Date:   Fri, 8 Oct 2021 14:30:07 +0800
From:   mingchuang qiao <mingchuang.qiao@...iatek.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     <kerun.zhu@...iatek.com>, <mingchuang.qiao@...iatek.com>,
        <linux-pci@...r.kernel.org>, <lambert.wang@...iatek.com>,
        <rjw@...ysocki.net>, <linux-kernel@...r.kernel.org>,
        <matthias.bgg@...il.com>, <alex.williamson@...hat.com>,
        <linux-mediatek@...ts.infradead.org>, <utkarsh.h.patel@...el.com>,
        <haijun.liu@...iatek.com>, <bhelgaas@...gle.com>,
        <mika.westerberg@...ux.intel.com>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [v4] PCI: Avoid unsync of LTR mechanism configuration

Hi Bjorn,

Much appreciate the comments. See below for my response.

On Thu, 2021-09-30 at 14:48 -0500, Bjorn Helgaas wrote:
> On Thu, Sep 30, 2021 at 03:02:24PM +0800, mingchuang qiao wrote:
> > Hi Bjorn,
> > 
> > A friendly ping.
> > Thanks.
> 
> I pointed out a couple issues, but you never responded.  See below.
> 
> > On Mon, 2021-09-06 at 13:36 +0800, mingchuang qiao wrote:
> > > Hi Bjorn,
> > > 
> > > On Thu, 2021-02-18 at 10:50 -0600, Bjorn Helgaas wrote:
> > > > On Thu, Feb 04, 2021 at 05:51:25PM +0800, mingchuang.qiao@...ia
> > > > tek.
> > > > co
> > > > m wrote:
> > > > > From: Mingchuang Qiao <mingchuang.qiao@...iatek.com>
> > > > > 
> > > > > In bus scan flow, the "LTR Mechanism Enable" bit of DEVCTL2
> > > > > register is
> > > > > configured in pci_configure_ltr(). If device and bridge both
> > > > > support LTR
> > > > > mechanism, the "LTR Mechanism Enable" bit of device and
> > > > > bridge
> > > > > will
> > > > > be
> > > > > enabled in DEVCTL2 register. And pci_dev->ltr_path will be
> > > > > set as
> > > > > 1.
> > > > > 
> > > > > If PCIe link goes down when device resets, the "LTR Mechanism
> > > > > Enable" bit
> > > > > of bridge will change to 0 according to PCIe r5.0, sec
> > > > > 7.5.3.16.
> > > > > However,
> > > > > the pci_dev->ltr_path value of bridge is still 1.
> > > > > 
> > > > > For following conditions, check and re-configure "LTR
> > > > > Mechanism
> > > > > Enable" bit
> > > > > of bridge to make "LTR Mechanism Enable" bit match ltr_path
> > > > > value.
> > > > >    -before configuring device's LTR for hot-remove/hot-add
> > > > >    -before restoring device's DEVCTL2 register when restore
> > > > > device
> > > > > state
> > > > 
> > > > There's definitely a bug here.  The commit log should say a
> > > > little
> > > > more about what it is.  I *think* if LTR is enabled and we
> > > > suspend
> > > > (putting the device in D3cold) and resume, LTR probably doesn't
> > > > work
> > > > after resume because LTR is disabled in the upstream bridge,
> > > > which
> > > > would be an obvious bug.
> 
> Here's one thing.  Above I was asking for more details.  In
> particular, how would a user notice this bug?  How did *you* notice
> the bug?
> 

I will update more details in the commit log. 

For the suspend(D3 cold) and resume case, the LTR enable bit value
of bridge is saved(by pci_save_state()) in suspend flow and restored(by
pci_restore_state()) in resume flow. 
  -If link goes down after bridge already does pci_save_state()
    LTR could work after resume due to pci_restore_state() will enable
the LTR of bridge.
  -If link goes down before bridge does pci_save_state()
    LTR probably doesn't work after resume due to the LTR bit is
already disable when pci_save_state() and will not enable after
pci_restore_sate().

The sequence of link goes down and brdige suspend maybe platform
specific.
 
The issue is noticed by AER log as following shows.

pcieport 0000:00:1d.0: AER: Uncorrected (Non-Fatal) error received:
id=00e8
pcieport 0000:00:1d.0: PCIe Bus Error: severity=Uncorrected (Non-
Fatal), type=Transaction Layer, id=00e8(Requester ID)
pcieport 0000:00:1d.0:   device [8086:9d18] error
status/mask=00100000/00010000
pcieport 0000:00:1d.0:    [20] Unsupported Request    (First)
pcieport 0000:00:1d.0:   TLP Header: 34000000 03000010 00000000
00000000

> > > > Also, if a device with LTR enabled is hot-removed, and we hot-
> > > > add a
> > > > device, I think LTR will not work on the new device.  Possibly
> > > > also
> > > > a
> > > > bug, although I'm not convinced we know how to configure LTR on
> > > > the
> > > > new device anyway.
> > > > 
> > > > So I'd *like* to merge the bug fix for v5.12, but I think I'll
> > > > wait
> > > > because of the issue below.
> > > > 
> > > 
> > > A friendly ping.
> > > Any further process shall I make to get this patch merged?
> > > 
> > > > > Signed-off-by: Mingchuang Qiao <mingchuang.qiao@...iatek.com>
> > > > > ---
> > > > > changes of v4
> > > > >  -fix typo of commit message
> > > > >  -rename: pci_reconfigure_bridge_ltr()-
> > > > > > pci_bridge_reconfigure_ltr()
> > > > > 
> > > > > changes of v3
> > > > >  -call pci_reconfigure_bridge_ltr() in probe.c
> > > > > changes of v2
> > > > >  -modify patch description
> > > > >  -reconfigure bridge's LTR before restoring device DEVCTL2
> > > > > register
> > > > > ---
> > > > >  drivers/pci/pci.c   | 25 +++++++++++++++++++++++++
> > > > >  drivers/pci/pci.h   |  1 +
> > > > >  drivers/pci/probe.c | 13 ++++++++++---
> > > > >  3 files changed, 36 insertions(+), 3 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > > index b9fecc25d213..6bf65d295331 100644
> > > > > --- a/drivers/pci/pci.c
> > > > > +++ b/drivers/pci/pci.c
> > > > > @@ -1437,6 +1437,24 @@ static int pci_save_pcie_state(struct
> > > > > pci_dev *dev)
> > > > >  	return 0;
> > > > >  }
> > > > >  
> > > > > +void pci_bridge_reconfigure_ltr(struct pci_dev *dev)
> > > > > +{
> > > > > +#ifdef CONFIG_PCIEASPM
> > > > > +	struct pci_dev *bridge;
> > > > > +	u32 ctl;
> > > > > +
> > > > > +	bridge = pci_upstream_bridge(dev);
> > > > > +	if (bridge && bridge->ltr_path) {
> > > > > +		pcie_capability_read_dword(bridge,
> > > > > PCI_EXP_DEVCTL2, &ctl);
> > > > > +		if (!(ctl & PCI_EXP_DEVCTL2_LTR_EN)) {
> > > > > +			pci_dbg(bridge, "re-enabling
> > > > > LTR\n");
> > > > > +			pcie_capability_set_word(bridge,
> > > > > PCI_EXP_DEVCTL2,
> > > > > +						 PCI_EXP_DEV
> > > > > CTL2
> > > > > _L
> > > > > TR_EN);
> > > > 
> > > > This pattern of updating the upstream bridge on behalf of "dev"
> > > > is
> > > > problematic because it's racy:
> > > > 
> > > >   CPU 1                     CPU 2
> > > >   -------------------       ---------------------
> > > >   ctl = read DEVCTL2        ctl = read(DEVCTL2)
> > > >   ctl |= DEVCTL2_LTR_EN     ctl |= DEVCTL2_ARI
> > > >   write(DEVCTL2, ctl)
> > > >                             write(DEVCTL2, ctl)
> > > > 
> > > > Now the bridge has ARI set, but not LTR_EN.
> > > > 
> > > > We have the same problem in the pci_enable_device() path.  The
> > > > most
> > > > recent try at fixing it is [1].
> 
> I was hoping you would respond with "yes, I understand the problem,
> but don't think it's likely" or "no, this isn't actually a problem
> because ..."
> 
> I think it *is* a problem, but we're probably unlikely to hit it, so
> we can probably live with it for now.  
> 

Yes, I understand the problem. I also think it unlikely to hit and we
can probably live with it for now.
Thanks. 

> > > > [1] https://lore.kernel.org/linux-pci/20201218174011.340514-2-
> > > > s.mir
> > > > os
> > > > hnichenko@...ro.com/
> > > > 
> > > > > +		}
> > > > > +	}
> > > > > +#endif
> > > > > +}
> > > > > +
> > > > >  static void pci_restore_pcie_state(struct pci_dev *dev)
> > > > >  {
> > > > >  	int i = 0;
> > > > > @@ -1447,6 +1465,13 @@ static void
> > > > > pci_restore_pcie_state(struct
> > > > > pci_dev *dev)
> > > > >  	if (!save_state)
> > > > >  		return;
> > > > >  
> > > > > +	/*
> > > > > +	 * Downstream ports reset the LTR enable bit when
> > > > > link
> > > > > goes down.
> > > > > +	 * Check and re-configure the bit here before
> > > > > restoring
> > > > > device.
> > > > > +	 * PCIe r5.0, sec 7.5.3.16.
> > > > > +	 */
> > > > > +	pci_bridge_reconfigure_ltr(dev);
> > > > > +
> > > > >  	cap = (u16 *)&save_state->cap.data[0];
> > > > >  	pcie_capability_write_word(dev, PCI_EXP_DEVCTL,
> > > > > cap[i++]);
> > > > >  	pcie_capability_write_word(dev, PCI_EXP_LNKCTL,
> > > > > cap[i++]);
> > > > > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > > > > index 5c59365092fa..b3a5e5287cb7 100644
> > > > > --- a/drivers/pci/pci.h
> > > > > +++ b/drivers/pci/pci.h
> > > > > @@ -111,6 +111,7 @@ void pci_free_cap_save_buffers(struct
> > > > > pci_dev
> > > > > *dev);
> > > > >  bool pci_bridge_d3_possible(struct pci_dev *dev);
> > > > >  void pci_bridge_d3_update(struct pci_dev *dev);
> > > > >  void pci_bridge_wait_for_secondary_bus(struct pci_dev *dev);
> > > > > +void pci_bridge_reconfigure_ltr(struct pci_dev *dev);
> > > > >  
> > > > >  static inline void pci_wakeup_event(struct pci_dev *dev)
> > > > >  {
> > > > > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > > > > index 953f15abc850..ade055e9fb58 100644
> > > > > --- a/drivers/pci/probe.c
> > > > > +++ b/drivers/pci/probe.c
> > > > > @@ -2132,9 +2132,16 @@ static void pci_configure_ltr(struct
> > > > > pci_dev
> > > > > *dev)
> > > > >  	 * Complex and all intermediate Switches indicate
> > > > > support
> > > > > for LTR.
> > > > >  	 * PCIe r4.0, sec 6.18.
> > > > >  	 */
> > > > > -	if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT ||
> > > > > -	    ((bridge = pci_upstream_bridge(dev)) &&
> > > > > -	      bridge->ltr_path)) {
> > > > > +	if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT) {
> > > > > +		pcie_capability_set_word(dev,
> > > > > PCI_EXP_DEVCTL2,
> > > > > +					 PCI_EXP_DEVCTL2_LTR
> > > > > _EN)
> > > > > ;
> > > > > +		dev->ltr_path = 1;
> > > > > +		return;
> > > > > +	}
> > > > > +
> > > > > +	bridge = pci_upstream_bridge(dev);
> > > > > +	if (bridge && bridge->ltr_path) {
> > > > > +		pci_bridge_reconfigure_ltr(dev);
> > > > >  		pcie_capability_set_word(dev,
> > > > > PCI_EXP_DEVCTL2,
> > > > >  					 PCI_EXP_DEVCTL2_LTR
> > > > > _EN)
> > > > > ;
> > > > >  		dev->ltr_path = 1;
> > > > > -- 
> > > > > 2.18.0
> > > > 
> > > > _______________________________________________
> > > > Linux-mediatek mailing list
> > > > Linux-mediatek@...ts.infradead.org
> > > > http://lists.infradead.org/mailman/listinfo/linux-mediatek

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