Re: [PATCH v2] PCI: Reset PCIe devices to stop ongoing DMA

[Eric W. Biederman]

Takao Indoh <indou.takao@xxxxxxxxxxxxxx> writes:

> This patch resets PCIe devices on boot to stop ongoing DMA. When
> "pci=pcie_reset_endpoint_devices" is specified, a hot reset is triggered
> on each PCIe root port and downstream port to reset its downstream
> endpoint.
>
> Problem:
> This patch solves the problem that kdump can fail when intel_iommu=on is
> specified. When intel_iommu=on is specified, many dma-remapping errors
> occur in second kernel and it causes problems like driver error or PCI
> SERR, at last kdump fails. This problem is caused as follows.
> 1) Devices are working on first kernel.
> 2) Switch to second kernel(kdump kernel). The devices are still working
>    and its DMA continues during this switch.
> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>    dma-remapping errors.
>
> Solution:
> All DMA transactions have to be stopped before iommu is initialized. By
> this patch devices are reset and in-flight DMA is stopped before
> pci_iommu_init.
>
> To invoke hot reset on an endpoint, its upstream link need to be reset.
> reset_pcie_endpoints() is called from fs_initcall_sync, and it finds
> root port/downstream port whose child is PCIe endpoint, and then reset
> link between them. If the endpoint is VGA device, it is skipped because
> the monitor blacks out if VGA controller is reset.

At a quick skim this patch looks reasonable.

Acked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

> Changelog:
> v2:
> - Read pci config before de-assert secondary bus reset to flush previous
>   write
> - Change function/variable name
> - Make a list of devices to be reset
>
> v1:
> https://patchwork.kernel.org/patch/2482291/
>
> Signed-off-by: Takao Indoh <indou.takao@xxxxxxxxxxxxxx>
> ---
>  Documentation/kernel-parameters.txt |    2 +
>  drivers/pci/pci.c                   |  113 +++++++++++++++++++++++++++++++++++
>  2 files changed, 115 insertions(+), 0 deletions(-)
>
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index c3bfacb..8c9e8e4 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -2321,6 +2321,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  				any pair of devices, possibly at the cost of
>  				reduced performance.  This also guarantees
>  				that hot-added devices will work.
> +		pcie_reset_endpoint_devices	Reset PCIe endpoint on boot by
> +				hot reset
>  		cbiosize=nn[KMG]	The fixed amount of bus space which is
>  				reserved for the CardBus bridge's IO window.
>  				The default value is 256 bytes.
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index a899d8b..70c1205 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3873,6 +3873,116 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>  }
>  EXPORT_SYMBOL(pci_fixup_cardbus);
>  
> +/*
> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
> + * endpoint except VGA device.
> + */
> +static int __init need_reset(struct pci_dev *dev)
> +{
> +	struct pci_bus *subordinate;
> +	struct pci_dev *child;
> +
> +	if (!pci_is_pcie(dev) || !dev->subordinate ||
> +	    list_empty(&dev->subordinate->devices) ||
> +	    ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT) &&
> +	     (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
> +		return 0;
> +
> +	subordinate = dev->subordinate;
> +	list_for_each_entry(child, &subordinate->devices, bus_list) {
> +		if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
> +		    (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
> +		    ((child->class >> 16) == PCI_BASE_CLASS_DISPLAY))
> +			/* Don't reset switch, bridge, VGA device */
> +			return 0;
> +	}
> +
> +	return 1;
> +}
> +
> +static void __init save_downstream_configs(struct pci_dev *dev)
> +{
> +	struct pci_bus *subordinate;
> +	struct pci_dev *child;
> +
> +	subordinate = dev->subordinate;
> +	list_for_each_entry(child, &subordinate->devices, bus_list) {
> +		dev_info(&child->dev, "save state\n");
> +		pci_save_state(child);
> +	}
> +}
> +
> +static void __init restore_downstream_configs(struct pci_dev *dev)
> +{
> +	struct pci_bus *subordinate;
> +	struct pci_dev *child;
> +
> +	subordinate = dev->subordinate;
> +	list_for_each_entry(child, &subordinate->devices, bus_list) {
> +		dev_info(&child->dev, "restore state\n");
> +		pci_restore_state(child);
> +	}
> +}
> +
> +static void __init do_downstream_device_reset(struct pci_dev *dev)
> +{
> +	u16 ctrl;
> +
> +	dev_info(&dev->dev, "Reset Secondary bus\n");
> +
> +	/* Assert Secondary Bus Reset */
> +	pci_read_config_word(dev, PCI_BRIDGE_CONTROL, &ctrl);
> +	ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
> +
> +	/* Read config again to flush previous write */
> +	pci_read_config_word(dev, PCI_BRIDGE_CONTROL, &ctrl);
> +
> +	msleep(2);
> +
> +	/* De-assert Secondary Bus Reset */
> +	ctrl &= ~PCI_BRIDGE_CTL_BUS_RESET;
> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
> +}
> +
> +struct pci_dev_entry {
> +	struct list_head list;
> +	struct pci_dev *dev;
> +};
> +static int __initdata pcie_reset_endpoint_devices;
> +static int __init reset_pcie_endpoints(void)
> +{
> +	struct pci_dev *dev = NULL;
> +	struct pci_dev_entry *pdev_entry, *tmp;
> +	LIST_HEAD(pdev_list);
> +
> +	if (!pcie_reset_endpoint_devices)
> +		return 0;
> +
> +	for_each_pci_dev(dev)
> +		if (need_reset(dev)) {
> +			pdev_entry = kmalloc(sizeof(*pdev_entry), GFP_KERNEL);
> +			pdev_entry->dev = dev;
> +			list_add(&pdev_entry->list, &pdev_list);
> +		}
> +
> +	list_for_each_entry(pdev_entry, &pdev_list, list)
> +		save_downstream_configs(pdev_entry->dev);
> +
> +	list_for_each_entry(pdev_entry, &pdev_list, list)
> +		do_downstream_device_reset(pdev_entry->dev);
> +
> +	msleep(1000);
> +
> +	list_for_each_entry_safe(pdev_entry, tmp, &pdev_list, list) {
> +		restore_downstream_configs(pdev_entry->dev);
> +		kfree(pdev_entry);
> +	}
> +
> +	return 0;
> +}
> +fs_initcall_sync(reset_pcie_endpoints);
> +
>  static int __init pci_setup(char *str)
>  {
>  	while (str) {
> @@ -3915,6 +4025,9 @@ static int __init pci_setup(char *str)
>  				pcie_bus_config = PCIE_BUS_PEER2PEER;
>  			} else if (!strncmp(str, "pcie_scan_all", 13)) {
>  				pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
> +			} else if (!strncmp(str, "pcie_reset_endpoint_devices",
> +						27)) {
> +				pcie_reset_endpoint_devices = 1;
>  			} else {
>  				printk(KERN_ERR "PCI: Unknown option `%s'\n",
>  						str);
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