Re: [PATCH v6 3/5] PCI: brcmstb: Set higher value for internal bus timeout

[Jim Quinlan]

On Mon, Aug 14, 2023 at 6:07 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> On Mon, Aug 14, 2023 at 03:30:07PM -0400, Jim Quinlan wrote:
> > On Fri, Jul 28, 2023 at 12:15 PM Jim Quinlan <james.quinlan@xxxxxxxxxxxx> wrote:
> > > On Thu, Jul 27, 2023, 10:44 PM Lorenzo Pieralisi <lpieralisi@xxxxxxxxxx> wrote:
> > >> On Fri, Jun 23, 2023 at 10:40:56AM -0400, Jim Quinlan wrote:
> > >> > During long periods of the PCIe RC HW being in an L1SS sleep state, there
> > >> > may be a timeout on an internal bus access, even though there may not be
> > >> > any PCIe access involved.  Such a timeout will cause a subsequent CPU
> > >> > abort.
> > >> >
> > >> > So, when "brcm,enable-l1ss" is observed, we increase the timeout value to
> > >> > four seconds instead of using its HW default.
> > >> >
> > >> > Signed-off-by: Jim Quinlan <james.quinlan@xxxxxxxxxxxx>
> > >> > Tested-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
> > >> > ---
> > >> >  drivers/pci/controller/pcie-brcmstb.c | 16 ++++++++++++++++
> > >> >  1 file changed, 16 insertions(+)
> > >> >
> > >> > diff --git a/drivers/pci/controller/pcie-brcmstb.c b/drivers/pci/controller/pcie-brcmstb.c
> > >> > index d30636a725d7..fe0415a98c63 100644
> > >> > --- a/drivers/pci/controller/pcie-brcmstb.c
> > >> > +++ b/drivers/pci/controller/pcie-brcmstb.c
> > >> > @@ -1034,6 +1034,21 @@ static int brcm_pcie_setup(struct brcm_pcie *pcie)
> > >> >       return 0;
> > >> >  }
> > >> >
> > >> > +/*
> > >> > + * This extends the timeout period for an access to an internal bus.  This
> > >> > + * access timeout may occur during L1SS sleep periods even without the
> > >> > + * presence of a PCIe access.
> > >> > + */
> > >> > +static void brcm_extend_rbus_timeout(struct brcm_pcie *pcie)
> > >> > +{
> > >> > +     /* TIMEOUT register is two registers before RGR1_SW_INIT_1 */
> > >> > +     const unsigned int REG_OFFSET = PCIE_RGR1_SW_INIT_1(pcie) - 8;
> > >>
> > >> Nit: you could define an offset for the TIMEOUT register, if that makes
> > >> it any cleaner, up to you.
> > >>
> > >> > +     u32 timeout_us = 4000000; /* 4 seconds, our setting for L1SS */
> > >>
> > >> It would be useful to describe why this has to be 4 seconds in case
> > >> someone in the future will have to change it.
> > >
> > > IIRC our customer requested 2s and we doubled it.  Bjorn, can you
> > > please add this comment or a paraphrase of it before applying --
> > > I'm currently on vacation.
> >
> > Is the above request okay with you?  What is the status of these
> > commits -- will they be applied to pci-next in the near future?
>
> The "brcm,enable-l1ss" DT property is either unnecessary or an
> indication of a hardware defect in the controller.

Agree.
>
> Requiring the property is a terrible user experience, completely
> antithetical to the PCI compatibility story, and per the conversation
> at [1], there are no known problems that would occur if we ignored
> "brcm,enable-l1ss" and always configured mode (c) ("Bidirectional
> CLKREQ# for L1SS capable devices").

Agree, but I don't believe this issue will be in the top five problems of RPi
folks getting their PCIe systems to work.

>
> Even when configured as mode (c), L1SS is not *always* enabled.  It's
> certainly not enabled before ASPM init, and users can always disable
> L1SS whenever they desire via the sysfs interfaces or pcie_aspm=off,
> so if there's some problem with running in mode (c) with L1SS
> disabled, we're still likely to see it.
>
> But if you want to require the DT property, I guess it's mainly an
> issue for you and your customers.

I believe this to be the best solution for the current HW.  As Cyril
and I have noted, it
allows some platforms to work that were not working previously.

So I am all for these commits being applied.

FWIW, I am currently advocating changing the PCIe HW core to
seamlessly handle all
of the ASPM (sub)states transitions w/o awkward SW driver
intervention.  I am also
advocating other changes as well.  So there is a possibility things will
be changed for the better in the future.

Regards,
Jim Quinlan
Broadcom STB

>
> So to answer your question, yes, I'm OK with this series.
>
> Bjorn
>
> [1] https://lore.kernel.org/r/20230428223500.23337-2-jim2101024@xxxxxxxxx
>
> > >> > +     /* Each unit in timeout register is 1/216,000,000 seconds */
> > >> > +     writel(216 * timeout_us, pcie->base + REG_OFFSET);
> > >> > +}
> > >> > +
> > >> >  static void brcm_config_clkreq(struct brcm_pcie *pcie)
> > >> >  {
> > >> >       bool l1ss = of_property_read_bool(pcie->np, "brcm,enable-l1ss");
> > >> > @@ -1059,6 +1074,7 @@ static void brcm_config_clkreq(struct brcm_pcie *pcie)
> > >> >                * of 400ns, as specified in 3.2.5.2.2 of the PCI Express
> > >> >                * Mini CEM 2.0 specification.
> > >> >                */
> > >> > +             brcm_extend_rbus_timeout(pcie);
> > >> >               clkreq_set |= PCIE_MISC_HARD_PCIE_HARD_DEBUG_L1SS_ENABLE_MASK;
> > >> >               dev_info(pcie->dev, "bi-dir CLKREQ# for L1SS power savings");
> > >> >       } else {

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