Re: [v3] PCI: Avoid unsync of LTR mechanism configuration

[Mingchuang Qiao]

Hi,

On Mon, 2021-02-01 at 13:32 +0200, Mika Westerberg wrote:
> Hi,
> 
> On Fri, Jan 29, 2021 at 03:11:37PM +0800, mingchuang.qiao@xxxxxxxxxxxx wrote:
> > From: Mingchuang Qiao <mingchuang.qiao@xxxxxxxxxxxx>
> > 
> > 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 mtach ltr_path value.
> 
> Typo mtach -> match.
> 
> >    -before configuring device's LTR for hot-remove/hot-add
> >    -before restoring device's DEVCTL2 register when restore device state
> > 
> > Signed-off-by: Mingchuang Qiao <mingchuang.qiao@xxxxxxxxxxxx>
> > ---
> > changes of v2
> >  -modify patch description
> >  -reconfigure bridge's LTR before restoring device DEVCTL2 register
> > changes of v3
> >  -call pci_reconfigure_bridge_ltr() in probe.c
> 
> Hmm, which part of this patch takes care of the reset path? It is not
> entirely clear to me at least.
> 

When device resets and link goes down, there seems to have two methods
to recover for software. 
   -One is that trigger device removal and rescan.
   -The other is that restore device with pci_restore_state() after link
comes back up.
For above both scenarios, we need check and reconfigure "LTR Mechanism
Enable" bit of bridge. It's also this patch intends to do.
   -For the rescan scenario, it's done in pci_configure_ltr().
   -For the restore scenario, it's done in pci_restore_pcie_state().

> > ---
> >  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..12b557c8f062 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_reconfigure_bridge_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_LTR_EN);
> > +		}
> > +	}
> > +#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_reconfigure_bridge_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..a660a01358c5 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_reconfigure_bridge_ltr(struct pci_dev *dev);
> 
> Nit: calling it pci_bridge_reconfigure_ltr() would match better with the
> other function names.
> 

Thanks for the suggestion. I will update the name in next patch :)

> >  
> >  static inline void pci_wakeup_event(struct pci_dev *dev)
> >  {
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 953f15abc850..fa6075093f3b 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_reconfigure_bridge_ltr(dev);
> >  		pcie_capability_set_word(dev, PCI_EXP_DEVCTL2,
> >  					 PCI_EXP_DEVCTL2_LTR_EN);
> >  		dev->ltr_path = 1;
> > -- 
> > 2.18.0