Message-ID: <ZC3Ev7qnUDdG0cFd@google.com>
Date:   Wed, 5 Apr 2023 11:58:07 -0700
From:   William McVicker <willmcvicker@...gle.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Sajid Dalvi <sdalvi@...gle.com>,
        Han Jingoo <jingoohan1@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rob Herring <robh@...nel.org>, kernel-team@...roid.com,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Subject: Re: [PATCH v2] PCI: dwc: Wait for link up only if link is started

On 04/05/2023, Bjorn Helgaas wrote:
> On Wed, Apr 05, 2023 at 03:24:36PM +0200, Lorenzo Pieralisi wrote:
> > On Thu, Mar 16, 2023 at 06:05:02PM -0500, Sajid Dalvi wrote:
> > > On Tue, Feb 28, 2023 at 10:36 PM Sajid Dalvi <sdalvi@...gle.com> wrote:
> > > >
> > > > Thanks for your review Jingoo.
> > > > Sajid
> > > >
> > > > On Tue, Feb 28, 2023 at 4:04 PM Han Jingoo <jingoohan1@...il.com> wrote:
> > > > >
> > > > > On Mon, Feb 27, 2023, Sajid Dalvi <sdalvi@...gle.com> wrote:
> > > > > >
> > > > > > In dw_pcie_host_init() regardless of whether the link has been started
> > > > > > or not, the code waits for the link to come up. Even in cases where
> > > > > > start_link() is not defined the code ends up spinning in a loop for 1
> > > > > > second. Since in some systems dw_pcie_host_init() gets called during
> > > > > > probe, this one second loop for each pcie interface instance ends up
> > > > > > extending the boot time.
> > > > > >
> > > > > > Call trace when start_link() is not defined:
> > > > > > dw_pcie_wait_for_link << spins in a loop for 1 second
> > > > > > dw_pcie_host_init
> > > > > >
> > > > > > Signed-off-by: Sajid Dalvi <sdalvi@...gle.com>
> > > > >
> > > > > (CC'ed Krzysztof Kozlowski)
> > > > >
> > > > > Acked-by: Jingoo Han <jingoohan1@...il.com>
> > > > >
> > > > > It looks good to me. I also checked the previous thread.
> > > > > I agree with Krzysztof's opinion that we should include
> > > > > only hardware-related features into DT.
> > > > > Thank you.
> > > > >
> > > > > Best regards,
> > > > > Jingoo Han
> > > > >
> > > > > > ---
> > > > > >  drivers/pci/controller/dwc/pcie-designware-host.c | 6 +++---
> > > > > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > > > index 9952057c8819..9709f69f173e 100644
> > > > > > --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > > > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > > > @@ -489,10 +489,10 @@ int dw_pcie_host_init(struct dw_pcie_rp *pp)
> > > > > >                 ret = dw_pcie_start_link(pci);
> > > > > >                 if (ret)
> > > > > >                         goto err_remove_edma;
> > > > > > -       }
> > > > > >
> > > > > > -       /* Ignore errors, the link may come up later */
> > > > > > -       dw_pcie_wait_for_link(pci);
> > > > > > +               /* Ignore errors, the link may come up later */
> > > > > > +               dw_pcie_wait_for_link(pci);
> > > > > > +       }
> > > > > >
> > > > > >         bridge->sysdata = pp;
> > > > > >
> > > > > > --
> > > > > > 2.39.2.722.g9855ee24e9-goog
> > > > > >
> > > 
> > > @bhelgaas Can this be picked up in your tree:
> > >  https://git.kernel.org/pub/scm/linux/kernel/git/helgaas/pci.git/
> > 
> > This patch seems fine to me. The question I have though is why the
> > *current* code is written the way it is. Perhaps it is just the way
> > it is, I wonder whether this change can trigger a regression though.
> 
> The new code will look basically like this:
> 
>   if (!dw_pcie_link_up(pci)) {
>     dw_pcie_start_link(pci);
>     dw_pcie_wait_for_link(pci);
>   }
> 
> If the link is already up by the time we get here, this change means
> we won't get this message emitted by dw_pcie_wait_for_link():
> 
>   dev_info(pci->dev, "PCIe Gen.%u x%u link up\n", ...)
> 
> I don't know how important that is, but I bet somebody cares about it.
> 
> From the commit log, I expected the patch to do something based on
> whether ->start_link() was defined, but there really isn't a direct
> connection, so maybe the log could be refined.
> 
> Bjorn
> 
> -- 
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> 

After taking a deeper dive into this patch, I found that [1] changes the
original intent which was to skip the call to dw_pcie_wait_for_link()
when pci->ops->start_link is NULL. I talked to Sajid offline and he
agreed we should put back the start_link NULL check. The updated patch
should look like this:

  if (!dw_pcie_link_up(pci) && pci->ops && pci->ops->start_link) {
    ret = dw_pcie_start_link(pci);
    if (ret)
      goto err_free_msi;
    dw_pcie_wait_for_link(pci);
  }


...which will ensure that we don't call dw_pcie_wait_for_link() when
pci->ops->start_link is NULL.

With regards to the log, I think there are 2 ways to solve this:

1) We could also call dw_pcie_wait_for_link() in a new else if
   dw_pcie_link_up() returns 1.
2) We could add this to the top of dw_pcie_wait_for_link() and leave the
   code as is:

   if (!pci->ops || !pci->ops->start_link)
     return 0;

I kind of like (2) since that solves both Sajid's original issue and
will keep the original log.

[1] https://lore.kernel.org/all/20220624143428.8334-14-Sergey.Semin@baikalelectronics.ru/

Regards,
Will

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