Re: [v5] PCI: Avoid unsync of LTR mechanism configuration

[Bjorn Helgaas]

On Tue, Oct 12, 2021 at 03:56:14PM +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, the "LTR Mechanism Enable" bit of bridge will 
> change to 0 according to PCIe r5.0, sec 7.5.3.16. However, the ->ltr_path 
> of bridge is still set.
> 
> Following shows two scenarios of this LTR issue:
> 
> -scenario of device restore
>  -- bridge LTR enabled
>  -- device LTR enabled
>  -- reset device
>  -- link goes down, bridge disables LTR
>  -- link comes back up, LTR disabled in both bridge and device
>  -- restore device state, including LTR enable
>  -- device sends LTR message
>  -- bridge reports Unsupported Request
> 
> -scenario of device hot-remove/hot-add
>  -- bridge LTR enabled
>  -- device LTR enabled
>  -- hot-remove device
>  -- link goes down, bridge disables LTR
>  -- hot-add device and link comes back up
>  -- scan device, set LTR enable bit of device
>  -- device sends LTR message
>  -- bridge reports Unsupported Request
> 
> This issue was 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)
> 
> It was also noticed when PCIe devices (thunderbolt docks) were hot removed 
> from chromebooks, and then hot-plugged back again. Once hotplugged back, 
> the newer Intel chromebooks fail to go into S0ix low power state because 
> of this LTR issue.(https://patchwork.kernel.org/project/linux-pci/patch/
> 20210204095125.9212-1-mingchuang.qiao@xxxxxxxxxxxx/)
> 
> To resolve this issue, check and re-configure "LTR Mechanism Enable" bit
> of bridge to make "LTR Mechanism Enable" bit match ltr_path value in
> following conditions.
>    -before configuring device's LTR for hot-remove/hot-add
>    -before restoring device's DEVCTL2 register when restore device state
> 
> Reviewed-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> Signed-off-by: Mingchuang Qiao <mingchuang.qiao@xxxxxxxxxxxx>

Applied to pci/aspm for v5.16, thanks!

> ---
> changes of v5
>  -add more details in commit message
> 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_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_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