Re: [PATCH v4 1/5] dt-bindings: PCI: brcmstb: brcm,{enable-l1ss,completion-timeout-us} props

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, May 03, 2023 at 05:38:15PM -0400, Jim Quinlan wrote:
> On Wed, May 3, 2023 at 2:07 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Wed, May 03, 2023 at 10:38:57AM -0400, Jim Quinlan wrote:
> > > On Sun, Apr 30, 2023 at 3:10 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > > > On Fri, Apr 28, 2023 at 06:34:55PM -0400, Jim Quinlan wrote:
> > > > > brcm,enable-l1ss (bool):
> > > > >
> > > > >   The Broadcom STB/CM PCIe HW -- a core that is also used by RPi SOCs --
> > > > >   requires the driver probe() to deliberately place the HW one of three
> > > > >   CLKREQ# modes:
> > > > >
> > > > >   (a) CLKREQ# driven by the RC unconditionally
> > > > >   (b) CLKREQ# driven by the EP for ASPM L0s, L1
> > > > >   (c) Bidirectional CLKREQ#, as used for L1 Substates (L1SS).
> > > > >
> > > > >   The HW+driver can tell the difference between downstream devices that
> > > > >   need (a) and (b), but does not know when to configure (c).  All devices
> > > > >   should work fine when the driver chooses (a) or (b), but (c) may be
> > > > >   desired to realize the extra power savings that L1SS offers.  So we
> > > > >   introduce the boolean "brcm,enable-l1ss" property to inform the driver
> > > > >   that (c) is desired.  Setting this property only makes sense when the
> > > > >   downstream device is L1SS-capable and the OS is configured to activate
> > > > >   this mode (e.g. policy==superpowersave).
> > > ...
> >
> > > > What bad things would happen if the driver always configured (c)?
> > >
> > > Well, our driver has traditionally only supported (b) and our
> > > existing boards have been designed with this in mind.  I would not
> > > want to switch modes w'o the user/customer/engineer opting-in to do
> > > so.  Further, the PCIe HW engineer told me defaulting to (c) was a
> > > bad idea and was "asking for trouble".  Note that the commit's
> > > comment has that warning about L1SS mode not meeting this 400ns
> > > spec, and I suspect that many of our existing designs have bumped
> > > into that.
> > >
> > > But to answer your question, I haven't found a scenario that did not
> > > work by setting mode (c).  That doesn't mean they are not out there.
> > >
> > > > Other platforms don't require this, and having to edit the DT
> > > > based on what PCIe device is plugged in seems wrong.  If brcmstb
> > > > does need it, that suggests a hardware defect.  If we need this to
> > > > work around a defect, that's OK, but we should acknowledge the
> > > > defect so we can stop using this for future hardware that doesn't
> > > > need it.
> > >
> > > All devices should work w/o the user having to change the DT.  Only
> > > if they desire L1SS must they add the "brcm,enable-l1ss" property.
> >
> > I thought the DT was supposed to describe properties of the
> > *hardware*, but this seems more like "use this untested clkreq
> > configuration," which maybe could be done via a module parameter?
>
> Electrically, it has been tested, but  specifically for L1SS capable
> devices.  What is untested AFAICT are platforms using this mode on
> non-L1SS capable devices.

Non-L1SS behavior is a subset of L1SS, so if you've tested with L1SS
enabled, I would think you'd be covered.

But I'm not a hardware engineer, so maybe there's some subtlety there.
The "asking for trouble" comment from your engineer is definitely
concerning, but I have no idea what's behind that.

And obviously even if we have "brcm,enable-l1ss", the user may decide
to disable L1SS administratively, so even if the Root Port and the
device both support L1SS, it may be never be enabled.

> WRT bootline param
> pci=[<domain>:]<bus>:<dev>.<func>[/<dev>.<func>]*pci:<vendor>:<device>[:<subvendor>:<subdevice>]:
> this does not look compatible for vendor specific DT options like
> "brcm,enable-l1ss".  I observe that pci_dev_str_match_path() is a
> static function and I don't see a single option in pci.c  that is
> vendor specific.  FWIW, moving something like this to the bootline
> would not be popular with our customers; for some reason they really
> don't like changes to the bootline.

They prefer editing the DT?

I agree the "pci=B:D.F" stuff is a bit ugly.  Do you have multiple
slots such that you would have to apply this parameter to some but not
others?  I guess I was imagining a single-slot system where you
wouldn't need to identify the specific device because there *is* only
one.

Bjorn