Re: [PATCH 1/1] i915/query: Correlate engine and cpu timestamps with better accuracy

[Jason Ekstrand <jason@jlekstrand.net>]

On Wed, Apr 28, 2021 at 7:34 PM Umesh Nerlige Ramappa
<umesh.nerlige.ramappa@intel.com> wrote:
>
> Perf measurements rely on CPU and engine timestamps to correlate
> events of interest across these time domains. Current mechanisms get
> these timestamps separately and the calculated delta between these
> timestamps lack enough accuracy.
>
> To improve the accuracy of these time measurements to within a few us,
> add a query that returns the engine and cpu timestamps captured as
> close to each other as possible.
>
> v2: (Tvrtko)
> - document clock reference used
> - return cpu timestamp always
> - capture cpu time just before lower dword of cs timestamp
>
> v3: (Chris)
> - use uncore-rpm
> - use __query_cs_timestamp helper
>
> v4: (Lionel)
> - Kernel perf subsytem allows users to specify the clock id to be used
>   in perf_event_open. This clock id is used by the perf subsystem to
>   return the appropriate cpu timestamp in perf events. Similarly, let
>   the user pass the clockid to this query so that cpu timestamp
>   corresponds to the clock id requested.
>
> v5: (Tvrtko)
> - Use normal ktime accessors instead of fast versions
> - Add more uApi documentation
>
> v6: (Lionel)
> - Move switch out of spinlock
>
> v7: (Chris)
> - cs_timestamp is a misnomer, use cs_cycles instead
> - return the cs cycle frequency as well in the query
>
> v8:
> - Add platform and engine specific checks
>
> v9: (Lionel)
> - Return 2 cpu timestamps in the query - captured before and after the
>   register read
>
> v10: (Chris)
> - Use local_clock() to measure time taken to read lower dword of
>   register and return it to user.
>
> v11: (Jani)
> - IS_GEN deprecated. User GRAPHICS_VER instead.
>
> v12: (Jason)
> - Split cpu timestamp array into timestamp and delta for cleaner API
>
> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
> Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_query.c | 148 ++++++++++++++++++++++++++++++
>  include/uapi/drm/i915_drm.h       |  52 +++++++++++
>  2 files changed, 200 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_query.c b/drivers/gpu/drm/i915/i915_query.c
> index fed337ad7b68..357c44e8177c 100644
> --- a/drivers/gpu/drm/i915/i915_query.c
> +++ b/drivers/gpu/drm/i915/i915_query.c
> @@ -6,6 +6,8 @@
>
>  #include <linux/nospec.h>
>
> +#include "gt/intel_engine_pm.h"
> +#include "gt/intel_engine_user.h"
>  #include "i915_drv.h"
>  #include "i915_perf.h"
>  #include "i915_query.h"
> @@ -90,6 +92,151 @@ static int query_topology_info(struct drm_i915_private *dev_priv,
>         return total_length;
>  }
>
> +typedef u64 (*__ktime_func_t)(void);
> +static __ktime_func_t __clock_id_to_func(clockid_t clk_id)
> +{
> +       /*
> +        * Use logic same as the perf subsystem to allow user to select the
> +        * reference clock id to be used for timestamps.
> +        */
> +       switch (clk_id) {
> +       case CLOCK_MONOTONIC:
> +               return &ktime_get_ns;
> +       case CLOCK_MONOTONIC_RAW:
> +               return &ktime_get_raw_ns;
> +       case CLOCK_REALTIME:
> +               return &ktime_get_real_ns;
> +       case CLOCK_BOOTTIME:
> +               return &ktime_get_boottime_ns;
> +       case CLOCK_TAI:
> +               return &ktime_get_clocktai_ns;
> +       default:
> +               return NULL;
> +       }
> +}
> +
> +static inline int
> +__read_timestamps(struct intel_uncore *uncore,
> +                 i915_reg_t lower_reg,
> +                 i915_reg_t upper_reg,
> +                 u64 *cs_ts,
> +                 u64 *cpu_ts,
> +                 u64 *cpu_delta,
> +                 __ktime_func_t cpu_clock)
> +{
> +       u32 upper, lower, old_upper, loop = 0;
> +
> +       upper = intel_uncore_read_fw(uncore, upper_reg);
> +       do {
> +               *cpu_delta = local_clock();
> +               *cpu_ts = cpu_clock();
> +               lower = intel_uncore_read_fw(uncore, lower_reg);
> +               *cpu_delta = local_clock() - *cpu_delta;
> +               old_upper = upper;
> +               upper = intel_uncore_read_fw(uncore, upper_reg);
> +       } while (upper != old_upper && loop++ < 2);
> +
> +       *cs_ts = (u64)upper << 32 | lower;
> +
> +       return 0;
> +}
> +
> +static int
> +__query_cs_cycles(struct intel_engine_cs *engine,
> +                 u64 *cs_ts, u64 *cpu_ts, u64 *cpu_delta,
> +                 __ktime_func_t cpu_clock)
> +{
> +       struct intel_uncore *uncore = engine->uncore;
> +       enum forcewake_domains fw_domains;
> +       u32 base = engine->mmio_base;
> +       intel_wakeref_t wakeref;
> +       int ret;
> +
> +       fw_domains = intel_uncore_forcewake_for_reg(uncore,
> +                                                   RING_TIMESTAMP(base),
> +                                                   FW_REG_READ);
> +
> +       with_intel_runtime_pm(uncore->rpm, wakeref) {
> +               spin_lock_irq(&uncore->lock);
> +               intel_uncore_forcewake_get__locked(uncore, fw_domains);
> +
> +               ret = __read_timestamps(uncore,
> +                                       RING_TIMESTAMP(base),
> +                                       RING_TIMESTAMP_UDW(base),
> +                                       cs_ts,
> +                                       cpu_ts,
> +                                       cpu_delta,
> +                                       cpu_clock);
> +
> +               intel_uncore_forcewake_put__locked(uncore, fw_domains);
> +               spin_unlock_irq(&uncore->lock);
> +       }
> +
> +       return ret;
> +}
> +
> +static int
> +query_cs_cycles(struct drm_i915_private *i915,
> +               struct drm_i915_query_item *query_item)
> +{
> +       struct drm_i915_query_cs_cycles __user *query_ptr;
> +       struct drm_i915_query_cs_cycles query;
> +       struct intel_engine_cs *engine;
> +       __ktime_func_t cpu_clock;
> +       int ret;
> +
> +       if (GRAPHICS_VER(i915) < 6)
> +               return -ENODEV;
> +
> +       query_ptr = u64_to_user_ptr(query_item->data_ptr);
> +       ret = copy_query_item(&query, sizeof(query), sizeof(query), query_item);
> +       if (ret != 0)
> +               return ret;
> +
> +       if (query.flags)
> +               return -EINVAL;
> +
> +       if (query.rsvd)
> +               return -EINVAL;
> +
> +       cpu_clock = __clock_id_to_func(query.clockid);
> +       if (!cpu_clock)
> +               return -EINVAL;
> +
> +       engine = intel_engine_lookup_user(i915,
> +                                         query.engine.engine_class,
> +                                         query.engine.engine_instance);
> +       if (!engine)
> +               return -EINVAL;
> +
> +       if (GRAPHICS_VER(i915) == 6 &&
> +           query.engine.engine_class != I915_ENGINE_CLASS_RENDER)
> +               return -ENODEV;
> +
> +       query.cs_frequency = engine->gt->clock_frequency;
> +       ret = __query_cs_cycles(engine,
> +                               &query.cs_cycles,
> +                               &query.cpu_timestamp,
> +                               &query.cpu_delta,
> +                               cpu_clock);
> +       if (ret)
> +               return ret;
> +
> +       if (put_user(query.cs_frequency, &query_ptr->cs_frequency))
> +               return -EFAULT;
> +
> +       if (put_user(query.cpu_timestamp, &query_ptr->cpu_timestamp))
> +               return -EFAULT;
> +
> +       if (put_user(query.cpu_delta, &query_ptr->cpu_delta))
> +               return -EFAULT;
> +
> +       if (put_user(query.cs_cycles, &query_ptr->cs_cycles))
> +               return -EFAULT;
> +
> +       return sizeof(query);
> +}
> +
>  static int
>  query_engine_info(struct drm_i915_private *i915,
>                   struct drm_i915_query_item *query_item)
> @@ -424,6 +571,7 @@ static int (* const i915_query_funcs[])(struct drm_i915_private *dev_priv,
>         query_topology_info,
>         query_engine_info,
>         query_perf_config,
> +       query_cs_cycles,
>  };
>
>  int i915_query_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index 6a34243a7646..0b4c27092d41 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -2230,6 +2230,10 @@ struct drm_i915_query_item {
>  #define DRM_I915_QUERY_TOPOLOGY_INFO    1
>  #define DRM_I915_QUERY_ENGINE_INFO     2
>  #define DRM_I915_QUERY_PERF_CONFIG      3
> +       /**
> +        * Query Command Streamer timestamp register.
> +        */
> +#define DRM_I915_QUERY_CS_CYCLES       4
>  /* Must be kept compact -- no holes and well documented */
>
>         /**
> @@ -2397,6 +2401,54 @@ struct drm_i915_engine_info {
>         __u64 rsvd1[4];
>  };
>
> +/**
> + * struct drm_i915_query_cs_cycles
> + *
> + * The query returns the command streamer cycles and the frequency that can be
> + * used to calculate the command streamer timestamp. In addition the query
> + * returns a set of cpu timestamps that indicate when the command streamer cycle
> + * count was captured.
> + */
> +struct drm_i915_query_cs_cycles {
> +       /** Engine for which command streamer cycles is queried. */
> +       struct i915_engine_class_instance engine;

I've checked with HW engineers and they're claiming that all CS
timestamp registers should report the same time modulo minor drift.
You're CC'd on the internal e-mail.  If this is really the case, then
I don't think we want to put an engine in this query.

--Jason

> +
> +       /** Must be zero. */
> +       __u32 flags;
> +
> +       /**
> +        * Command streamer cycles as read from the command streamer
> +        * register at 0x358 offset.
> +        */
> +       __u64 cs_cycles;
> +
> +       /** Frequency of the cs cycles in Hz. */
> +       __u64 cs_frequency;
> +
> +       /**
> +        * CPU timestamp in ns. The timestamp is captured before reading the
> +        * cs_cycles register using the reference clockid set by the user.
> +        */
> +       __u64 cpu_timestamp;
> +
> +       /**
> +        * Time delta in ns captured around reading the lower dword of the
> +        * cs_cycles register.
> +        */
> +       __u64 cpu_delta;
> +
> +       /**
> +        * Reference clock id for CPU timestamp. For definition, see
> +        * clock_gettime(2) and perf_event_open(2). Supported clock ids are
> +        * CLOCK_MONOTONIC, CLOCK_MONOTONIC_RAW, CLOCK_REALTIME, CLOCK_BOOTTIME,
> +        * CLOCK_TAI.
> +        */
> +       __s32 clockid;
> +
> +       /** Must be zero. */
> +       __u32 rsvd;
> +};
> +
>  /**
>   * struct drm_i915_query_engine_info
>   *
> --
> 2.20.1
>
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel