linux-kernel - Re: [PATCH V16 13/13] PCI: tegra: Add Tegra194 PCIe support

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Date: Wed, 14 Feb 2024 13:12:49 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Vidya Sagar <vidyas@...dia.com>
Cc: Lorenzo Pieralisi <lpieralisi@...nel.org>, bhelgaas@...gle.com,
	robh+dt@...nel.org, mark.rutland@....com, thierry.reding@...il.com,
	jonathanh@...dia.com, Kishon Vijay Abraham I <kishon@...nel.org>,
	catalin.marinas@....com, Will Deacon <will@...nel.org>,
	jingoohan1@...il.com, gustavo.pimentel@...opsys.com,
	digetx@...il.com, mperttunen@...dia.com, linux-pci@...r.kernel.org,
	devicetree@...r.kernel.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kthota@...dia.com, mmaddireddy@...dia.com, sagar.tv@...il.com
Subject: Re: [PATCH V16 13/13] PCI: tegra: Add Tegra194 PCIe support

Hi Vidya, question about ancient history:

On Tue, Aug 13, 2019 at 05:06:27PM +0530, Vidya Sagar wrote:
> Add support for Synopsys DesignWare core IP based PCIe host controller
> present in Tegra194 SoC.
> ...

> +static int tegra_pcie_dw_host_init(struct pcie_port *pp)
> +{
> +	struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
> +	struct tegra_pcie_dw *pcie = to_tegra_pcie(pci);
> +	u32 val, tmp, offset, speed;
> +
> +	tegra_pcie_prepare_host(pp);
> +
> +	if (dw_pcie_wait_for_link(pci)) {
> +		/*
> +		 * There are some endpoints which can't get the link up if
> +		 * root port has Data Link Feature (DLF) enabled.
> +		 * Refer Spec rev 4.0 ver 1.0 sec 3.4.2 & 7.7.4 for more info
> +		 * on Scaled Flow Control and DLF.
> +		 * So, need to confirm that is indeed the case here and attempt
> +		 * link up once again with DLF disabled.

This comment suggests that there's an issue with *Endpoints*, not an
issue with the Root Port.  If so, it seems like this problem could
occur with all Root Ports, not just Tegra194.  Do you remember any
details about this?

I don't remember hearing about any similar issues, and this driver is
the only place PCI_EXT_CAP_ID_DLF is referenced, so maybe it is
actually something related to Tegra194?

> +		val = appl_readl(pcie, APPL_DEBUG);
> +		val &= APPL_DEBUG_LTSSM_STATE_MASK;
> +		val >>= APPL_DEBUG_LTSSM_STATE_SHIFT;
> +		tmp = appl_readl(pcie, APPL_LINK_STATUS);
> +		tmp &= APPL_LINK_STATUS_RDLH_LINK_UP;
> +		if (!(val == 0x11 && !tmp)) {
> +			/* Link is down for all good reasons */
> +			return 0;
> +		}
> +
> +		dev_info(pci->dev, "Link is down in DLL");
> +		dev_info(pci->dev, "Trying again with DLFE disabled\n");
> +		/* Disable LTSSM */
> +		val = appl_readl(pcie, APPL_CTRL);
> +		val &= ~APPL_CTRL_LTSSM_EN;
> +		appl_writel(pcie, val, APPL_CTRL);
> +
> +		reset_control_assert(pcie->core_rst);
> +		reset_control_deassert(pcie->core_rst);
> +
> +		offset = dw_pcie_find_ext_capability(pci, PCI_EXT_CAP_ID_DLF);
> +		val = dw_pcie_readl_dbi(pci, offset + PCI_DLF_CAP);
> +		val &= ~PCI_DLF_EXCHANGE_ENABLE;
> +		dw_pcie_writel_dbi(pci, offset, val);
> +
> +		tegra_pcie_prepare_host(pp);
> +
> +		if (dw_pcie_wait_for_link(pci))
> +			return 0;
> +	}
> +
> +	speed = dw_pcie_readw_dbi(pci, pcie->pcie_cap_base + PCI_EXP_LNKSTA) &
> +		PCI_EXP_LNKSTA_CLS;
> +	clk_set_rate(pcie->core_clk, pcie_gen_freq[speed - 1]);
> +
> +	tegra_pcie_enable_interrupts(pp);
> +
> +	return 0;
> +}