Re: [PATCH v3 04/17] dt-bindings: PCI: dwc: Add max-link-speed common property

[Serge Semin <fancer.lancer@gmail.com>]

Rob,
Could you please get your attention back to this this thread?

-Sergey

On Sun, Jun 19, 2022 at 05:27:20PM +0300, Serge Semin wrote:
> On Wed, Jun 15, 2022 at 08:55:50AM -0600, Rob Herring wrote:
> > On Fri, Jun 10, 2022 at 11:56:52AM +0300, Serge Semin wrote:
> > > In accordance with [1] DW PCIe controllers support up to Gen5 link speed.
> > > Let's add the max-link-speed property upper bound to 5 then. The DT
> > > bindings of the particular devices are expected to setup more strict
> > > constraint on that parameter.
> > > 
> > > [1] Synopsys DesignWare Cores PCI Express Controller Databook, Version
> > > 5.40a, March 2019, p. 27
> > > 
> > > Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
> > > 
> > > ---
> > > 
> > > Changelog v3:
> > > - This is a new patch unpinned from the next one:
> > >   https://lore.kernel.org/linux-pci/20220503214638.1895-2-Sergey.Semin@baikalelectronics.ru/
> > >   by the Rob' request. (@Rob)
> > > ---
> > >  Documentation/devicetree/bindings/pci/snps,dw-pcie-common.yaml | 3 +++
> > >  Documentation/devicetree/bindings/pci/snps,dw-pcie-ep.yaml     | 2 ++
> > >  Documentation/devicetree/bindings/pci/snps,dw-pcie.yaml        | 1 +
> > >  3 files changed, 6 insertions(+)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/pci/snps,dw-pcie-common.yaml b/Documentation/devicetree/bindings/pci/snps,dw-pcie-common.yaml
> > > index 627a5d6625ba..b2fbe886981b 100644
> > > --- a/Documentation/devicetree/bindings/pci/snps,dw-pcie-common.yaml
> > > +++ b/Documentation/devicetree/bindings/pci/snps,dw-pcie-common.yaml
> > > @@ -45,6 +45,9 @@ properties:
> > >        the peripheral devices available on the PCIe bus.
> > >      maxItems: 1
> > >  
> > > +  max-link-speed:
> > > +    maximum: 5
> > 
> 
> > Unless the default is less than the max, shouldn't the max here be 1 
> > less than the h/w max?
> 
> Why? AFAIU max-link-speed semantics it works as less-than-or-equal
> operator isn't it? The modern DW PCIe Root ports and Endpoints
> IP-cores support up to Gen5 PCIe speed including the Gen5 mode (see
> the CX_MAX_PCIE_SPEED IP-core synthesize paramter). It's reasonable to
> set the max-link-speed here to be in coherency with the IP-core
> reference manual.
> 
> > 
> > > +
> > >    num-lanes:
> > >      description:
> > >        Number of PCIe link lanes to use. Can be omitted should the already
> > > diff --git a/Documentation/devicetree/bindings/pci/snps,dw-pcie-ep.yaml b/Documentation/devicetree/bindings/pci/snps,dw-pcie-ep.yaml
> > > index dcd521aed213..fc3b5d4ac245 100644
> > > --- a/Documentation/devicetree/bindings/pci/snps,dw-pcie-ep.yaml
> > > +++ b/Documentation/devicetree/bindings/pci/snps,dw-pcie-ep.yaml
> > > @@ -55,4 +55,6 @@ examples:
> > >  
> > >        phys = <&pcie_phy0>, <&pcie_phy1>, <&pcie_phy2>, <&pcie_phy3>;
> > >        phy-names = "pcie0", "pcie1", "pcie2", "pcie3";
> > > +
> > > +      max-link-speed = <3>;
> > >      };
> > > diff --git a/Documentation/devicetree/bindings/pci/snps,dw-pcie.yaml b/Documentation/devicetree/bindings/pci/snps,dw-pcie.yaml
> > > index 4a5c8b933b52..01cedf51e0f8 100644
> > > --- a/Documentation/devicetree/bindings/pci/snps,dw-pcie.yaml
> > > +++ b/Documentation/devicetree/bindings/pci/snps,dw-pcie.yaml
> > > @@ -74,4 +74,5 @@ examples:
> > >        phy-names = "pcie";
> > >  
> > >        num-lanes = <1>;
> > > +      max-link-speed = <3>;
> > 
> 
> > This should give you an error because pci-bus.yaml only goes up to 4. 
> 
> I've set max-link-speed to "3" here. So no error will be caused neither
> by this schema nor by the pci-bus.yaml bindings.
> 
> * Though these examples won't be evaluated because the generic DW PCIe
> RP and EP schemas have been marked as "select: false".
> 
> > 
> > I'm not really sure that limiting it in the common schema is too useful. 
> > We're going to be updating it one step at a time. Limiting it is really 
> > only helpful for specific implementations.
> > 
> 
> I disagree. As I said above the max PCIe speed limit set here has been
> taken from the HW reference manual so it describes the modern DW PCIe
> controllers capability. No mater what value is set by the pci-bus.yaml
> schema (eventually we'll get to have it increased to Gen5 too) we can
> use the DW PCIe-specific limitation here as a known upper capabilities
> bound.
> 
> > Patch 1 didn't apply for me, so none of the checks ran.
> 
> I see. The series will be re-based onto 5.19-rc1 in the next patchset
> revision.
> 
> -Sergey
> 
> > 
> > Rob