linux-kernel - Re: [V3] PCI: fu740: Drop to 2.5GT/s to fix initial device probing on some boards

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Date:   Mon, 21 Mar 2022 14:49:41 +0100
From:   Alexandre Ghiti <alex@...ti.fr>
To:     Ben Dooks <ben.dooks@...ethink.co.uk>, linux-pci@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Rob Herring <robh@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Greentime Hu <greentime.hu@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        David Abdurachmanov <david.abdurachmanov@...ive.com>
Subject: Re: [V3] PCI: fu740: Drop to 2.5GT/s to fix initial device probing on
 some boards

Hi Ben,

On 3/18/22 16:24, Ben Dooks wrote:
> The fu740 PCIe core does not probe any devices on the SiFive Unmatched
> board without this fix (or having U-Boot explicitly start the PCIe via
> either boot-script or user command). The fix is to start the link at
> 2.5GT/s speeds and once the link is up then change the maximum speed back
> to the default.
>
> The U-Boot driver claims to set the link-speed to 2.5GT/s to get the probe
> to work (and U-Boot does print link up at 2.5GT/s) in the following code:
> https://source.denx.de/u-boot/u-boot/-/blob/master/drivers/pci/pcie_dw_sifive.c?id=v2022.01#L271
>
> Signed-off-by: Ben Dooks <ben.dooks@...ethink.co.uk>
> --
> Note, this patch has had significant re-work since the previous 4
> sets, including trying to fix style, message, reliance on the U-Boot
> fix and the comments about usage of LINK_CAP and reserved fields.
>
> v2:
> - fix issues with Gen1/2.5GTs
> - updated comment on the initial probe
> - run tests with both uninitialised and initialsed pcie from uboot
> ---
>   drivers/pci/controller/dwc/pcie-fu740.c | 52 ++++++++++++++++++++++++-
>   1 file changed, 51 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/controller/dwc/pcie-fu740.c b/drivers/pci/controller/dwc/pcie-fu740.c
> index 842b7202b96e..ecac0364178a 100644
> --- a/drivers/pci/controller/dwc/pcie-fu740.c
> +++ b/drivers/pci/controller/dwc/pcie-fu740.c
> @@ -181,10 +181,60 @@ static int fu740_pcie_start_link(struct dw_pcie *pci)
>   {
>   	struct device *dev = pci->dev;
>   	struct fu740_pcie *afp = dev_get_drvdata(dev);
> +	u8 cap_exp = dw_pcie_find_capability(pci, PCI_CAP_ID_EXP);
> +	int ret;
> +	u32 orig, tmp;
> +
> +	/*
> +	 * Force 2.5GT/s when starting the link, due to some devices not
> +	 * probing at higher speeds. This happens with the PCIe switch
> +	 * on the Unmatched board when U-Boot has not initialised the PCIe.
> +	 * The fix in U-Boot is to force 2.5GT/s, which then gets cleared
> +	 * by the soft reset does by this driver.
> +	 */
> +
> +	dev_dbg(dev, "cap_exp at %x\n", cap_exp);
> +	dw_pcie_dbi_ro_wr_en(pci);
> +
> +	tmp = dw_pcie_readl_dbi(pci, cap_exp + PCI_EXP_LNKCAP);
> +	orig = tmp & PCI_EXP_LNKCAP_SLS;
> +	tmp &= ~PCI_EXP_LNKCAP_SLS;
> +	tmp |= PCI_EXP_LNKCAP_SLS_2_5GB;
> +	dw_pcie_writel_dbi(pci, cap_exp + PCI_EXP_LNKCAP, tmp);
>   
>   	/* Enable LTSSM */
>   	writel_relaxed(0x1, afp->mgmt_base + PCIEX8MGMT_APP_LTSSM_ENABLE);
> -	return 0;
> +
> +	ret = dw_pcie_wait_for_link(pci);
> +	if (ret) {
> +		dev_err(dev, "error: link did not start\n");
> +		goto err;
> +	}
> +
> +	tmp = dw_pcie_readl_dbi(pci, cap_exp + PCI_EXP_LNKCAP);
> +	if ((tmp & PCI_EXP_LNKCAP_SLS) != orig) {
> +		dev_dbg(dev, "changing speed back to original\n");
> +
> +		tmp &= ~PCI_EXP_LNKCAP_SLS;
> +		tmp |= orig;
> +		dw_pcie_writel_dbi(pci, cap_exp + PCI_EXP_LNKCAP, tmp);
> +
> +		tmp = dw_pcie_readl_dbi(pci, PCIE_LINK_WIDTH_SPEED_CONTROL);
> +		tmp |= PORT_LOGIC_SPEED_CHANGE;
> +		dw_pcie_writel_dbi(pci, PCIE_LINK_WIDTH_SPEED_CONTROL, tmp);
> +
> +		ret = dw_pcie_wait_for_link(pci);
> +		if (ret) {
> +			dev_err(dev, "error: link did not start at new speed\n");
> +			goto err;
> +		}
> +	}
> +
> +	ret = 0;
> +err:
> +	WARN_ON(ret);	/* we assume that errors will be very rare */
> +	dw_pcie_dbi_ro_wr_dis(pci);
> +	return ret;
>   }
>   
>   static int fu740_pcie_host_init(struct pcie_port *pp)

+cc Maciej and David as there is this other fix that seems to do the 
same but differently, it's been under review for some time now: 
https://lore.kernel.org/all/20220302000043.GA662523@bhelgaas/t/

I fell onto this issue recently, I'll give your patch and the above a 
try soon.

Thanks

Alex