Re: [v4] PCI: Avoid unsync of LTR mechanism configuration

[Bjorn Helgaas <helgaas@kernel.org>]

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@mediatek.
> > > co
> > > m wrote:
> > > > From: Mingchuang Qiao <mingchuang.qiao@mediatek.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?

> > > 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@mediatek.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_DEVCTL2
> > > > _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.  

> > > [1] https://lore.kernel.org/linux-pci/20201218174011.340514-2-s.mir
> > > os
> > > hnichenko@yadro.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
> > > 
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