linux-kernel - RE: [PATCH 3/4] ARM: dts: ls1021a: Remove num-lanes property from PCIe nodes

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Date:   Tue, 20 Aug 2019 00:12:31 +0000
From:   "Z.q. Hou" <zhiqiang.hou@....com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "gustavo.pimentel@...opsys.com" <gustavo.pimentel@...opsys.com>,
        "jingoohan1@...il.com" <jingoohan1@...il.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        Leo Li <leoyang.li@....com>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "M.h. Lian" <minghuan.lian@....com>
Subject: RE: [PATCH 3/4] ARM: dts: ls1021a: Remove num-lanes property from
 PCIe nodes

Hi Bjorn,

Thanks a lot for your comments!

> -----Original Message-----
> From: Bjorn Helgaas [mailto:helgaas@...nel.org]
> Sent: 2019年8月20日 3:25
> To: Z.q. Hou <zhiqiang.hou@....com>
> Cc: linux-pci@...r.kernel.org; devicetree@...r.kernel.org;
> linux-kernel@...r.kernel.org; gustavo.pimentel@...opsys.com;
> jingoohan1@...il.com; robh+dt@...nel.org; mark.rutland@....com;
> shawnguo@...nel.org; Leo Li <leoyang.li@....com>;
> lorenzo.pieralisi@....com; M.h. Lian <minghuan.lian@....com>
> Subject: Re: [PATCH 3/4] ARM: dts: ls1021a: Remove num-lanes property
> from PCIe nodes
> 
> On Mon, Aug 12, 2019 at 04:22:27AM +0000, Z.q. Hou wrote:
> > From: Hou Zhiqiang <Zhiqiang.Hou@....com>
> >
> > On FSL Layerscape SoCs, the number of lanes assigned to PCIe
> > controller is not fixed, it is determined by the selected SerDes
> > protocol in the RCW (Reset Configuration Word), and the PCIe link
> > training is completed automatically base on the selected SerDes
> > protocol, and the link width set-up is updated by hardware. So the
> > num-lanes is not needed to specify the link width.
> >
> > The current num-lanes indicates the max lanes PCIe controller can
> > support up to, instead of the lanes assigned to the PCIe controller.
> > This can result in PCIe link training fail after hot-reset. So remove
> > the num-lanes to avoid set-up to incorrect link width.
> 
> It would be useful to explain *why* "num-lanes" in DT causes a link training
> failure.  Maybe the code programs "num-lanes" somewhere, overriding what
> hardware automatically sensed?  Maybe the code tries to bring up exactly
> "num-lanes" lanes even if not all are present?

Will add in v2.
As the Layerscape PCIe controller link training is completed automatically during
the power on reset. It doesn't need software to bring up. So I think it should be the
former.

Thanks,
Zhiqiang

> > Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@....com>
> > ---
> >  arch/arm/boot/dts/ls1021a.dtsi | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/arch/arm/boot/dts/ls1021a.dtsi
> > b/arch/arm/boot/dts/ls1021a.dtsi index 464df4290ffc..2f6977ada447
> > 100644
> > --- a/arch/arm/boot/dts/ls1021a.dtsi
> > +++ b/arch/arm/boot/dts/ls1021a.dtsi
> > @@ -874,7 +874,6 @@
> >  			#address-cells = <3>;
> >  			#size-cells = <2>;
> >  			device_type = "pci";
> > -			num-lanes = <4>;
> >  			num-viewport = <6>;
> >  			bus-range = <0x0 0xff>;
> >  			ranges = <0x81000000 0x0 0x00000000 0x40 0x00010000 0x0
> 0x00010000   /* downstream I/O */
> > @@ -899,7 +898,6 @@
> >  			#address-cells = <3>;
> >  			#size-cells = <2>;
> >  			device_type = "pci";
> > -			num-lanes = <4>;
> >  			num-viewport = <6>;
> >  			bus-range = <0x0 0xff>;
> >  			ranges = <0x81000000 0x0 0x00000000 0x48 0x00010000 0x0
> 0x00010000   /* downstream I/O */
> > --
> > 2.17.1
> >