linux-kernel - Re: [v8,3/7] PCI: mediatek-gen3: Add MediaTek Gen3 driver for MT8192

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Date:   Thu, 11 Mar 2021 13:38:44 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Jianjun Wang <jianjun.wang@...iatek.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh+dt@...nel.org>, maz@...nel.org,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Ryder Lee <ryder.lee@...iatek.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-pci@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Sj Huang <sj.huang@...iatek.com>, youlin.pei@...iatek.com,
        chuanjia.liu@...iatek.com, qizhong.cheng@...iatek.com,
        sin_jieyang@...iatek.com, drinkcat@...omium.org,
        Rex-BC.Chen@...iatek.com, anson.chuang@...iatek.com
Subject: Re: [v8,3/7] PCI: mediatek-gen3: Add MediaTek Gen3 driver for MT8192

On Wednesday 24 February 2021 14:11:28 Jianjun Wang wrote:
> +static int mtk_pcie_startup_port(struct mtk_pcie_port *port)
> +{
...
> +
> +	/* Delay 100ms to wait the reference clocks become stable */
> +	msleep(100);
> +
> +	/* De-assert PERST# signal */
> +	val &= ~PCIE_PE_RSTB;
> +	writel_relaxed(val, port->base + PCIE_RST_CTRL_REG);

Hello! This is a new driver which introduce yet another custom timeout
prior PERST# signal for PCIe card is de-asserted. Timeouts for other
drivers I collected in older email [2].

Please look at my email [1] about PCIe Warm Reset if you have any clue
about it. Lorenzo and Rob already expressed that this timeout should not
be driver specific. But nobody was able to "decode" and "understand"
PCIe spec yet about these timeouts.

> +
> +	/* Check if the link is up or not */
> +	err = readl_poll_timeout(port->base + PCIE_LINK_STATUS_REG, val,
> +				 !!(val & PCIE_PORT_LINKUP), 20,
> +				 50 * USEC_PER_MSEC);

IIRC, you need to wait at least 100ms after de-asserting PERST# signal
as it is required by PCIe specs and also because experiments proved that
some Compex wifi cards (e.g. WLE900VX) are not detected if you do not
wait this minimal time.

> +	if (err) {
> +		val = readl_relaxed(port->base + PCIE_LTSSM_STATUS_REG);
> +		dev_err(port->dev, "PCIe link down, ltssm reg val: %#x\n", val);
> +		return err;
> +	}

[1] - https://lore.kernel.org/linux-pci/20210310110535.zh4pnn4vpmvzwl5q@pali/
[2] - https://lore.kernel.org/linux-pci/20200424092546.25p3hdtkehohe3xw@pali/