linux-kernel - Re: [PATCH v8 2/6] PCI/DPC: define a function to check and wait till port finish DPC handling

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Date:   Thu, 8 Oct 2020 13:49:05 +0800
From:   Ethan Zhao <xerces.zhao@...il.com>
To:     "Kuppuswamy, Sathyanarayanan" <sathyanarayanan.kuppuswamy@...el.com>
Cc:     Ethan Zhao <haifeng.zhao@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, Oliver <oohall@...il.com>,
        ruscur@...sell.cc, Lukas Wunner <lukas@...ner.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Stuart Hayes <stuart.w.hayes@...il.com>,
        Alexandru Gagniuc <mr.nuke.me@...il.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Raj, Ashok" <ashok.raj@...ux.intel.com>
Subject: Re: [PATCH v8 2/6] PCI/DPC: define a function to check and wait till
 port finish DPC handling

On Thu, Oct 8, 2020 at 2:16 AM Kuppuswamy, Sathyanarayanan
<sathyanarayanan.kuppuswamy@...el.com> wrote:
>
>
> On 10/7/20 4:31 AM, Ethan Zhao wrote:
> > Once root port DPC capability is enabled and triggered, at the beginning
> > of DPC is triggered, the DPC status bits are set by hardware and then
> > sends DPC/DLLSC/PDC interrupts to OS DPC and pciehp drivers, it will
> > take the port and software DPC interrupt handler 10ms to 50ms (test data
> > on ICS(Ice Lake SP platform, see
> > https://en.wikichip.org/wiki/intel/microarchitectures/ice_lake_(server)
> > & stable 5.9-rc6) to complete the DPC containment procedure
> This data is based on one particular architecture. So using this
> to create a timed loop in pci_wait_port_outdpc() looks incorrect.
>
> I still recommend looking for some locking model to fix this
> issue (may be atomic state flag or lock).
 It is actually a device semaphore. DLLSC/PDC handler needs to wait
for the critical
area is clear to enter by monitoring the DPC triggered status is
cleaned or not.

 Another  problem is,  DPC reset/interrupt is initiated by hardware,
you couldn't place
 a software lock between interrupt handler and device resetting.

While device semaphore--- DPC triggered status is the right one to wait for.

Better idea ?

Thanks,
Ethan

> > till the DPC status is cleared at the end of the DPC interrupt handler.
> >
> > We use this function to check if the root port is in DPC handling status
> > and wait till the hardware and software completed the procedure.
> >
> > Signed-off-by: Ethan Zhao <haifeng.zhao@...el.com>
> > Tested-by: Wen Jin <wen.jin@...el.com>
> > Tested-by: Shanshan Zhang <ShanshanX.Zhang@...el.com>
> > ---
> > changes:
> >   v2：align ICS code name to public doc.
> >   v3: no change.
> >   v4: response to Christoph's (Christoph Hellwig <hch@...radead.org>)
> >       tip, move pci_wait_port_outdpc() to DPC driver and its declaration
> >       to pci.h.
> >   v5: fix building issue reported by lkp@...el.com with some config.
> >   v6: move from [1/5] to [2/5].
> >   v7: no change.
> >   v8: no change.
> >
> >   drivers/pci/pci.h      |  2 ++
> >   drivers/pci/pcie/dpc.c | 27 +++++++++++++++++++++++++++
> >   2 files changed, 29 insertions(+)
> >
> > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > index fa12f7cbc1a0..455b32187abd 100644
> > --- a/drivers/pci/pci.h
> > +++ b/drivers/pci/pci.h
> > @@ -455,10 +455,12 @@ void pci_restore_dpc_state(struct pci_dev *dev);
> >   void pci_dpc_init(struct pci_dev *pdev);
> >   void dpc_process_error(struct pci_dev *pdev);
> >   pci_ers_result_t dpc_reset_link(struct pci_dev *pdev);
> > +bool pci_wait_port_outdpc(struct pci_dev *pdev);
> >   #else
> >   static inline void pci_save_dpc_state(struct pci_dev *dev) {}
> >   static inline void pci_restore_dpc_state(struct pci_dev *dev) {}
> >   static inline void pci_dpc_init(struct pci_dev *pdev) {}
> > +static inline bool pci_wait_port_outdpc(struct pci_dev *pdev) { return false; }
> >   #endif
> >
> >   #ifdef CONFIG_PCI_ATS
> > diff --git a/drivers/pci/pcie/dpc.c b/drivers/pci/pcie/dpc.c
> > index daa9a4153776..2e0e091ce923 100644
> > --- a/drivers/pci/pcie/dpc.c
> > +++ b/drivers/pci/pcie/dpc.c
> > @@ -71,6 +71,33 @@ void pci_restore_dpc_state(struct pci_dev *dev)
> >       pci_write_config_word(dev, dev->dpc_cap + PCI_EXP_DPC_CTL, *cap);
> >   }
> >
> > +bool pci_wait_port_outdpc(struct pci_dev *pdev)
> > +{
> > +     u16 cap = pdev->dpc_cap, status;
> > +     u16 loop = 0;
> > +
> > +     if (!cap) {
> > +             pci_WARN_ONCE(pdev, !cap, "No DPC capability initiated\n");
> > +             return false;
> > +     }
> > +     pci_read_config_word(pdev, cap + PCI_EXP_DPC_STATUS, &status);
> > +     pci_dbg(pdev, "DPC status %x, cap %x\n", status, cap);
> > +
> > +     while (status & PCI_EXP_DPC_STATUS_TRIGGER && loop < 100) {
> > +             msleep(10);
> > +             loop++;
> > +             pci_read_config_word(pdev, cap + PCI_EXP_DPC_STATUS, &status);
> > +     }
> > +
> > +     if (!(status & PCI_EXP_DPC_STATUS_TRIGGER)) {
> > +             pci_dbg(pdev, "Out of DPC %x, cost %d ms\n", status, loop*10);
> > +             return true;
> > +     }
> > +
> > +     pci_dbg(pdev, "Timeout to wait port out of DPC status\n");
> > +     return false;
> > +}
> > +
> >   static int dpc_wait_rp_inactive(struct pci_dev *pdev)
> >   {
> >       unsigned long timeout = jiffies + HZ;
>
> --
> Sathyanarayanan Kuppuswamy
> Linux Kernel Developer
>