linux-kernel - Re: [PATCH v7 2/5] drm/panfrost: Add fdinfo support GPU load metrics

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Date:   Thu, 28 Sep 2023 13:24:55 +0200
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Adrián Larumbe <adrian.larumbe@...labora.com>,
        maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...il.com, daniel@...ll.ch,
        robdclark@...il.com, quic_abhinavk@...cinc.com,
        dmitry.baryshkov@...aro.org, sean@...rly.run,
        marijn.suijten@...ainline.org, robh@...nel.org,
        steven.price@....com
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, freedreno@...ts.freedesktop.org,
        healych@...zon.com, kernel@...labora.com,
        tvrtko.ursulin@...ux.intel.com, boris.brezillon@...labora.com
Subject: Re: [PATCH v7 2/5] drm/panfrost: Add fdinfo support GPU load metrics

Il 27/09/23 23:29, Adrián Larumbe ha scritto:
> The drm-stats fdinfo tags made available to user space are drm-engine,
> drm-cycles, drm-max-freq and drm-curfreq, one per job slot.
> 
> This deviates from standard practice in other DRM drivers, where a single
> set of key:value pairs is provided for the whole render engine. However,
> Panfrost has separate queues for fragment and vertex/tiler jobs, so a
> decision was made to calculate bus cycles and workload times separately.
> 
> Maximum operating frequency is calculated at devfreq initialisation time.
> Current frequency is made available to user space because nvtop uses it
> when performing engine usage calculations.
> 
> It is important to bear in mind that both GPU cycle and kernel time numbers
> provided are at best rough estimations, and always reported in excess from
> the actual figure because of two reasons:
>   - Excess time because of the delay between the end of a job processing,
>     the subsequent job IRQ and the actual time of the sample.
>   - Time spent in the engine queue waiting for the GPU to pick up the next
>     job.
> 
> To avoid race conditions during enablement/disabling, a reference counting
> mechanism was introduced, and a job flag that tells us whether a given job
> increased the refcount. This is necessary, because user space can toggle
> cycle counting through a debugfs file, and a given job might have been in
> flight by the time cycle counting was disabled.
> 
> The main goal of the debugfs cycle counter knob is letting tools like nvtop
> or IGT's gputop switch it at any time, to avoid power waste in case no
> engine usage measuring is necessary.
> 
> Add also documentation file explaining the possible values for fdinfo's

Also add a documentation file explaining .....

or

Moreover, add a documentation file explaining .....

> engine keystrings and Panfrost-specific drm-curfreq-<keystr> pairs.
> 
> Signed-off-by: Adrián Larumbe <adrian.larumbe@...labora.com>
> Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
> Reviewed-by: Steven Price <steven.price@....com>

Nitpick apart:

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>