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

[Jim Quinlan]

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.
> >
> > 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.

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?

Regards,
Jim Quinlan
Broadcom STB/CM


>
> 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
> > > >
> > >
> > >
>
>

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