[PATCH v6 3/5] drm/doc: Document DRM_WEDGE_RECOVERY_COLD_RESET recovery method

[Mallesh Koujalagi <mallesh.koujalagi@intel.com>]

When ``WEDGED=cold-reset`` is sent, it indicates that the device has
encountered an error condition that cannot be resolved through other
recovery methods such as driver rebind or bus reset, and requires a
complete device power cycle to restore functionality.

Signed-off-by: Mallesh Koujalagi <mallesh.koujalagi@intel.com>
---
v2:
- Add several instead of number to avoid update. (Jani)

v3:
- Update document with generic scenario. (Riana)
- Consistent with terminology. (Raag)
- Remove already covered information.

v4:
- Update doc. (Raag/Riana)
- Change commit message.

v5:
- Update commit message. (Raag)
- Remove unbind.
- Simplify cold-reset script.

v6:
- Look up the hotplug slot in DEVPATH instead of scanning
  every PCI slot on the system. (Raag)
- Drop arbitrary sleep values from the example script.
---
 Documentation/gpu/drm-uapi.rst | 60 +++++++++++++++++++++++++++++++++-
 1 file changed, 59 insertions(+), 1 deletion(-)

diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
index 2c2f939322fb..665c66782504 100644
--- a/Documentation/gpu/drm-uapi.rst
+++ b/Documentation/gpu/drm-uapi.rst
@@ -424,7 +424,7 @@ needed.
 Recovery
 --------
 
-Current implementation defines four recovery methods, out of which, drivers
+Current implementation defines several recovery methods, out of which, drivers
 can use any one, multiple or none. Method(s) of choice will be sent in the
 uevent environment as ``WEDGED=<method1>[,..,<methodN>]`` in order of less to
 more side-effects. See the section `Vendor Specific Recovery`_
@@ -441,6 +441,7 @@ following expectations.
     rebind          unbind + bind driver
     bus-reset       unbind + bus reset/re-enumeration + bind
     vendor-specific vendor specific recovery method
+    cold-reset      remove device + slot power cycle + rescan
     unknown         consumer policy
     =============== ========================================
 
@@ -453,6 +454,17 @@ debug purpose in order to root cause the hang. This is useful because the first
 hang is usually the most critical one which can result in consequential hangs
 or complete wedging.
 
+Cold Reset Recovery
+-------------------
+
+When ``WEDGED=cold-reset`` is sent, it indicates that the device has
+encountered an error condition that cannot be resolved through other
+recovery methods such as driver rebind or bus reset, and requires a complete
+device power cycle to restore functionality.
+
+This method is used by devices that are plugged directly into the PCIe slot
+which supports removing the power.
+
 Vendor Specific Recovery
 ------------------------
 
@@ -530,6 +542,52 @@ Recovery script::
     echo -n $DEVICE > $DRIVER/unbind
     echo -n $DEVICE > $DRIVER/bind
 
+Example - cold-reset
+--------------------
+
+Udev rule::
+
+    SUBSYSTEM=="drm", ENV{WEDGED}=="cold-reset", DEVPATH=="*/drm/card[0-9]",
+    RUN+="/path/to/cold-reset.sh $env{DEVPATH}"
+
+Recovery script::
+
+    #!/bin/sh
+
+    [ -z "$1" ] && echo "Usage: $0 <device-path>" && exit 1
+
+    # Get device
+    DEVPATH=$(readlink -f /sys/$1/device 2>/dev/null || readlink -f /sys/$1)
+    DEVICE=$(basename $DEVPATH)
+
+    echo "Cold reset: $DEVICE"
+
+    # The PCI core exposes a 'slot' symlink on the device that sits in a
+    # registered hotplug slot. Use it directly instead of scanning every
+    # slot on the system.
+    SLOT=""
+    if [ -L "$DEVPATH/slot" ]; then
+	    SLOT=$(basename "$(readlink -f "$DEVPATH/slot")")
+    fi
+
+    if [ -n "$SLOT" ]; then
+	echo "Using slot $SLOT"
+
+	# Remove device
+	echo 1 > /sys/bus/pci/devices/$DEVICE/remove
+
+	# Power cycle slot. A platform-specific settle delay may be required
+	# between power off and power on; tune to the hardware as needed.
+	echo 0 > /sys/bus/pci/slots/$SLOT/power
+	echo 1 > /sys/bus/pci/slots/$SLOT/power
+
+	# Rescan
+	echo 1 > /sys/bus/pci/rescan
+	echo "Done!"
+    else
+	echo "No slot found"
+    fi
+
 Customization
 -------------
 
-- 
2.34.1