drm/i915: Add a kerneldoc summary for i915_perf.c

 *   Robert Bragg <robert@sixbynine.org>

 */

/**

 * DOC: i915 Perf, streaming API for GPU metrics

 *

 * Gen graphics supports a large number of performance counters that can help

 * driver and application developers understand and optimize their use of the

 * GPU.

 *

 * This i915 perf interface enables userspace to configure and open a file

 * descriptor representing a stream of GPU metrics which can then be read() as

 * a stream of sample records.

 *

 * The interface is particularly suited to exposing buffered metrics that are

 * captured by DMA from the GPU, unsynchronized with and unrelated to the CPU.

 *

 * Streams representing a single context are accessible to applications with a

 * corresponding drm file descriptor, such that OpenGL can use the interface

 * without special privileges. Access to system-wide metrics requires root

 * privileges by default, unless changed via the dev.i915.perf_event_paranoid

 * sysctl option.

 *

 *

 * The interface was initially inspired by the core Perf infrastructure but

 * some notable differences are:

 *

 * i915 perf file descriptors represent a "stream" instead of an "event"; where

 * a perf event primarily corresponds to a single 64bit value, while a stream

 * might sample sets of tightly-coupled counters, depending on the

 * configuration.  For example the Gen OA unit isn't designed to support

 * orthogonal configurations of individual counters; it's configured for a set

 * of related counters. Samples for an i915 perf stream capturing OA metrics

 * will include a set of counter values packed in a compact HW specific format.

 * The OA unit supports a number of different packing formats which can be

 * selected by the user opening the stream. Perf has support for grouping

 * events, but each event in the group is configured, validated and

 * authenticated individually with separate system calls.

 *

 * i915 perf stream configurations are provided as an array of u64 (key,value)

 * pairs, instead of a fixed struct with multiple miscellaneous config members,

 * interleaved with event-type specific members.

 *

 * i915 perf doesn't support exposing metrics via an mmap'd circular buffer.

 * The supported metrics are being written to memory by the GPU unsynchronized

 * with the CPU, using HW specific packing formats for counter sets. Sometimes

 * the constraints on HW configuration require reports to be filtered before it

 * would be acceptable to expose them to unprivileged applications - to hide

 * the metrics of other processes/contexts. For these use cases a read() based

 * interface is a good fit, and provides an opportunity to filter data as it

 * gets copied from the GPU mapped buffers to userspace buffers.

 *

 *

 * Some notes regarding Linux Perf:

 * --------------------------------

 *

 * The first prototype of this driver was based on the core perf

 * infrastructure, and while we did make that mostly work, with some changes to

 * perf, we found we were breaking or working around too many assumptions baked

 * into perf's currently cpu centric design.

 *

 * In the end we didn't see a clear benefit to making perf's implementation and

 * interface more complex by changing design assumptions while we knew we still

 * wouldn't be able to use any existing perf based userspace tools.

 *

 * Also considering the Gen specific nature of the Observability hardware and

 * how userspace will sometimes need to combine i915 perf OA metrics with

 * side-band OA data captured via MI_REPORT_PERF_COUNT commands; we're

 * expecting the interface to be used by a platform specific userspace such as

 * OpenGL or tools. This is to say; we aren't inherently missing out on having

 * a standard vendor/architecture agnostic interface by not using perf.

 *

 *

 * For posterity, in case we might re-visit trying to adapt core perf to be

 * better suited to exposing i915 metrics these were the main pain points we

 * hit:

 *

 * - The perf based OA PMU driver broke some significant design assumptions:

 *

 *   Existing perf pmus are used for profiling work on a cpu and we were

 *   introducing the idea of _IS_DEVICE pmus with different security

 *   implications, the need to fake cpu-related data (such as user/kernel

 *   registers) to fit with perf's current design, and adding _DEVICE records

 *   as a way to forward device-specific status records.

 *

 *   The OA unit writes reports of counters into a circular buffer, without

 *   involvement from the CPU, making our PMU driver the first of a kind.

 *

 *   Given the way we were periodically forward data from the GPU-mapped, OA

 *   buffer to perf's buffer, those bursts of sample writes looked to perf like

 *   we were sampling too fast and so we had to subvert its throttling checks.

 *

 *   Perf supports groups of counters and allows those to be read via

 *   transactions internally but transactions currently seem designed to be

 *   explicitly initiated from the cpu (say in response to a userspace read())

 *   and while we could pull a report out of the OA buffer we can't

 *   trigger a report from the cpu on demand.

 *

 *   Related to being report based; the OA counters are configured in HW as a

 *   set while perf generally expects counter configurations to be orthogonal.

 *   Although counters can be associated with a group leader as they are

 *   opened, there's no clear precedent for being able to provide group-wide

 *   configuration attributes (for example we want to let userspace choose the

 *   OA unit report format used to capture all counters in a set, or specify a

 *   GPU context to filter metrics on). We avoided using perf's grouping

 *   feature and forwarded OA reports to userspace via perf's 'raw' sample

 *   field. This suited our userspace well considering how coupled the counters

 *   are when dealing with normalizing. It would be inconvenient to split

 *   counters up into separate events, only to require userspace to recombine

 *   them. For Mesa it's also convenient to be forwarded raw, periodic reports

 *   for combining with the side-band raw reports it captures using

 *   MI_REPORT_PERF_COUNT commands.

 *

 *   _ As a side note on perf's grouping feature; there was also some concern

 *     that using PERF_FORMAT_GROUP as a way to pack together counter values

 *     would quite drastically inflate our sample sizes, which would likely

 *     lower the effective sampling resolutions we could use when the available

 *     memory bandwidth is limited.

 *

 *     With the OA unit's report formats, counters are packed together as 32

 *     or 40bit values, with the largest report size being 256 bytes.

 *

 *     PERF_FORMAT_GROUP values are 64bit, but there doesn't appear to be a

 *     documented ordering to the values, implying PERF_FORMAT_ID must also be

 *     used to add a 64bit ID before each value; giving 16 bytes per counter.

 *

 *   Related to counter orthogonality; we can't time share the OA unit, while

 *   event scheduling is a central design idea within perf for allowing

 *   userspace to open + enable more events than can be configured in HW at any

 *   one time.  The OA unit is not designed to allow re-configuration while in

 *   use. We can't reconfigure the OA unit without losing internal OA unit

 *   state which we can't access explicitly to save and restore. Reconfiguring

 *   the OA unit is also relatively slow, involving ~100 register writes. From

 *   userspace Mesa also depends on a stable OA configuration when emitting

 *   MI_REPORT_PERF_COUNT commands and importantly the OA unit can't be

 *   disabled while there are outstanding MI_RPC commands lest we hang the

 *   command streamer.

 *

 *   The contents of sample records aren't extensible by device drivers (i.e.

 *   the sample_type bits). As an example; Sourab Gupta had been looking to

 *   attach GPU timestamps to our OA samples. We were shoehorning OA reports

 *   into sample records by using the 'raw' field, but it's tricky to pack more

 *   than one thing into this field because events/core.c currently only lets a

 *   pmu give a single raw data pointer plus len which will be copied into the

 *   ring buffer. To include more than the OA report we'd have to copy the

 *   report into an intermediate larger buffer. I'd been considering allowing a

 *   vector of data+len values to be specified for copying the raw data, but

 *   it felt like a kludge to being using the raw field for this purpose.

 *

 * - It felt like our perf based PMU was making some technical compromises

 *   just for the sake of using perf:

 *

 *   perf_event_open() requires events to either relate to a pid or a specific

 *   cpu core, while our device pmu related to neither.  Events opened with a

 *   pid will be automatically enabled/disabled according to the scheduling of

 *   that process - so not appropriate for us. When an event is related to a

 *   cpu id, perf ensures pmu methods will be invoked via an inter process

 *   interrupt on that core. To avoid invasive changes our userspace opened OA

 *   perf events for a specific cpu. This was workable but it meant the

 *   majority of the OA driver ran in atomic context, including all OA report

 *   forwarding, which wasn't really necessary in our case and seems to make

 *   our locking requirements somewhat complex as we handled the interaction

 *   with the rest of the i915 driver.

 */

#include <linux/anon_inodes.h>

#include <linux/sizes.h>