Re: [PATCH 3/5] drm/vc4: Use drm_atomic_helper_check_plane_state() to simplify the logic

[Boris Brezillon <boris.brezillon@bootlin.com>]

On Fri, 27 Jul 2018 13:38:08 -0700
Eric Anholt <eric@anholt.net> wrote:

> Boris Brezillon <boris.brezillon@bootlin.com> writes:
> 
> > From: Eric Anholt <eric@anholt.net>
> >
> > drm_atomic_helper_check_plane_state() takes care of checking the
> > scaling capabilities and calculating the clipped X/Y offsets for us.
> >
> > Rely on this function instead of open-coding the logic. While at it, we
> > get rid of a few fields in vc4_plane_state that can be easily extracted
> > from drm_plane_state.
> >
> > Incidentally, it seems to fix a problem we had with negative X/Y
> > positioning of YUV planes.
> >
> > Signed-off-by: Eric Anholt <eric@anholt.net>
> > Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
> > ---
> >  drivers/gpu/drm/vc4/vc4_drv.h   |   9 --
> >  drivers/gpu/drm/vc4/vc4_plane.c | 210 +++++++++++++++++++++-------------------
> >  2 files changed, 111 insertions(+), 108 deletions(-)  
> 
> I feel like you ought to grab authorship of this patch -- you've made a
> bunch of good changes since my WIP patch.
> 
> >  	if (num_planes > 1) {
> > -		vc4_state->is_yuv = true;
> > -
> > -		h_subsample = drm_format_horz_chroma_subsampling(format);
> > -		v_subsample = drm_format_vert_chroma_subsampling(format);
> > -		vc4_state->src_w[1] = vc4_state->src_w[0] / h_subsample;
> > -		vc4_state->src_h[1] = vc4_state->src_h[0] / v_subsample;
> > +		src_w /= h_subsample;
> > +		src_h /= v_subsample;  
> 
> I'm a little nervous about leaving src_w/src_h divided after this block,
> in case we end up using them in some other calculation in a later
> change.  Could we just move the divide into the vc4_get_scaling_mode()
> call?
> 
> In general, I'm not enthusiastic about having src_w computations in 5
> places now.  Was keeping it in the vc4 state that much of a problem?

No strong objection to keeping ->src_{h,w}[] arrays in vc4_plane_state.

> 
> I'm not saying no, but copy-and-paste of these kinds of calculations are
> also a frequent source of bugs when you miss a x->y or w->h change.

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