linux-kernel - [PATCH 4.19 27/69] PCI: aardvark: Update comment about disabling link training

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Message-Id: <20211129181704.557091873@linuxfoundation.org>
Date:   Mon, 29 Nov 2021 19:18:09 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Pali Rohár <pali@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Marek Behún <kabel@...nel.org>
Subject: [PATCH 4.19 27/69] PCI: aardvark: Update comment about disabling link training

From: Pali Rohár <pali@...nel.org>

commit 1d1cd163d0de22a4041a6f1aeabcf78f80076539 upstream.

According to PCI Express Base Specifications (rev 4.0, 6.6.1
"Conventional reset"), after fundamental reset a 100ms delay is needed
prior to enabling link training.

Update comment in code to reflect this requirement.

Link: https://lore.kernel.org/r/20201202184659.3795-1-pali@kernel.org
Signed-off-by: Pali Rohár <pali@...nel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Signed-off-by: Marek Behún <kabel@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/pci/controller/pci-aardvark.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/drivers/pci/controller/pci-aardvark.c
+++ b/drivers/pci/controller/pci-aardvark.c
@@ -236,7 +236,14 @@ static void advk_pcie_issue_perst(struct
 	if (!pcie->reset_gpio)
 		return;
 
-	/* PERST does not work for some cards when link training is enabled */
+	/*
+	 * As required by PCI Express spec (PCI Express Base Specification, REV.
+	 * 4.0 PCI Express, February 19 2014, 6.6.1 Conventional Reset) a delay
+	 * for at least 100ms after de-asserting PERST# signal is needed before
+	 * link training is enabled. So ensure that link training is disabled
+	 * prior de-asserting PERST# signal to fulfill that PCI Express spec
+	 * requirement.
+	 */
 	reg = advk_readl(pcie, PCIE_CORE_CTRL0_REG);
 	reg &= ~LINK_TRAINING_EN;
 	advk_writel(pcie, reg, PCIE_CORE_CTRL0_REG);