linux-kernel - Re: [PATCH v4 04/12] dt-bindings: PCI: mvebu: Add num-lanes property

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Date:   Fri, 25 Feb 2022 13:58:10 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Marek Behún <kabel@...nel.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 04/12] dt-bindings: PCI: mvebu: Add num-lanes property

On Thursday 24 February 2022 18:02:26 Bjorn Helgaas wrote:
> On Tue, Feb 22, 2022 at 04:50:22PM +0100, Pali Rohár wrote:
> > Controller driver needs to correctly configure PCIe link if it contains 1
> > or 4 SerDes PCIe lanes. Therefore add a new 'num-lanes' DT property for
> > mvebu PCIe controller. Property 'num-lanes' seems to be de-facto standard
> > way how number of lanes is specified in other PCIe controllers.
> > 
> > Signed-off-by: Pali Rohár <pali@...nel.org>
> > Acked-by: Rob Herring <robh@...nel.org>
> > ---
> >  Documentation/devicetree/bindings/pci/mvebu-pci.txt | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/pci/mvebu-pci.txt b/Documentation/devicetree/bindings/pci/mvebu-pci.txt
> > index 6173af6885f8..24225852bce0 100644
> > --- a/Documentation/devicetree/bindings/pci/mvebu-pci.txt
> > +++ b/Documentation/devicetree/bindings/pci/mvebu-pci.txt
> > @@ -77,6 +77,7 @@ and the following optional properties:
> >  - marvell,pcie-lane: the physical PCIe lane number, for ports having
> >    multiple lanes. If this property is not found, we assume that the
> >    value is 0.
> > +- num-lanes: number of SerDes PCIe lanes for this link (1 or 4)
> >  - reset-gpios: optional GPIO to PERST#
> >  - reset-delay-us: delay in us to wait after reset de-assertion, if not
> >    specified will default to 100ms, as required by the PCIe specification.
> > @@ -141,6 +142,7 @@ pcie-controller {
> >  		interrupt-map = <0 0 0 0 &mpic 58>;
> >  		marvell,pcie-port = <0>;
> >  		marvell,pcie-lane = <0>;
> > +		num-lanes = <1>;
> 
> Is this patch really necessary?

This is just documentation patch. And I think that documentation is
always important.

> AFAICS, the related driver change
> only sets "port->is_x4 = true" when "num-lanes = <4>", and in all
> other cases it defaults to a Max Link Width of 1:
> 
>   lnkcap |= (port->is_x4 ? 4 : 1) << 4;

Yes!

And this registers configures number of lanes in HW.

> I don't see the point of adding a value that we don't validate or do
> anything with.  E.g., I don't see an error message that would catch
> "num-lanes = <3>".
> 
> Bjorn

In past I was told that kernel should not do validation of DT properties
and it is job of some DT schema validation. That is why I did not added
code into kernel which show error message when value different than 1
and 4 is specified in DT.

But issue here is that there is no DT schema for pci-mvebu as above
.txt file was not converted to YAML schema yet. This is something which
should be improved...