Re: [PATCH v2] PCI: dwc: Wait for link up only if link is started

William McVicker
2023-04-05T18:58:20Z

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@google.com> wrote:
> > > >
> > > > Thanks for your review Jingoo.
> > > > Sajid
> > > >
> > > > On Tue, Feb 28, 2023 at 4:04 PM Han Jingoo <jingoohan1@gmail.com> wrote:
> > > > >
> > > > > On Mon, Feb 27, 2023, Sajid Dalvi <sdalvi@google.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@google.com>
> > > > >
> > > > > (CC'ed Krzysztof Kozlowski)
> > > > >
> > > > > Acked-by: Jingoo Han <jingoohan1@gmail.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
>
> --
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.
>

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