Re: [PATCH v1 4/4] PCI: brcmstb: add shutdown call to driver

[Bjorn Helgaas]

On Thu, Jun 03, 2021 at 10:30:37AM -0700, Florian Fainelli wrote:
> On 6/3/21 10:23 AM, Bjorn Helgaas wrote:
> > On Wed, May 26, 2021 at 10:03:47AM -0700, Florian Fainelli wrote:
> >> On 5/25/21 2:18 PM, Bjorn Helgaas wrote:
> >>> On Tue, Apr 27, 2021 at 01:51:39PM -0400, Jim Quinlan wrote:
> >>>> The shutdown() call is similar to the remove() call except the former does
> >>>> not need to invoke pci_{stop,remove}_root_bus(), and besides, errors occur
> >>>> if it does.
> >>>
> >>> This doesn't explain why shutdown() is necessary.  "errors occur"
> >>> might be a hint, except that AFAICT, many similar drivers do invoke
> >>> pci_stop_root_bus() and pci_remove_root_bus() (several of them while
> >>> holding pci_lock_rescan_remove()), without implementing .shutdown().
> >>
> >> We have to implement .shutdown() in order to meet a certain power budget
> >> while the chip is being put into S5 (soft off) state and still support
> >> Wake-on-WLAN, for our latest chips this translates into roughly 200mW of
> >> power savings at the wall. We could probably add a word or two in a v2
> >> that indicates this is done for power savings.
> > 
> > "Saving power" is a great reason to do this.  But we still need to
> > connect this to the driver model and the system-level behavior
> > somehow.
> > 
> > The pci_driver comment says @shutdown is to "stop idling DMA
> > operations" and it hooks into reboot_notifier_list in kernel/sys.c.
> > That's incorrect or at least incomplete because reboot_notifier_list
> > isn't mentioned at all in kernel/sys.c, and I don't see the connection
> > between @shutdown and reboot_notifier_list.
> > 
> > AFAICT, @shutdown is currently used in this path:
> > 
> >   kernel_restart_prepare or kernel_shutdown_prepare
> >     device_shutdown
> >       dev->bus->shutdown
> >         pci_device_shutdown                     # pci_bus_type.shutdown
> >           drv->shutdown
> > 
> > so we're going to either reboot or halt/power-off the entire system,
> > and we're not going to use this device again until we're in a
> > brand-new kernel and we re-enumerate the device and re-register the
> > driver.
> > 
> > I'm not quite sure how either of those fits into the power-saving
> > reason.  I guess going to S5 is probably via the kernel_power_off()
> > path and that by itself doesn't turn off as much power to the PCIe
> > controller as it could?  And this new .shutdown() method will get
> > called in that path and will turn off more power, but will still leave
> > enough for wake-on-LAN to work?  And when we *do* wake from S5,
> > obviously that means a complete boot with a new kernel.
> 
> Correct, the S5 shutdown is via kernel_power_off() and will turn off all
> that we can in the PCIe root complex and its PHY, drop the PCIe link to
> the end-point which signals that the end-point can enter its own suspend
> logic, too. And yes, when we do wake-up from S5 it means booting a
> completely new kernel. S5 is typically implemented in our chips by
> keeping just a little bit of logic active to service wake-up events
> (infrared remotes, GPIOs, RTC, etc.).

Which part of that does this patch change?  Is it that the new
.shutdown() turns off more power than machine_power_off() does by
itself?

Bjorn