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

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Date:   Sun, 19 Jun 2022 17:27:20 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Frank Li <Frank.Li@....com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        linux-pci@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/17] dt-bindings: PCI: dwc: Add max-link-speed
 common property

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@...kalelectronics.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