Re: [PATCH 1/3] drm/arm: hdlcd: properly validate plane state

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Fri, Mar 31, 2017 at 12:41:30PM +0100, Liviu Dudau wrote:
> On Fri, Mar 31, 2017 at 11:27:51AM +0100, Russell King - ARM Linux wrote:
> > On Fri, Mar 31, 2017 at 11:23:45AM +0100, Liviu Dudau wrote:
> > > On Fri, Mar 31, 2017 at 11:20:35AM +0100, Russell King - ARM Linux wrote:
> > > > On Fri, Mar 31, 2017 at 11:18:50AM +0100, Liviu Dudau wrote:
> > > > > Hi Russell,
> > > > > 
> > > > > You were Cc-ed in a patch from March 8th that did all this:
> > > > > 
> > > > > https://lists.freedesktop.org/archives/dri-devel/2017-March/135172.html
> > > > 
> > > > I'm aware of that (you may notice that this was threaded to that patch.)
> > > > 
> > > > > I have not received any response from you, so I have already pushed the
> > > > > patch in my public repo:
> > > > > 
> > > > > git://linux-arm.org/linux-ld.git for-upstream/hdlcd
> > > > > 
> > > > > It has been included into linux-next for at least a couple of weeks now.
> > > > 
> > > > I've not had a chance to test any of this, but I believe that your
> > > > patch does not fully address the issue, due to bits missing from
> > > > the validation path.
> > > 
> > > Care to point out which bits were missing from my patch that are in yours?
> > 
> > The visible check?
> 
> A plane's ->atomic_check() hook can be called with TEST_ONLY to figure out from
> userspace if the given configuration is a valid one that can be accepted by
> the hardware. There should be no error if the plane will not be visible, as we
> are not programming anything yet.
> 
> I would also argue that the test that you remove and replace with state->visible
> is important. We can't do *any* scaling, while with your patch we could accept
> src_w != crtc_w as long as it is visible. Hardware is not capable of handling that.

That's what the "DRM_PLANE_HELPER_NO_SCALING" arguments to
drm_plane_helper_check_state() are doing:

	drm_plane_helper_check_state()
		drm_rect_calc_hscale()

		        if (hscale < min_hscale || hscale > max_hscale)
                		return -ERANGE;

		drm_rect_calc_vscale()

		        if (vscale < min_vscale || vscale > max_vscale)
                		return -ERANGE;

where DRM_PLANE_HELPER_NO_SCALING is 1.0 in 16:16 format.  So, this
ensures that the scaling factor is 1.0, returning -ERANGE if it isn't.

If this lets through a scaled source, then there's a bug that needs
fixing in the helper.

-- 
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