Re: [PATCH v4] drm/imx: ipuv3-plane: use drm_plane_helper_check_state, clipped coordinates

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

On Tue, Feb 07, 2017 at 09:51:50AM +0100, Philipp Zabel wrote:
> On Fri, 2017-02-03 at 15:20 +0200, Ville Syrjälä wrote:
> > On Fri, Feb 03, 2017 at 10:31:39AM +0100, Philipp Zabel wrote:
> > > Use drm_plane_helper_check_state to clip raw user coordinates to crtc
> > > bounds. This checks for full plane coverage and scaling already, so
> > > we can drop some custom checks. Use the clipped coordinates everywhere.
> > > 
> > > Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
> > > ---
> > > Changes since v3:
> > >  - Disallow target frames that start offscreen (state->crtc_x/y < 0), to avoid
> > >    confusing userspace: due to the necessary line start address alignment, we
> > >    could only support very specific negative values for crtc_x/y, depending on
> > >    the pixel format.
> > 
> > That's really no different to the user specifying non-zero src
> > coordinates. So I'm wondering what's the point of special casing
> > crtc coordinates this way because you'll need to check the src
> > coordinates anyway.
> 
> User expectations.
> 
> For the src_x/y / src.x1/y1 source coordinates we have format specific
> alignment requirements due to limitations of the DMA unit, there is no
> way around it.
> For the crtc_x/y plane coordinates there are no alignment requirements
> at all, the partial plane can be positioned at any integer x/y position.
> If we allow to simulate a partially offscreen plane at negative
> positions by clipping and setting src.x1/y1, suddenly the crtc_x/y have
> alignment requirements, but only if the values are negative.
> I assume that would rather confuse any user space application that tries
> to reason about which crtc_x/y values are valid, so it's probably better
> to disallow it altogether.

I don't really agree. Userspace has to be prepared for anything
to fail really. Whether it fails every time or sometimes is not that
much different IMO. Given the simple return value userspace can't
really draw any real conclusions about the reason for the failure.

A related question is how strict we should really be. In the past we've
been very lax in fudging the coordinates to make something appear on the
screen. Probably I went a bit too far in the laxness actually, and with
atomic we want to tighten things up a little. Just no one has managed to
really answer how tight we should make things. Is it OK to round things
to pixel boundaries perhaps? Maybe even macropixel boundaries?

-- 
Ville Syrjälä
Intel OTC
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