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

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Date:   Mon, 21 Aug 2023 17:41:34 +0200
From:   Lorenzo Pieralisi <lpieralisi@...nel.org>
To:     Jim Quinlan <james.quinlan@...adcom.com>
Cc:     linux-pci@...r.kernel.org,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Cyril Brulebois <kibi@...ian.org>,
        Phil Elwell <phil@...pberrypi.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Jim Quinlan <jim2101024@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        "moderated list:BROADCOM BCM7XXX ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-rpi-kernel@...ts.infradead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/5] dt-bindings: PCI: brcmstb: Add brcm,enable-l1ss
 property

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@...nel.org> 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@...adcom.com>
> > > Reviewed-by: Rob Herring <robh@...nel.org>
> > > ---
> > >  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.
> 
> 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.

Thanks,
Lorenzo

> 
> Regards,
> Jim
> 
> > Again - it depends on what DT should be used for, I am not claiming to
> > have any authority on that, just asking.
> >
> > Thanks,
> > Lorenzo
> >
> > > +      using a mini-card connector, this mode may not meet the
> > > +      TCRLon maximum time of 400ns, as specified in 3.2.5.2.2
> > > +      of the PCI Express Mini CEM 2.0 specification.
> > > +    type: boolean
> > > +
> > >    brcm,scb-sizes:
> > >      description: u64 giving the 64bit PCIe memory
> > >        viewport size of a memory controller.  There may be up to
> > > --
> > > 2.17.1
> > >
> >
> >