linux-kernel - Re: [PATCHv2 0/4] Layerscape: Remove num-lanes property from PCIe nodes

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Message-ID: <20190823094424.GB14582@e119886-lin.cambridge.arm.com>
Date:   Fri, 23 Aug 2019 10:44:25 +0100
From:   Andrew Murray <andrew.murray@....com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:     "Z.q. Hou" <zhiqiang.hou@....com>,
        "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>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.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>,
        "M.h. Lian" <minghuan.lian@....com>
Subject: Re: [PATCHv2 0/4] Layerscape: Remove num-lanes property from PCIe
 nodes

On Thu, Aug 22, 2019 at 05:48:15PM +0100, Lorenzo Pieralisi wrote:
> On Tue, Aug 20, 2019 at 07:28:37AM +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. 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.
> > 
> > Hou Zhiqiang (4):
> >   dt-bindings: PCI: designware: Remove the num-lanes from Required
> >     properties
> >   PCI: dwc: Return directly when num-lanes is not found
> >   ARM: dts: ls1021a: Remove num-lanes property from PCIe nodes
> >   arm64: dts: fsl: Remove num-lanes property from PCIe nodes
> > 
> >  Documentation/devicetree/bindings/pci/designware-pcie.txt | 1 -
> >  arch/arm/boot/dts/ls1021a.dtsi                            | 2 --
> >  arch/arm64/boot/dts/freescale/fsl-ls1012a.dtsi            | 1 -
> >  arch/arm64/boot/dts/freescale/fsl-ls1043a.dtsi            | 3 ---
> >  arch/arm64/boot/dts/freescale/fsl-ls1046a.dtsi            | 6 ------
> >  arch/arm64/boot/dts/freescale/fsl-ls1088a.dtsi            | 3 ---
> >  arch/arm64/boot/dts/freescale/fsl-ls208xa.dtsi            | 4 ----
> >  drivers/pci/controller/dwc/pcie-designware.c              | 6 ++++--
> >  8 files changed, 4 insertions(+), 22 deletions(-)
> 
> What a mess.
> 
> I am going to apply these but first if anyone can explain to
> me what commit 907fce090253 was _supposed_ to to I would
> be grateful, I read it multiple times but I still have not
> understood it. This series does the right thing but why things

The DWC controller drivers all implement a .host_init callback -
some of the drivers choose to call dw_pcie_setup_rc from their
callback which, amongst other things will set up/train the link.

As far as I can tell, dw_pcie_setup_rc is the only user of pp->lanes.
Therefore for hardware where the link is already set up by firmware
and thus dw_pcie_setup_rc is never called - it is unnecessary to
read the DT value for pp->lanes. So the first hunk in 907fce090253
gets rid of the error and makes the num-lanes property optional.

However this opens up the possibility of a DT misconfiguration for
other controllers that do call dw_pcie_setup_rc, i.e. they set
num-lanes to 0 when it is required. Therefore the second hunk
ensures that an error is emitted when num-lanes was needed but not
provided.

> are they way they are in the mainline honestly I have no
> idea, this does not make any sense in the slightest:
> 
> ret = of_property_read_u32(np, "num-lanes", &lanes);
> if (ret)
> 	lanes = 0;

Please note that the code below is in a different function to the
code above.

> 
> /* Set the number of lanes */
> val = dw_pcie_readl_dbi(pci, PCIE_PORT_LINK_CONTROL);
> val &= ~PORT_LINK_MODE_MASK;
> switch (lanes) {
> case 1:
> 	val |= PORT_LINK_MODE_1_LANES;
> 	break;
> case 2:
> 	val |= PORT_LINK_MODE_2_LANES;
> 	break;
> case 4:
> 	val |= PORT_LINK_MODE_4_LANES;
> 	break;
> case 8:
> 	val |= PORT_LINK_MODE_8_LANES;
> 	break;
> default:
> 	dev_err(pci->dev, "num-lanes %u: invalid value\n", lanes);
> 	return;
> }
> 
> why do we need to set lanes to 0 if num-lanes is not present ? To print
> an error message ?

At this point in time, the controller is trying to train the link but
it doesn't know how many lanes, so we need to error. We don't error when
reading the device tree earlier - because at that point in time we don't
know if num-lanes is optional or not.

Thanks,

Andrew Murray

> 
> I really do not understand this code.
> 
> Lorenzo