linux-kernel - Re: [PATCH v4 05/12] PCI: mvebu: Correctly configure x1/x4 mode

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Message-ID: <20220225000800.GA304526@bhelgaas>
Date:   Thu, 24 Feb 2022 18:08:00 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Pali Rohár <pali@...nel.org>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Marek Behún <kabel@...nel.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 05/12] PCI: mvebu: Correctly configure x1/x4 mode

On Tue, Feb 22, 2022 at 04:50:23PM +0100, Pali Rohár wrote:
> If x1/x4 mode is not set correctly then link with endpoint card is not
> established.
> 
> Use DTS property 'num-lanes' to deteriminate x1/x4 mode.

I know this is already merged, but if tweaking for any other reason,
s/deteriminate/determine/

> +	 * Set Maximum Link Width to X1 or X4 in Root Port's PCIe Link
> +	 * Capability register. This register is defined by PCIe specification
> +	 * as read-only but this mvebu controller has it as read-write and must
> +	 * be set to number of SerDes PCIe lanes (1 or 4). If this register is
> +	 * not set correctly then link with endpoint card is not established.

True, everything in Link Capability is RO or HwInit, but that's for
the architected access via config space.  I think a device-specific
mechanism like this is fair game as long as you do it before anybody
can read it via config space.

> +	 */
> +	lnkcap = mvebu_readl(port, PCIE_CAP_PCIEXP + PCI_EXP_LNKCAP);
> +	lnkcap &= ~PCI_EXP_LNKCAP_MLW;
> +	lnkcap |= (port->is_x4 ? 4 : 1) << 4;
> +	mvebu_writel(port, lnkcap, PCIE_CAP_PCIEXP + PCI_EXP_LNKCAP);