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

[Brian Norris <briannorris@chromium.org>]

Hi,

On Tue, Feb 26, 2019 at 05:14:00PM +0000, Marc Zyngier wrote:
> On 26/02/2019 16:21, Ard Biesheuvel wrote:
> > On Mon, 25 Feb 2019 at 15:53, Marc Zyngier <marc.zyngier@arm.com> wrote:
> >> It outlines one thing: If you have to interpret per-device PCI
> >> properties from DT, you're in for serious trouble. I should get some
> >> better HW.
> >>
> > 
> > Yeah, it obviously makes no sense at all for the interrupt parent of a
> > PCI device to deviate from the host bridge's interrupt parent, and
> > it's quite unfortunate that we can't simply ban it now that the cat is
> > out of the bag already.
> > 
> > Arguably, the wake up widget is not part of the PCI device, but I have
> > no opinion as to whether it is better modeling it as a sub device as
> > you are proposing or as an entirely separate device referenced via a
> > phandle.
> 
> It is not that clear. The widget seems to be an integral part of the
> device, as it is the same basic IP that is used for SDIO and USB.

It's not really a widget specific to this IP. It's just a GPIO. It so
happens that both SDIO and PCIe designs have wanted to use a GPIO for
wakeup, as many other devices do. (Note: it's not just cheap ARM
devices; pulling up some Intel Chromebook designs, I see the exact same
WAKE# GPIO on their PCIe WiFi as well.)

> It looks like the good old pre-PCI-2.2 days, where you had to have a
> separate cable between your network card and the base-board for the
> wake-up interrupt to be delivered. Starting with PCI-2.2, the bus can
> carry the signal just fine. With PCIe, it should just be an interrupt
> TLP sent to the RC, but that's obviously not within the capabilities of
> the HW.

You should search the PCI Express specification for WAKE#. There is a
clearly-documented "side-band wake" feature that is part of the
standard, as an alternative to in-band TLP wakeup. While you claim this
is an ancient thing, it in fact still in use on many systems -- it's
just usually abstracted better by ACPI firmware, whereas the dirty
laundry is aired a bit more on a Device Tree system. And we got it
wrong.

> Anyway, it'd be good if the Marvell people could chime in and let us
> know how they'd prefer to handle this.

I'm not sure this is really a Marvell-specific problem. (Well, except
for the marvell,wakeup-pin silliness, which is somewhat orthogonal.) In
fact, if we cared a little more about Wake-on-WiFi, we'd be trying to
support the same (out-of-band WAKE#) with other WiFi drivers.

Brian