Re: [PATCH 0/4] mwifiex PCI/wake-up interrupt fixes

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Wed, 27 Feb 2019 at 11:02, Marc Zyngier <marc.zyngier@arm.com> wrote:
>
> + Lorenzo
>
> Hi Brian,
>
> On 26/02/2019 23:28, Brian Norris wrote:
> > + others
> >
> > Hi Marc,
> >
> > Thanks for the series. I have a few bits of history to add to this, and
> > some comments.
> >
> > On Sun, Feb 24, 2019 at 02:04:22PM +0000, Marc Zyngier wrote:
> >> For quite some time, I wondered why the PCI mwifiex device built in my
> >> Chromebook was unable to use the good old legacy interrupts. But as MSIs
> >> were working fine, I never really bothered investigating. I finally had a
> >> look, and the result isn't very pretty.
> >>
> >> On this machine (rk3399-based kevin), the wake-up interrupt is described as
> >> such:
> >>
> >> &pci_rootport {
> >>      mvl_wifi: wifi@0,0 {
> >>              compatible = "pci1b4b,2b42";
> >>              reg = <0x83010000 0x0 0x00000000 0x0 0x00100000
> >>                     0x83010000 0x0 0x00100000 0x0 0x00100000>;
> >>              interrupt-parent = <&gpio0>;
> >>              interrupts = <8 IRQ_TYPE_LEVEL_LOW>;
> >>              pinctrl-names = "default";
> >>              pinctrl-0 = <&wlan_host_wake_l>;
> >>              wakeup-source;
> >>      };
> >> };
> >>
> >> Note how the interrupt is part of the properties directly attached to the
> >> PCI node. And yet, this interrupt has nothing to do with a PCI legacy
> >> interrupt, as it is attached to the wake-up widget that bypasses the PCIe RC
> >> altogether (Yay for the broken design!). This is in total violation of the
> >> IEEE Std 1275-1994 spec[1], which clearly documents that such interrupt
> >> specifiers describe the PCI device interrupts, and must obey the
> >> INT-{A,B,C,D} mapping. Oops!
> >
> > You're not the first person to notice this. All the motivations are not
> > necessarily painted clearly in their cover letter, but here are some
> > previous attempts at solving this problem:
> >
> > [RFC PATCH v11 0/5] PCI: rockchip: Move PCIe WAKE# handling into pci core
> > https://lkml.kernel.org/lkml/20171225114742.18920-1-jeffy.chen@rock-chips.com/
> > http://lkml.kernel.org/lkml/20171226023646.17722-1-jeffy.chen@rock-chips.com/
> >
> > As you can see by the 12th iteration, it wasn't left unsolved for lack
> > of trying...
>
> I wasn't aware of this. That's definitely a better approach than my
> hack, and I would really like this to be revived.
>

I don't think this approach is entirely sound either.

From the side of the PCI device, WAKE# is just a GPIO line, and how it
is wired into the system is an entirely separate matter. So I don't
think it is justified to overload the notion of legacy interrupts with
some other pin that may behave in a way that is vaguely similar to how
a true wake-up capable interrupt works.

So I'd argue that we should add an optional 'wake-gpio' DT property
instead to the generic PCI device binding, and leave the interrupt
binding and discovery alone.