linux-kernel - Re: [PATCH v5] PCI: cadence: Retrain Link to work around Gen2 training defect.

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Message-ID: <CAL_JsqLepmopGObX_r+7gtR+keaNtEAA3WA1j697T4jAWP8DHA@mail.gmail.com>
Date:   Wed, 16 Dec 2020 11:01:51 -0600
From:   Rob Herring <robh@...nel.org>
To:     Kishon Vijay Abraham I <kishon@...com>
Cc:     Tom Joseph <tjoseph@...ence.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Nadeem Athani <nadeem@...ence.com>,
        linux-omap <linux-omap@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Milind Parab <mparab@...ence.com>,
        Swapnil Kashinath Jakhade <sjakhade@...ence.com>,
        Parshuram Raju Thombare <pthombar@...ence.com>
Subject: Re: [PATCH v5] PCI: cadence: Retrain Link to work around Gen2
 training defect.

On Wed, Dec 16, 2020 at 9:01 AM Kishon Vijay Abraham I <kishon@...com> wrote:
>
> Hi Rob,
>
> On 15/12/20 9:23 pm, Rob Herring wrote:
> > On Tue, Dec 15, 2020 at 1:00 AM Kishon Vijay Abraham I <kishon@...com> wrote:
> >>
> >> From: Nadeem Athani <nadeem@...ence.com>
> >>
> >> Cadence controller will not initiate autonomous speed change if strapped as
> >> Gen2. The Retrain Link bit is set as quirk to enable this speed change.
> >>
> >> Signed-off-by: Nadeem Athani <nadeem@...ence.com>
> >> [kishon@...com: Enable the workaround for TI's J721E SoC]
> >> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
> >> ---
> >> Hi Lorenzo,
> >> The previous version of the patch can be found at [1].
> >> I slightly re-worked the patch from Nadeem
> >> *) Removed additional Link Up Check
> >> *) Removed quirk from pcie-cadence-plat.c
> >> *) Also removed additional compatible
> >>    "cdns,cdns-pcie-host-quirk-retrain" added in that series
> >> *) Enabled the quirk for J721E
> >> [1] -> http://lore.kernel.org/r/20201211144236.3825-1-nadeem@cadence.com
> >>
> >>  drivers/pci/controller/cadence/pci-j721e.c    |  3 +
> >>  .../controller/cadence/pcie-cadence-host.c    | 67 ++++++++++++++-----
> >>  drivers/pci/controller/cadence/pcie-cadence.h | 11 ++-
> >>  3 files changed, 62 insertions(+), 19 deletions(-)
> >>
> >> diff --git a/drivers/pci/controller/cadence/pci-j721e.c b/drivers/pci/controller/cadence/pci-j721e.c
> >> index dac1ac8a7615..baf729850cb1 100644
> >> --- a/drivers/pci/controller/cadence/pci-j721e.c
> >> +++ b/drivers/pci/controller/cadence/pci-j721e.c
> >> @@ -64,6 +64,7 @@ enum j721e_pcie_mode {
> >>
> >>  struct j721e_pcie_data {
> >>         enum j721e_pcie_mode    mode;
> >> +       bool                    quirk_retrain_flag;
> >>  };
> >>
> >>  static inline u32 j721e_pcie_user_readl(struct j721e_pcie *pcie, u32 offset)
> >> @@ -280,6 +281,7 @@ static struct pci_ops cdns_ti_pcie_host_ops = {
> >>
> >>  static const struct j721e_pcie_data j721e_pcie_rc_data = {
> >>         .mode = PCI_MODE_RC,
> >> +       .quirk_retrain_flag = true,
> >>  };
> >>
> >>  static const struct j721e_pcie_data j721e_pcie_ep_data = {
> >> @@ -388,6 +390,7 @@ static int j721e_pcie_probe(struct platform_device *pdev)
> >>
> >>                 bridge->ops = &cdns_ti_pcie_host_ops;
> >>                 rc = pci_host_bridge_priv(bridge);
> >> +               rc->quirk_retrain_flag = data->quirk_retrain_flag;
> >>
> >>                 cdns_pcie = &rc->pcie;
> >>                 cdns_pcie->dev = dev;
> >> diff --git a/drivers/pci/controller/cadence/pcie-cadence-host.c b/drivers/pci/controller/cadence/pcie-cadence-host.c
> >> index 811c1cb2e8de..773c0d1137ed 100644
> >> --- a/drivers/pci/controller/cadence/pcie-cadence-host.c
> >> +++ b/drivers/pci/controller/cadence/pcie-cadence-host.c
> >> @@ -77,6 +77,50 @@ static struct pci_ops cdns_pcie_host_ops = {
> >>         .write          = pci_generic_config_write,
> >>  };
> >>
> >> +static int cdns_pcie_host_wait_for_link(struct cdns_pcie *pcie)
> >> +{
> >> +       struct device *dev = pcie->dev;
> >> +       int retries;
> >> +
> >> +       /* Check if the link is up or not */
> >> +       for (retries = 0; retries < LINK_WAIT_MAX_RETRIES; retries++) {
> >> +               if (cdns_pcie_link_up(pcie)) {
> >> +                       dev_info(dev, "Link up\n");
> >> +                       return 0;
> >> +               }
> >> +               usleep_range(LINK_WAIT_USLEEP_MIN, LINK_WAIT_USLEEP_MAX);
> >> +       }
> >> +
> >> +       return -ETIMEDOUT;
> >> +}
> >> +
> >> +static void cdns_pcie_retrain(struct cdns_pcie *pcie)
> >> +{
> >> +       u32 lnk_cap_sls, pcie_cap_off = CDNS_PCIE_RP_CAP_OFFSET;
> >> +       u16 lnk_stat, lnk_ctl;
> >> +
> >> +       /*
> >> +        * Set retrain bit if current speed is 2.5 GB/s,
> >> +        * but the PCIe root port support is > 2.5 GB/s.
> >
> > If you don't have the retrain quirk, wouldn't this condition never
> > happen and then the function is just a nop? So this could just be
> > called unconditionally.
>
> Yeah, but only for the quirk we have to retrain to go to GEN2 speed
> mode. Else the HW will automatically retrain and go to GEN2.

Again, so you don't need a flag for this. Comparing the speed is
enough. IOW, all you need is:

if (current speed < advertised speed)
  do retrain

The question is the condition ever true and you don't want to do a
retrain? I could see higher speeds being unstable or something, but
then 'advertised speed' would be lowered in that case (to prevent auto
retraining, right?) and the condition would be false.

Rob