Re: [PATCH v6 1/5] dt-bindings: PCI: brcmstb: Add brcm,enable-l1ss property

From: Rob Herring
Date: Wed Aug 23 2023 - 10:49:00 EST


On Wed, Aug 23, 2023 at 2:30 AM Lorenzo Pieralisi <lpieralisi@xxxxxxxxxx> wrote:
>
> On Mon, Aug 21, 2023 at 12:01:50PM -0400, Jim Quinlan wrote:
> > On Mon, Aug 21, 2023 at 11:41 AM Lorenzo Pieralisi
> > <lpieralisi@xxxxxxxxxx> wrote:
> > >
> > > On Mon, Aug 21, 2023 at 11:25:11AM -0400, Jim Quinlan wrote:
> > > > On Mon, Aug 21, 2023 at 10:47 AM Lorenzo Pieralisi
> > > > <lpieralisi@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Fri, Jun 23, 2023 at 10:40:54AM -0400, Jim Quinlan wrote:
> > > > > > This commit adds the boolean "brcm,enable-l1ss" property:
> > > > > >
> > > > > > 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==powersupersave).
> > > > > >
> > > > > > This property is already present in the Raspian version of Linux, but the
> > > > > > upstream driver implementation that follows adds more details and
> > > > > > discerns between (a) and (b).
> > > > > >
> > > > > > Signed-off-by: Jim Quinlan <james.quinlan@xxxxxxxxxxxx>
> > > > > > Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
> > > > > > ---
> > > > > > Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml | 9 +++++++++
> > > > > > 1 file changed, 9 insertions(+)
> > > > > >
> > > > > > diff --git a/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml b/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
> > > > > > index 7e15aae7d69e..8b61c2179608 100644
> > > > > > --- a/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
> > > > > > +++ b/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
> > > > > > @@ -64,6 +64,15 @@ properties:
> > > > > >
> > > > > > aspm-no-l0s: true
> > > > > >
> > > > > > + brcm,enable-l1ss:
> > > > > > + description: Indicates that PCIe L1SS power savings
> > > > > > + are desired, the downstream device is L1SS-capable, and the
> > > > > > + OS has been configured to enable this mode. For boards
> > > > >
> > > > > What does this mean ? I don't think DT properties are supposed
> > > > > to carry information related to how the OS is configured.
> > > >
> > > > The DT setting in question is unrelated to the statement "and the OS
> > > > has been configured to
> > > > enable this mode".
> > > >
> > > > This is merely saying that even if you enable "brcm,l1ss-enable"
> > > > that you may not get L1SS power savings w/o setting
> > > > "CONFIG_PCIEASPM_POWER_SUPERSAVE=y".
> > > > I mentioned that exact term but a reviewer nakked it because
> > > > apparently DT descriptions should not be OS specific.

Yeah, probably the OS part should be dropped.

> > > >
> > > > I am actually open for this to be a command-line option but I wanted to honor
> > > > what the Raspian OS folks have already done. RaspianOS already has
> > > > "brcm,enable-l1ss"
> > > > set in their DTS files.
> > >
> > > This is about the mainline kernel, I don't have any visibility into
> > > downstream kernels (where that property management was added without DT
> > > and PCI maintainers supervision).
> > >
> > > Raspian OS folks' choice is theirs but it can't and it shouldn't override
> > > the mainline review process even though I understand the position you
> > > are in.

Sure, but we shouldn't change things just to be different from
downstream. If we're only discussing the color of the shed, then no
point changing it.

> >
> > Understood, but using the command line has its warts as well; I now recall the
> > discussion Bjorn and I had regarding this option. I'm pretty sure
> > that upstreaam will not allow the following
> > possible command line kernel params:
> >
> > brcm,enable-l1ss
> > pci=brcm,entable-l1ss
> >
> > Bjorn suggested using the documented but (IMO) obscure and rarely
> > used format
> >
> > pci=[<domain>:]<bus>:<dev>.<func>[/<dev>.<func>]*pci:<vendor>:<device>[:<subvendor>:<subdevice>]
> >
> > but this is just going in the wrong direction; here's why. Using the
> > above iformat s completely dependent on the
> > PCI "linux-domaiin" property, a non-HW related DT property I might
> > add. Since "linux-domain" is already
> > a valid and well-used DT property, and the value of the above
> > command line format is dependent
> > on the value of the "linux-domain", why not be consistent and let
> > "brcm,enable-l1ss" be a Broadcom specific property?
>
> I am just asking to add a module_param to the host controller driver.

FWIW, gregkh pretty much always nacks new module parameters.

>
> Anyway - time is running out for v6.6, I need Rob's feedback, if I don't
> hear from him today I will merge the last three patches and postpone the
> discussion.

You've already got my reviewed-by. If you aren't happy with this, then
fine. I don't know enough about L1SS to comment further. Is it
normally always supported or discoverable? If so, then I'd think we'd
want it default enabled with a disable override flag. Maybe it should
be generic?

Rob