Message-ID: <1923573.HjpiSnITgW@en-pc05>
Date:   Wed, 14 Jun 2017 05:52:58 +0000
From:   Schöfegger Stefan 
        <Stefan.Schoefegger@...zinger.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Richard Zhu <hongxing.zhu@....com>,
        Arnd Bergmann <arnd@...db.de>,
        open list <linux-kernel@...r.kernel.org>,
        Kishon Vijay Abraham I <kishon@...com>,
        Jingoo Han <jingoohan1@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "moderated list:PCI DRIVER FOR IMX6" 
        <linux-arm-kernel@...ts.infradead.org>,
        Lucas Stach <l.stach@...gutronix.de>
Subject: Re: [PATCH v2 1/1] PCI: imx6: Add pcie compliance test option

On Tuesday, June 13, 2017 8:58:47 AM CEST Bjorn Helgaas wrote:
> On Tue, Jun 13, 2017 at 05:43:14AM +0000, Schöfegger Stefan wrote:
> > On Monday, June 12, 2017 6:49:24 PM CEST Bjorn Helgaas wrote:
> > > On Wed, Jun 07, 2017 at 01:36:11PM +0200, Stefan Schoefegger wrote:
> > > > Link speed must not be limited to gen1 during link test for compliance
> > > > tests
> > > > 
> > > > Signed-off-by: Stefan Schoefegger <stefan.schoefegger@...zinger.com>
> > > > ---
> > > > 
> > > > Changes since v1:
> > > >  - pci-imx6.c moved to dwc directory
> > > >  
> > > >  drivers/pci/dwc/Kconfig    | 10 ++++++++++
> > > >  drivers/pci/dwc/pci-imx6.c | 21 ++++++++++++---------
> > > >  2 files changed, 22 insertions(+), 9 deletions(-)
> > > > 
> > > > diff --git a/drivers/pci/dwc/Kconfig b/drivers/pci/dwc/Kconfig
> > > > index b7e15526d676..b6e9ced5a45d 100644
> > > > --- a/drivers/pci/dwc/Kconfig
> > > > +++ b/drivers/pci/dwc/Kconfig
> > > > @@ -77,6 +77,16 @@ config PCI_IMX6
> > > > 
> > > >  	select PCIEPORTBUS
> > > >  	select PCIE_DW_HOST
> > > > 
> > > > +config PCI_IMX6_COMPLIANCE_TEST
> > > > +	bool "Enable pcie compliance tests on imx6"
> > > > +	depends on PCI_IMX6
> > > > +	default n
> > > > +	help
> > > > +	  Enables support for pcie compliance test on FSL iMX SoCs.
> > > > +	  The link speed wouldn't be limited to gen1 when enabled.
> > > > +	  Enable only during compliance tests, otherwise
> > > > +	  link detection will fail on some peripherals.
> > > 
> > > I'm puzzled about why we would want to merge this patch.  It looks
> > > like we're trying to game the system to make the device pass
> > > compliance testing when it isn't really compliant.  Is this config
> > > option useful to users, or is it only useful during internal
> > > development of iMX SoCs?
> > 
> > It's not for passing compliance tests, it is necessary to do the
> > compliance
> > tests. Without this patch only gen 1 speed is possible to test. Also i.mx6
> > is not fully gen2 compliant (withour external clk) we should have the
> > option to do gen2 tests. Switching from gen1 to gen2 is done with a
> > 100MHz (1ms) clk pulse on the receiver. Without this patch link speed is
> > forced to gen1 afterwards.
> 
> I don't understand the purpose of this yet, so maybe all we need is a
> better description.

This patch enabled compliance tests for pcie gen2 published by pci-sig. 
(Signal level, signal timing, jitter and rise/fall times on tx lines). Special 
hardware is needed to do this tests (compliance load board, oszilloscope with 
about 10GHz bandwith). The pcie phy switches in a complinace test state where 
the phy outputs a special test pattern (without having a real link to a 
device). The bitrate and de-emphasis must be switched. The driver (without 
this patches) does not allow to switch to gen2 because it falls back to gen1. 
It is impossible to generate the gen2 test pattern.
The patch now removes the forced gen1 start to allow generating gen2 test 
pattern. gen1 / gen2 switch is done through signals generated on the 
compliance load board (which triggers switching in the phy).
> 
> "It's not for passing compliance testing, it is necessary to do the
> compliance tests" doesn't make sense to me -- it seems
> self-contradictory.
> 
> The Kconfig text says "enable only for testing because it makes link
> detection fail."  To me that means this option is not useful for
> users.  We need some justification for why it should be in the
> mainline kernel, where users and distros may enable it.

It use useless for users and distros. Only board designer want this option. If 
this is not a reason for applying it's ok for me.
> 
> If you can only support gen2 in certain board configurations, maybe
> you should add a config option that can always be enabled for those
> boards.
> 
> > Yes it is only useful for board setup, it is comparable to
> > CONFIG_USB_EHSET_TEST_FIXTURE for usb (ok, this is more general and not
> > host specific).
> 
> USB_EHSET_TEST_FIXTURE (added by 1353aa53851e ("usb: misc: EHSET Test
> Fixture device driver for host compliance")) looks like just another
> driver in the sense that it's always safe to enable it and it doesn't
> hurt anything if you enable it without having the hardware.
> 
> This patch doesn't seem comparable to USB_EHSET_TEST_FIXTURE because
> this new option apparently breaks link detection in some cases.

This depends on the point of view (Black box vs. white box) :-) Both are for 
doing compliance tests on signal level. But I think we don't need to discuss 
this further.
> 
> > > >  config PCIE_SPEAR13XX
> > > >  
> > > >  	bool "STMicroelectronics SPEAr PCIe controller"
> > > >  	depends on PCI
> > > > 
> > > > diff --git a/drivers/pci/dwc/pci-imx6.c b/drivers/pci/dwc/pci-imx6.c
> > > > index 19a289b8cc94..b0fbe52e25b0 100644
> > > > --- a/drivers/pci/dwc/pci-imx6.c
> > > > +++ b/drivers/pci/dwc/pci-imx6.c
> > > > @@ -533,15 +533,18 @@ static int imx6_pcie_establish_link(struct
> > > > imx6_pcie
> > > > *imx6_pcie)>
> > > > 
> > > >  	u32 tmp;
> > > >  	int ret;
> > > > 
> > > > -	/*
> > > > -	 * Force Gen1 operation when starting the link.  In case the link is
> > > > -	 * started in Gen2 mode, there is a possibility the devices on the
> > > > -	 * bus will not be detected at all.  This happens with PCIe
> > > > switches.
> > > > -	 */
> > > > -	tmp = dw_pcie_readl_dbi(pci, PCIE_RC_LCR);
> > > > -	tmp &= ~PCIE_RC_LCR_MAX_LINK_SPEEDS_MASK;
> > > > -	tmp |= PCIE_RC_LCR_MAX_LINK_SPEEDS_GEN1;
> > > > -	dw_pcie_writel_dbi(pci, PCIE_RC_LCR, tmp);
> > > > +	if (!IS_ENABLED(CONFIG_PCI_IMX6_COMPLIANCE_TEST)) {
> > > > +		/*
> > > > +		 * Force Gen1 operation when starting the link.  In case the
> > > > +		 * link is started in Gen2 mode, there is a possibility the
> > > > +		 * devices on the bus will not be detected at all.  This
> > > > +		 * happens with PCIe switches.
> > > > +		 */
> > > > +		tmp = dw_pcie_readl_dbi(pci, PCIE_RC_LCR);
> > > > +		tmp &= ~PCIE_RC_LCR_MAX_LINK_SPEEDS_MASK;
> > > > +		tmp |= PCIE_RC_LCR_MAX_LINK_SPEEDS_GEN1;
> > > > +		dw_pcie_writel_dbi(pci, PCIE_RC_LCR, tmp);
> > > > +	}
> > > > 
> > > >  	/* Start LTSSM. */
> > > >  	if (imx6_pcie->variant == IMX7D)

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