DRM client usage stats

DRM drivers can choose to export partly standardised text output via the fops->show_fdinfo() as part of the driver specific file operations registered in the struct drm_driver object registered with the DRM core.

One purpose of this output is to enable writing as generic as practicaly feasible top(1) like userspace monitoring tools.

Given the differences between various DRM drivers the specification of the output is split between common and driver specific parts. Having said that, wherever possible effort should still be made to standardise as much as possible.

File format specification

  • File shall contain one key value pair per one line of text.
  • Colon character (:) must be used to delimit keys and values.
  • All keys shall be prefixed with drm-.
  • Whitespace between the delimiter and first non-whitespace character shall be ignored when parsing.
  • Neither keys or values are allowed to contain whitespace characters.
  • Numerical key value pairs can end with optional unit string.
  • Data type of the value is fixed as defined in the specification.

Key types

  1. Mandatory, fully standardised.
  2. Optional, fully standardised.
  3. Driver specific.

Data types

  • <uint> - Unsigned integer without defining the maximum value.
  • <str> - String excluding any above defined reserved characters or whitespace.

Mandatory fully standardised keys

  • drm-driver: <str>

String shall contain the name this driver registered as via the respective struct drm_driver data structure.

Optional fully standardised keys

  • drm-pdev: <aaaa:bb.cc.d>

For PCI devices this should contain the PCI slot address of the device in question.

  • drm-client-id: <uint>

Unique value relating to the open DRM file descriptor used to distinguish duplicated and shared file descriptors. Conceptually the value should map 1:1 to the in kernel representation of struct drm_file instances.

Uniqueness of the value shall be either globally unique, or unique within the scope of each device, in which case drm-pdev shall be present as well.

Userspace should make sure to not double account any usage statistics by using the above described criteria in order to associate data to individual clients.

  • drm-engine-<str>: <uint> ns

GPUs usually contain multiple execution engines. Each shall be given a stable and unique name (str), with possible values documented in the driver specific documentation.

Value shall be in specified time units which the respective GPU engine spent busy executing workloads belonging to this client.

Values are not required to be constantly monotonic if it makes the driver implementation easier, but are required to catch up with the previously reported larger value within a reasonable period. Upon observing a value lower than what was previously read, userspace is expected to stay with that larger previous value until a monotonic update is seen.

  • drm-engine-capacity-<str>: <uint>

Engine identifier string must be the same as the one specified in the drm-engine-<str> tag and shall contain a greater than zero number in case the exported engine corresponds to a group of identical hardware engines.

In the absence of this tag parser shall assume capacity of one. Zero capacity is not allowed.

  • drm-memory-<str>: <uint> [KiB|MiB]

Each possible memory type which can be used to store buffer objects by the GPU in question shall be given a stable and unique name to be returned as the string here.

Value shall reflect the amount of storage currently consumed by the buffer object belong to this client, in the respective memory region.

Default unit shall be bytes with optional unit specifiers of ‘KiB’ or ‘MiB’ indicating kibi- or mebi-bytes.

Driver specific implementations

i915 DRM client usage stats implementation