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From:   Daniel Vetter <daniel@ffwll.ch>
Date:   Tue, 6 Aug 2019 13:45:53 +0200
Message-ID: <CAKMK7uHjz=t2dCVwCbEf5qUkcEcyUSqL=hjsHb-ZJ0pD2w7rvQ@mail.gmail.com>
Subject: Re: [PATCH v2] drm/crc-debugfs: Add notes about CRC<->commit interactions
To:     Brian Starkey <brian.starkey@arm.com>
Cc:     Liviu Dudau <Liviu.Dudau@arm.com>,
        "james qian wang (Arm Technology China)" <james.qian.wang@arm.com>,
        "Lowry Li (Arm Technology China)" <Lowry.Li@arm.com>,
        nd <nd@arm.com>, dri-devel <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Aug 6, 2019 at 12:48 PM Brian Starkey <brian.starkey@arm.com> wrote:
>
> CRC generation can be impacted by commits coming from userspace, and
> enabling CRC generation may itself trigger a commit. Add notes about
> this to the kerneldoc.
>
> Signed-off-by: Brian Starkey <brian.starkey@arm.com>

lgtm, my earlier r-b holds. Well maybe should have a v2 here somewhere
with what you changed.
-Daniel
> ---
>  drivers/gpu/drm/drm_debugfs_crc.c | 17 +++++++++++++----
>  include/drm/drm_crtc.h            |  4 ++++
>  2 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_debugfs_crc.c b/drivers/gpu/drm/drm_debugfs_crc.c
> index 7ca486d750e9..77159b6e77c3 100644
> --- a/drivers/gpu/drm/drm_debugfs_crc.c
> +++ b/drivers/gpu/drm/drm_debugfs_crc.c
> @@ -65,10 +65,19 @@
>   * it submits. In this general case, the maximum userspace can do is to compare
>   * the reported CRCs of frames that should have the same contents.
>   *
> - * On the driver side the implementation effort is minimal, drivers only need to
> - * implement &drm_crtc_funcs.set_crc_source. The debugfs files are automatically
> - * set up if that vfunc is set. CRC samples need to be captured in the driver by
> - * calling drm_crtc_add_crc_entry().
> + * On the driver side the implementation effort is minimal, drivers only need
> + * to implement &drm_crtc_funcs.set_crc_source. The debugfs files are
> + * automatically set up if that vfunc is set. CRC samples need to be captured
> + * in the driver by calling drm_crtc_add_crc_entry(). Depending on the driver
> + * and HW requirements, &drm_crtc_funcs.set_crc_source may result in a commit
> + * (even a full modeset).
> + *
> + * CRC results must be reliable across non-full-modeset atomic commits, so if a
> + * commit via DRM_IOCTL_MODE_ATOMIC would disable or otherwise interfere with
> + * CRC generation, then the driver must mark that commit as a full modeset
> + * (drm_atomic_crtc_needs_modeset() should return true). As a result, to ensure
> + * consistent results, generic userspace must re-setup CRC generation after a
> + * legacy SETCRTC or an atomic commit with DRM_MODE_ATOMIC_ALLOW_MODESET.
>   */
>
>  static int crc_control_show(struct seq_file *m, void *data)
> diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
> index 128d8b210621..7d14c11bdc0a 100644
> --- a/include/drm/drm_crtc.h
> +++ b/include/drm/drm_crtc.h
> @@ -756,6 +756,9 @@ struct drm_crtc_funcs {
>          * provided from the configured source. Drivers must accept an "auto"
>          * source name that will select a default source for this CRTC.
>          *
> +        * This may trigger an atomic modeset commit if necessary, to enable CRC
> +        * generation.
> +        *
>          * Note that "auto" can depend upon the current modeset configuration,
>          * e.g. it could pick an encoder or output specific CRC sampling point.
>          *
> @@ -767,6 +770,7 @@ struct drm_crtc_funcs {
>          * 0 on success or a negative error code on failure.
>          */
>         int (*set_crc_source)(struct drm_crtc *crtc, const char *source);
> +
>         /**
>          * @verify_crc_source:
>          *
> --
> 2.17.1
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch