linux-kernel - Re: [PATCH v2] PCI: cadence: Fix Gen2 Link Retraining process

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Message-ID: <20230330170218.GA3155390@bhelgaas>
Date:   Thu, 30 Mar 2023 12:02:18 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Siddharth Vadapalli <s-vadapalli@...com>
Cc:     tjoseph@...ence.com, lpieralisi@...nel.org, robh@...nel.org,
        kw@...ux.com, bhelgaas@...gle.com, nadeem@...ence.com,
        "Raghavendra, Vignesh" <vigneshr@...com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, srk@...com, nm@...com
Subject: Re: [PATCH v2] PCI: cadence: Fix Gen2 Link Retraining process

On Thu, Mar 30, 2023 at 09:52:06AM +0530, Siddharth Vadapalli wrote:
> Hello Bjorn,
> 
> On 29/03/23 22:38, Bjorn Helgaas wrote:
> > On Wed, Mar 29, 2023 at 08:11:25PM +0530, Raghavendra, Vignesh wrote:
> >> Hi Lorenzo, Bjorn,
> >>
> >> On 3/15/2023 12:38 PM, Siddharth Vadapalli wrote:
> >>> The Link Retraining process is initiated to account for the Gen2 defect in
> >>> the Cadence PCIe controller in J721E SoC. The errata corresponding to this
> >>> is i2085, documented at:
> >>> https://www.ti.com/lit/er/sprz455c/sprz455c.pdf
> >>>
> >>> The existing workaround implemented for the errata waits for the Data Link
> >>> initialization to complete and assumes that the link retraining process
> >>> at the Physical Layer has completed. However, it is possible that the
> >>> Physical Layer training might be ongoing as indicated by the
> >>> PCI_EXP_LNKSTA_LT bit in the PCI_EXP_LNKSTA register.
> >>>
> >>> Fix the existing workaround, to ensure that the Physical Layer training
> >>> has also completed, in addition to the Data Link initialization.
> >>>
> >>> Fixes: 4740b969aaf5 ("PCI: cadence: Retrain Link to work around Gen2 training defect")
> >>> Signed-off-by: Siddharth Vadapalli <s-vadapalli@...com>
> >>> Reviewed-by: Vignesh Raghavendra <vigneshr@...com>
> >>> ---
> >>> Changes from v1:
> >>> 1. Collect Reviewed-by tag from Vignesh Raghavendra.
> >>> 2. Rebase on next-20230315.
> >>>
> >>> v1:
> >>> https://lore.kernel.org/r/20230102075656.260333-1-s-vadapalli@ti.com
> >>>
> >>>  .../controller/cadence/pcie-cadence-host.c    | 27 +++++++++++++++++++
> >>>  1 file changed, 27 insertions(+)
> >>
> >> Wondering do one of you be pulling this patch in? This patch was never
> >> picked for 6.3-rc1 merge cycle... Just want to make sure
> >> pcie-cadence*.c and pci-j721e.c patches have a path to reach pci tree.
> > 
> > Yes, Lorenzo or Krzysztof will likely pick this up.  I think Lorenzo
> > is out of the office this week.
> > 
> > Drive-by comment: the current patch doesn't seem to give any
> > indication to the user when cdns_pcie_host_training_complete() times
> > out.  Is that timeout potentially of interest to a user?  Should there
> > be a log message there?
> 
> Thank you for reviewing the patch. The return value of -ETIMEDOUT from the
> function cdns_pcie_host_training_complete() added by this patch will be handled
> similar to the -ETIMEDOUT from the cdns_pcie_host_wait_for_link() function that
> is already present.
> 
> If cdns_pcie_host_training_complete() returns -ETIMEDOUT, it is returned to
> cdns_pcie_host_start_link() function which is called within
> cdns_pcie_host_setup() function. In the cdns_pcie_host_setup() function, there
> is already a dev_dbg() print for handling the case where
> cdns_pcie_host_wait_for_link() times out. For this reason, I felt that for both
> cases, the dev_dbg() print can be used to debug without the need for an extra
> log message. Please let me know if that's fine.

Sounds good.

dev_dbg() wouldn't be the right thing if we *expect* the link to come
up, but ISTR that maybe you can't detect device presence directly.  If
that's the case, all you can do is try to bring the link up and assume
the slot is empty if it doesn't come up.  If the usual reason for the
timeout is that the slot is empty, dev_dbg() should be fine.

Another drive-by comment, no action needed, seems slightly strange to
have two "start_link" functions called one after the other:

  cdns_pcie_host_setup
    cdns_pcie_start_link
    cdns_pcie_host_start_link

I assume both are for the same link, so it's weird to have two
functions for it.

Bjorn