> in theory.  If the client OS leaving the hypervisor needs to
> > > >> worry about what state it leaves behind, the design of the
> > > >> hypervisor is sort of
> > > questionable IMO.
> > > >
> > > > This is valid concern. But moving the power domain logic into
> > > > hypervisor mostly micro-kernel design will introduce more
> > > > complexity and
> > > make certification harder.
> > > > Currently, Let Linux shutdown it's power domain is the easiest way
> > > > to me and make things work well after reboot.
> > >
> > > Well, to put it bluntly, if your hypervisor depends on the guest to
> > > do the right thing on exit, it doesn't do its job.  I wouldn't have
> > > certified it for you if that was my decision.
> >
> > It is guest os not work well after guest os reboot. The hypervisor is not
> affected.
> >
> > Thinking another case without hypervisor, M4 core run RTOS, A35 Core
> > run Linux, when Linux rebooting, RTOS should not be affected. After
> > Linux reboot itself, because its power domain is not paired with
> open/shutdown, some devices not function well.
> 
> The question boils down to whether or not devices should be detached from PM
> domains on shutdown IMO.
> 
> They are detached from PM domains on driver removal, so I guess one answer is
> "yes, in analogy with that".  However, the point about performace brought up
> by Ulf seems to be valid too.
> 
> In any case, the change should be made for all of the bus types using PM
> domains, not just one.

Understand, it will increase shutdown time. How about shutdown the power domain
in platform_driver->shutdown, let the driver handle it's power domain sthudown by itself?
Then no need common framework change.

Thanks,
Peng