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

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Message-ID: <20190819192440.GT253360@google.com>
Date:   Mon, 19 Aug 2019 14:24:40 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Z.q. Hou" <zhiqiang.hou@....com>
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

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?

> 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
>