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

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Date:   Thu, 3 Jun 2021 14:01:53 -0700
From:   Florian Fainelli <florian.fainelli@...adcom.com>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>
Cc:     Jim Quinlan <jim2101024@...il.com>, linux-pci@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        bcm-kernel-feedback-list@...adcom.com, james.quinlan@...adcom.com,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-rpi-kernel@...ts.infradead.org>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 4/4] PCI: brcmstb: add shutdown call to driver

On 6/3/21 1:58 PM, Bjorn Helgaas wrote:
> 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?

Yes, with pcie-brcmstb.c providing a .shutdow() callback we have a
chance to turn off our PCIe PHY and the RC's digital clock which would
not be able to do otherwise.
-- 
Florian

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