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

From: "Pali Rohár" <pali@kernel.org>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Rob Herring <robh@kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] PCI: aardvark: Update comment about disabling link training
Date: Sun, 11 Oct 2020 19:21:49 +0200	[thread overview]
Message-ID: <20201011172149.x7crspugv2xne6ui@pali> (raw)
In-Reply-To: <20200924152232.ecoxpmxdc5iyrz76@pali>

On Thursday 24 September 2020 17:22:32 Pali Rohár wrote:
> 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@kernel.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

Bjorn or Lorenzo, do you need something else for this patch? It just
updates comment and basically clarify why PERST does not work for some
cards when link training is enabled.

> > 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
> > > 