[PATCH] drm/crc-debugfs: Add notes about CRC<->commit interactions

[Brian Starkey]

On Mon, Aug 05, 2019 at 06:24:17PM +0200, Daniel Vetter wrote:
> On Mon, Aug 05, 2019 at 04:11:43PM +0100, Brian Starkey 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 at arm.com>
> > ---
> >
> > I might have got the wrong end of the stick, but this is what I
> > understood from what you said.
> >
> > Cheers,
> > -Brian
> >
> >  drivers/gpu/drm/drm_debugfs_crc.c | 15 +++++++++++----
> >  include/drm/drm_crtc.h            |  3 +++
> >  2 files changed, 14 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_debugfs_crc.c b/drivers/gpu/drm/drm_debugfs_crc.c
> > index 7ca486d750e9..1dff956bcc74 100644
> > --- a/drivers/gpu/drm/drm_debugfs_crc.c
> > +++ b/drivers/gpu/drm/drm_debugfs_crc.c
> > @@ -65,10 +65,17 @@
> >   * 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).
> > + *
> > + * It's also possible that a "normal" commit via DRM_IOCTL_MODE_ATOMIC or the
> > + * legacy paths may interfere with CRC generation. So, in the general case,
> > + * userspace can't rely on the values in crtc-N/crc/data being valid
> > + * across a commit.
>
> It's not just the values, but the generation itself might get disabled
> (e.g. on i915 if you select "auto" on some chips you get the DP port
> sampling point, but for HDMI mode you get a different sampling ploint, and
> if you get the wrong one there won't be any crc for you). Also it's not
> just any atomic commit, only the ones with ALLOW_MODESET.

Is the meaning of ALLOW_MODESET actually defined somewhere then? I
thought it was broadly speaking "anything that would cause a flicker
on the output" - but that needn't be the same set of things that break
CRC generation.

>
> Maybe something like the below text:
>
> "Please note that an atomic modeset commit with the
> DRM_MODE_ATOMIC_ALLOW_MODESET, or a call to the legacy SETCRTR ioctl
> (which amounts to the same), can destry the CRC setup due to hardware
> requirements. This results in inconsistent CRC values or not even any CRC
> values generated. Generic userspace therefore needs to re-setup the CRC
> after each such 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..0f7ea094a900 100644
> > --- a/include/drm/drm_crtc.h
> > +++ b/include/drm/drm_crtc.h
> > @@ -756,6 +756,8 @@ 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 a commit if necessary, to enable CRC generation.
>
> I'd clarify this as "atomic modeset commit" just to be sure.

Ack.

Thanks,
-Brian

>
> With these two comments addressed somehow:
>
> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
>
>
> > + *
> >   * 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 +769,7 @@ 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
> http://blog.ffwll.ch
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