linux-kernel - Re: Re: Re: [RFC 8/9] PCI/pwrctl: add PCI power control core code

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Date: Fri, 9 Feb 2024 17:43:56 -0600
From: Bjorn Andersson <andersson@...nel.org>
To: Manivannan Sadhasivam <mani@...nel.org>
Cc: Bartosz Golaszewski <brgl@...ev.pl>, 
	Konrad Dybcio <konrad.dybcio@...aro.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, 
	Marcel Holtmann <marcel@...tmann.org>, Luiz Augusto von Dentz <luiz.dentz@...il.com>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Alex Elder <elder@...aro.org>, Srini Kandagatla <srinivas.kandagatla@...aro.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arnd Bergmann <arnd@...db.de>, Abel Vesa <abel.vesa@...aro.org>, 
	Lukas Wunner <lukas@...ner.de>, linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-bluetooth@...r.kernel.org, linux-pci@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: Re: Re: [RFC 8/9] PCI/pwrctl: add PCI power control core code

On Thu, Feb 08, 2024 at 05:02:01PM +0530, Manivannan Sadhasivam wrote:
> On Fri, Feb 02, 2024 at 10:52:11AM -0600, Bjorn Andersson wrote:
> > On Fri, Feb 02, 2024 at 10:11:42AM +0100, Bartosz Golaszewski wrote:
> > > On Fri, Feb 2, 2024 at 4:53 AM Bjorn Andersson <andersson@...nel.org> wrote:
> > [..]
> > > > > +             break;
> > > > > +     }
> > > > > +
> > > > > +     return NOTIFY_DONE;
> > > > > +}
> > > > > +
> > > > > +int pci_pwrctl_device_enable(struct pci_pwrctl *pwrctl)
> > > >
> > > > This function doesn't really "enable the device", looking at the example
> > > > driver it's rather "device_enabled" than "device_enable"...
> > > >
> > > 
> > > I was also thinking about pci_pwrctl_device_ready() or
> > > pci_pwrctl_device_prepared().
> > 
> > I like both of these.
> > 
> > I guess the bigger question is how the flow would look like in the event
> > that we need to power-cycle the attached PCIe device, e.g. because
> > firmware has gotten into a really bad state.
> > 
> > Will we need an operation that removes the device first, and then cut
> > the power, or do we cut the power and then call unprepared()?
> > 
> 
> Currently, we don't power cycle while resetting the devices. Most of the drivers
> just do a software reset using some register writes. Part of the reason for
> that is, the drivers themselves don't control the power to the devices and there
> would be no way to let the parent know about the firmware crash.
> 

I don't know what the appropriate design for this is, but we do have a
need for being able to recover from this state by the means of
power-cycling the device.

If it's not possible to let the device do it (in any fashion), then
perhaps a user-space-assisted model is needed?

Turning on power is an important first step, but please do consider the
full scope of the known problem space.

Regards,
Bjorn