Re: [PATCH v5 3/5] drm/doc: Document DRM_WEDGE_RECOVERY_COLD_RESET recovery method

[Raag Jadav <raag.jadav@intel.com>]

On Tue, May 12, 2026 at 06:56:18PM +0530, Mallesh Koujalagi wrote:
> 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.
> ---
>  Documentation/gpu/drm-uapi.rst | 64 +++++++++++++++++++++++++++++++++-
>  1 file changed, 63 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
> index 32206ce62931..809386f16521 100644
> --- a/Documentation/gpu/drm-uapi.rst
> +++ b/Documentation/gpu/drm-uapi.rst
> @@ -422,7 +422,7 @@ 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`_
> @@ -439,6 +439,7 @@ 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
>      =============== ========================================
>  
> @@ -451,6 +452,17 @@ 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.

This looks good to me.

>  Vendor Specific Recovery
>  ------------------------
>  
> @@ -528,6 +540,56 @@ 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"
> +
> +    # Try slot power reset first
> +    SLOT=""
> +    for addr_file in /sys/bus/pci/slots/*/address; do

This looks like we're peeking into every slot present on the machine which
is not very optimal. Can't we just follow device hierarchy in DEVPATH to
look for the slot that is relevant for us?

> +	    [ -f "$addr_file" ] || break
> +	    addr=$(cat "$addr_file" 2>/dev/null)
> +	    [ -n "$addr" ] || continue
> +	    case "$DEVICE" in
> +		    "$addr"*) SLOT=$(basename "$(dirname "$addr_file")"); break ;;
> +	    esac
> +    done
> +
> +    if [ -n "$SLOT" ]; then
> +	echo "Using slot $SLOT"
> +
> +	# Remove device
> +	echo 1 > /sys/bus/pci/devices/$DEVICE/remove
> +
> +	# Power cycle slot
> +	echo 0 > /sys/bus/pci/slots/$SLOT/power
> +	sleep 2

Is this tested on actual machine? Is there an explanation for the amount
of sleep needed that serves all cases generically?

> +	echo 1 > /sys/bus/pci/slots/$SLOT/power
> +	sleep 1

Ditto.

Raag

> +	# Rescan
> +	echo 1 > /sys/bus/pci/rescan
> +	echo "Done!"
> +    else
> +	echo "No slot found"
> +    fi
> +
>  Customization
>  -------------
>  
> -- 
> 2.34.1
>