Re: [PATCH v4 6/6] drm/i915: Add skl_check_nv12_surface for NV12

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Apr 18, 2018 at 08:06:57PM +0200, Maarten Lankhorst wrote:
> Op 18-04-18 om 17:32 schreef Ville Syrjälä:
> > On Wed, Apr 18, 2018 at 09:38:13AM +0530, Vidya Srinivas wrote:
> >> From: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> >>
> >> We skip src trunction/adjustments for
> >> NV12 case and handle the sizes directly.
> >> Without this, pipe fifo underruns are seen on APL/KBL.
> >>
> >> v2: For NV12, making the src coordinates multiplier of 4
> >>
> >> v3: Moving all the src coords handling code for NV12
> >> to skl_check_nv12_surface
> >>
> >> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> >> Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
> >> ---
> >>  drivers/gpu/drm/i915/intel_display.c | 39 ++++++++++++++++++++++++++++++++++++
> >>  drivers/gpu/drm/i915/intel_sprite.c  | 15 ++++++++++----
> >>  2 files changed, 50 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> >> index 925402e..b8dbaca 100644
> >> --- a/drivers/gpu/drm/i915/intel_display.c
> >> +++ b/drivers/gpu/drm/i915/intel_display.c
> >> @@ -3118,6 +3118,42 @@ static int skl_check_main_surface(const struct intel_crtc_state *crtc_state,
> >>  	return 0;
> >>  }
> >>  
> >> +static int
> >> +skl_check_nv12_surface(const struct intel_crtc_state *crtc_state,
> >> +		       struct intel_plane_state *plane_state)
> >> +{
> >> +	int crtc_x2 = plane_state->base.crtc_x + plane_state->base.crtc_w;
> >> +	int crtc_y2 = plane_state->base.crtc_y + plane_state->base.crtc_h;
> >> +
> >> +	if (((plane_state->base.src_x >> 16) % 4) != 0 ||
> >> +	    ((plane_state->base.src_y >> 16) % 4) != 0 ||
> >> +	    ((plane_state->base.src_w >> 16) % 4) != 0 ||
> >> +	    ((plane_state->base.src_h >> 16) % 4) != 0) {
> >> +		DRM_DEBUG_KMS("src coords must be multiple of 4 for NV12\n");
> >> +		return -EINVAL;
> >> +	}
> > I don't really see why we should check these. The clipped coordinates
> > are what matters.
> 
> To propagate our limits to the userspace. I think we should do it for all formats,
> but NV12 is the first YUV format we have tests for. If we could we should do
> something similar for the other YUV formats, but they have different requirements.
> 
> In case of NV12 we don't have existing userspace, there will be nothing that
> breaks if we enforce limits from the start.

But what about sub-pixel coordinates? You're totally ignoring them here.
We need to come up with some proper rules for this stuff.

> 
> >> +
> >> +	/* Clipping would cause a 1-3 pixel gap at the edge of the screen? */
> >> +	if ((crtc_x2 > crtc_state->pipe_src_w && crtc_state->pipe_src_w % 4) ||
> >> +	    (crtc_y2 > crtc_state->pipe_src_h && crtc_state->pipe_src_h % 4)) {
> >> +		DRM_DEBUG_KMS("It's not possible to clip %u,%u to %u,%u\n",
> >> +			      crtc_x2, crtc_y2,
> >> +			      crtc_state->pipe_src_w, crtc_state->pipe_src_h);
> >> +		return -EINVAL;
> >> +	}
> > Why should we care? The current code already plays it fast and loose
> > and allows the dst rectangle to shrink to accomodate the hw limits.
> > If we want to change that we should change it universally.
> 
> Unfortunately for the other formats we already have an existing userspace
> (X.org) that doesn't perform any validation. We can't change it for that,
> but we can prevent future mistakes.

We should do it uniformly. Not per-format. That will make the code
unmaintainable real quick.

> 
> >> +
> >> +	plane_state->base.src.x1 =
> >> +		DIV_ROUND_CLOSEST(plane_state->base.src.x1, 1 << 18) << 18;
> >> +	plane_state->base.src.x2 =
> >> +		DIV_ROUND_CLOSEST(plane_state->base.src.x2, 1 << 18) << 18;
> >> +	plane_state->base.src.y1 =
> >> +		DIV_ROUND_CLOSEST(plane_state->base.src.y1, 1 << 18) << 18;
> >> +	plane_state->base.src.y2 =
> >> +		DIV_ROUND_CLOSEST(plane_state->base.src.y2, 1 << 18) << 18;
> > Since this can now increase the size of the source rectangle our
> > scaling factor checks are no longer 100% valid. We might end up with
> > a scaling factor that is too high.
> >
> > I don't really like any of these "let's make NV12 behave special"
> > tricks. We should make the code behave the same way for all pixel
> > formats instead of adding format specific hacks.
> 
> This is not nivalid because we restrict the original src coordinates to be
> a multiple of 4, you can only clip to something smaller, not to something
> bigger. :)

The clipped coordinates can be whatever thanks to scaling/etc.

Also why are we trying to make everything a multiple of four? I don't
remember any hw restrictions like that.

-- 
Ville Syrjälä
Intel
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