drm/doc: Document device wedged event

Add documentation for device wedged event in a new "Device wedging"
chapter. This describes basic definitions, prerequisites and consumer
expectations along with an example.

v8: Improve introduction (Christian, Rodrigo)
v9: Add prerequisites section (Christian)
v10: Clarify mmap cleanup and consumer prerequisites (Christian, Aravind)
v11: Reference wedged event in device reset chapter (André)
v12: Refine consumer expectations and terminologies (Xaver, Pekka)

Signed-off-by: Raag Jadav <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Reviewed-by: André Almeida <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Rodrigo Vivi <[e-mail]>

Documentation/gpu/drm-uapi.rst

···
 
 Apart from propagating the reset through the stack so apps can recover, it's
 really useful for driver developers to learn more about what caused the reset in
-the first place. DRM devices should make use of devcoredump to store relevant
-information about the reset, so this information can be added to user bug
-reports.
+the first place. For this, drivers can make use of devcoredump to store relevant
+information about the reset and send device wedged event with ``none`` recovery
+method (as explained in "Device Wedging" chapter) to notify userspace, so this
+information can be collected and added to user bug reports.
+
+Device Wedging
+==============
+
+Drivers can optionally make use of device wedged event (implemented as
+drm_dev_wedged_event() in DRM subsystem), which notifies userspace of 'wedged'
+(hanged/unusable) state of the DRM device through a uevent. This is useful
+especially in cases where the device is no longer operating as expected and has
+become unrecoverable from driver context. Purpose of this implementation is to
+provide drivers a generic way to recover the device with the help of userspace
+intervention, without taking any drastic measures (like resetting or
+re-enumerating the full bus, on which the underlying physical device is sitting)
+in the driver.
+
+A 'wedged' device is basically a device that is declared dead by the driver
+after exhausting all possible attempts to recover it from driver context. The
+uevent is the notification that is sent to userspace along with a hint about
+what could possibly be attempted to recover the device from userspace and bring
+it back to usable state. Different drivers may have different ideas of a
+'wedged' device depending on hardware implementation of the underlying physical
+device, and hence the vendor agnostic nature of the event. It is up to the
+drivers to decide when they see the need for device recovery and how they want
+to recover from the available methods.
+
+Driver prerequisites
+--------------------
+
+The driver, before opting for recovery, needs to make sure that the 'wedged'
+device doesn't harm the system as a whole by taking care of the prerequisites.
+Necessary actions must include disabling DMA to system memory as well as any
+communication channels with other devices. Further, the driver must ensure
+that all dma_fences are signalled and any device state that the core kernel
+might depend on is cleaned up. All existing mmaps should be invalidated and
+page faults should be redirected to a dummy page. Once the event is sent, the
+device must be kept in 'wedged' state until the recovery is performed. New
+accesses to the device (IOCTLs) should be rejected, preferably with an error
+code that resembles the type of failure the device has encountered. This will
+signify the reason for wedging, which can be reported to the application if
+needed.
+
+Recovery
+--------
+
+Current implementation defines three 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. If driver is unsure about recovery or method is unknown
+(like soft/hard system reboot, firmware flashing, physical device replacement
+or any other procedure which can't be attempted on the fly), ``WEDGED=unknown``
+will be sent instead.
+
+Userspace consumers can parse this event and attempt recovery as per the
+following expectations.
+
+    =============== ========================================
+    Recovery method Consumer expectations
+    =============== ========================================
+    none            optional telemetry collection
+    rebind          unbind + bind driver
+    bus-reset       unbind + bus reset/re-enumeration + bind
+    unknown         consumer policy
+    =============== ========================================
+
+The only exception to this is ``WEDGED=none``, which signifies that the device
+was temporarily 'wedged' at some point but was recovered from driver context
+using device specific methods like reset. No explicit recovery is expected from
+the consumer in this case, but it can still take additional steps like gathering
+telemetry information (devcoredump, syslog). This is useful because the first
+hang is usually the most critical one which can result in consequential hangs or
+complete wedging.
+
+Consumer prerequisites
+----------------------
+
+It is the responsibility of the consumer to make sure that the device or its
+resources are not in use by any process before attempting recovery. With IOCTLs
+erroring out, all device memory should be unmapped and file descriptors should
+be closed to prevent leaks or undefined behaviour. The idea here is to clear the
+device of all user context beforehand and set the stage for a clean recovery.
+
+Example
+-------
+
+Udev rule::
+
+    SUBSYSTEM=="drm", ENV{WEDGED}=="rebind", DEVPATH=="*/drm/card[0-9]",
+    RUN+="/path/to/rebind.sh $env{DEVPATH}"
+
+Recovery script::
+
+    #!/bin/sh
+
+    DEVPATH=$(readlink -f /sys/$1/device)
+    DEVICE=$(basename $DEVPATH)
+    DRIVER=$(readlink -f $DEVPATH/driver)
+
+    echo -n $DEVICE > $DRIVER/unbind
+    echo -n $DEVICE > $DRIVER/bind
+
+Customization
+-------------
+
+Although basic recovery is possible with a simple script, consumers can define
+custom policies around recovery. For example, if the driver supports multiple
+recovery methods, consumers can opt for the suitable one depending on scenarios
+like repeat offences or vendor specific failures. Consumers can also choose to
+have the device available for debugging or telemetry collection and base their
+recovery decision on the findings. This is useful especially when the driver is
+unsure about recovery or method is unknown.
 
 .. _drm_driver_ioctl: