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

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Wed, Aug 23, 2023 at 06:29:08PM +0200, Lorenzo Pieralisi wrote:
> On Wed, Aug 23, 2023 at 09:48:40AM -0500, Rob Herring wrote:
> > On Wed, Aug 23, 2023 at 2:30 AM Lorenzo Pieralisi <lpieralisi@kernel.org> 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@kernel.org> 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@kernel.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@broadcom.com>
> > > > > > > > Reviewed-by: Rob Herring <robh@kernel.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.
> > 
> > Yeah, probably the OS part should be dropped.
> 
> I will drop it - if you don't mind.
> 
> > > > > > 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.
> 
> Sure, no problem.
> 
> > > > 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.
> 
> Fine by me.
> 
> > > 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?
> 
> It is not that I am not happy with it - we asked questions to understand
> what this property was for, I think that's normal.
> 
> I will merge this series as-is.
> 

There is a discussion ongoing here: https://lore.kernel.org/linux-pci/20230823181650.GL3737@thinkpad/T/#u

Can you please hold on to merging this series until we've reached a conclusion?

- Mani

> Lorenzo

-- 
மணிவண்ணன் சதாசிவம்