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Add reset_method sysfs attribute to enable user to query and set user
preferred device reset methods and their ordering.

Co-developed-by: Alex Williamson <redacted>
Signed-off-by: Alex Williamson <redacted>
Signed-off-by: Amey Narkhede <redacted>
---
 Documentation/ABI/testing/sysfs-bus-pci |  19 +++++
 drivers/pci/pci-sysfs.c                 |   1 +
 drivers/pci/pci.c                       | 105 ++++++++++++++++++++++++
 drivers/pci/pci.h                       |   2 +
 4 files changed, 127 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
index ef00fada2efb..ef66b62bf025 100644
--- a/Documentation/ABI/testing/sysfs-bus-pci
+++ b/Documentation/ABI/testing/sysfs-bus-pci

@@ -121,6 +121,25 @@ Description:
                child buses, and re-discover devices removed earlier
                from this part of the device tree.

+What:          /sys/bus/pci/devices/.../reset_method
+Date:          March 2021
+Contact:       Amey Narkhede <ameynarkhede03@gmail.com>
+Description:
+               Some devices allow an individual function to be reset
+               without affecting other functions in the same slot.
+
+               For devices that have this support, a file named
+               reset_method will be present in sysfs. Initially reading
+               this file will give names of the device supported reset
+               methods and their ordering. After write, this file will
+               give names and ordering of currently enabled reset methods.
+               Writing the name or space separated list of names of any of
+               the device supported reset methods to this file will set
+               the reset methods and their ordering to be used when
+               resetting the device. Writing empty string to this file
+               will disable ability to reset the device and writing
+               "default" will return to the original value.
+
 What:          /sys/bus/pci/devices/.../reset
 Date:          July 2009
 Contact:       Michael S. Tsirkin <mst@redhat.com>

diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index 316f70c3e3b4..54ee7193b463 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c

@@ -1491,6 +1491,7 @@ const struct attribute_group *pci_dev_groups[] = {
        &pci_dev_config_attr_group,
        &pci_dev_rom_attr_group,
        &pci_dev_reset_attr_group,
+       &pci_dev_reset_method_attr_group,
        &pci_dev_vpd_attr_group,
 #ifdef CONFIG_DMI
        &pci_dev_smbios_attr_group,

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 932dd21e759b..c496cd164aca 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c

@@ -5132,6 +5132,111 @@ static const struct pci_reset_fn_method pci_reset_fn_methods[] = {
        { pci_reset_bus_function, .name = "bus" },
 };

+static ssize_t reset_method_show(struct device *dev,
+                                struct device_attribute *attr,
+                                char *buf)
+{
+       struct pci_dev *pdev = to_pci_dev(dev);
+       ssize_t len = 0;
+       int i, m;
+
+       for (i = 0; i < PCI_NUM_RESET_METHODS; i++) {
+               m = pdev->reset_methods[i];
+               if (!m)
+                       break;
+
+               len += sysfs_emit_at(buf, len, "%s%s", len ? " " : "",
+                                    pci_reset_fn_methods[m].name);
+       }
+
+       if (len)
+               len += sysfs_emit_at(buf, len, "\n");
+
+       return len;
+}
+
+static ssize_t reset_method_store(struct device *dev,
+                                 struct device_attribute *attr,
+                                 const char *buf, size_t count)
+{
+       struct pci_dev *pdev = to_pci_dev(dev);
+       int i = 0;
+       char *name, *options = NULL;
+
+       if (count >= (PAGE_SIZE - 1))
+               return -EINVAL;
+
+       if (sysfs_streq(buf, "")) {
+               pdev->reset_methods[0] = 0;
+               pci_warn(pdev, "All device reset methods disabled by user");
+               return count;
+       }
+
+       if (sysfs_streq(buf, "default")) {
+               pci_init_reset_methods(pdev);
+               return count;
+       }
+
+       options = kstrndup(buf, count, GFP_KERNEL);
+       if (!options)
+               return -ENOMEM;
+
+       while ((name = strsep(&options, " ")) != NULL) {
+               int m;
+
+               if (sysfs_streq(name, ""))
+                       continue;
+
+               name = strim(name);
+
+               for (m = 1; m < PCI_NUM_RESET_METHODS && i < PCI_NUM_RESET_METHODS; m++) {
+                       if (sysfs_streq(name, pci_reset_fn_methods[m].name) &&
+                           !pci_reset_fn_methods[m].reset_fn(pdev, 1)) {
+                               pdev->reset_methods[i++] = m;
+                               break;
+                       }
+               }
+