linux-kernel - Re: [PATCH] PCI: aardvark: Update comment about disabling link training

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Date:   Thu, 24 Sep 2020 17:22:32 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: aardvark: Update comment about disabling link
 training

On Thursday 24 September 2020 10:11:06 Bjorn Helgaas wrote:
> On Thu, Sep 24, 2020 at 10:46:18AM +0200, Pali Rohár wrote:
> > It is not HW bug or workaround for some cards but it is requirement by PCI
> > Express spec. After fundamental reset is needed 100ms delay prior enabling
> > link training. So update comment in code to reflect this requirement.
> > 
> > Signed-off-by: Pali Rohár <pali@...nel.org>
> > ---
> >  drivers/pci/controller/pci-aardvark.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/controller/pci-aardvark.c b/drivers/pci/controller/pci-aardvark.c
> > index 50ab6d7519ae..19b9b79226e5 100644
> > --- a/drivers/pci/controller/pci-aardvark.c
> > +++ b/drivers/pci/controller/pci-aardvark.c
> > @@ -259,7 +259,12 @@ static void advk_pcie_issue_perst(struct advk_pcie *pcie)
> >  	if (!pcie->reset_gpio)
> >  		return;
> >  
> > -	/* PERST does not work for some cards when link training is enabled */
> > +	/*
> > +	 * As required by PCI Express spec 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.
> 
> Can you please include the spec citation here?  In the PCIe base spec,
> PERST# is only mentioned in PCIe r5.0, sec 6.6.1, and I don't see the
> connection there to 100ms between de-assert of PERST# and enabling
> link training.

Hello! I copied this "comment" from other place in pci-aardvark.c where
that timeout 100ms is already applied. Timeout with explanation comment
was introduced in following commit:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f4c7d053d7f7

Here are links to discussions about that patch:

https://lore.kernel.org/linux-pci/20190313213752.1246-1-repk@triplefau.lt/T/#u
https://lore.kernel.org/linux-pci/20190522213351.21366-2-repk@triplefau.lt/T/#u

> Sec 6.1.1 does talk about 100ms before sending config requests (for
> ports that support <= 5 GT/s), and 100ms after link training completes
> (for ports that support > 5 GT/s).
> 
> Maybe there's more language in a form-factor spec or something?
> 
> > +	 */
> >  	reg = advk_readl(pcie, PCIE_CORE_CTRL0_REG);
> >  	reg &= ~LINK_TRAINING_EN;
> >  	advk_writel(pcie, reg, PCIE_CORE_CTRL0_REG);
> > -- 
> > 2.20.1
> >