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Date:   Wed, 9 Jan 2019 11:37:19 +0000
From:   Peter Rosin <peda@...ntia.se>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        David Airlie <airlied@...ux.ie>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] drm/atmel-hlcdc: prevent divide by zero

On 2019-01-09 11:12, Daniel Vetter wrote:
> On Tue, Jan 08, 2019 at 12:31:36PM +0000, Peter Rosin wrote:
>> While trying to temporarily hide a plane, one thing that was attempted
>> was to call (from libdrm)
>>
>> 	drmModeSetPlane(fd, plane_id, crtc_id, fb_id, 0,
>> 			0, 0, 0, 0, 0, 0, 0, 0);
>>
>> This call causes a pair of "Division by zero in kernel." messages. Kill
>> those messages, but preserve the current behaviour that also happen to
>> make the plane disappear with the above call.
>>
>> Signed-off-by: Peter Rosin <peda@...ntia.se>
>> ---
>>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c | 14 ++++++++++----
>>  1 file changed, 10 insertions(+), 4 deletions(-)
>>
>> Side note, when comparing with drm_atomic_helper_check_plane_state(), I get
>> the feeling that the src rect should be clipped together with the crtc rect
>> if/when clipping is needed. That function calls drm_rect_clip_scaled(), and
>> the equivalent does not seem to happen here. Should clipping be performed
>> on the src rect?
> 
> Any reasons atmel can't switch over to that helper? Would compute a nice
> ->visible state variable for you ...
> -Daniel

I have not researched that, and as stated above, I was not sure if that was the
correct approach to begin with. Boris, any insights? Does this code predate the
helper or something?

Maybe there's some register bit that hides a plane explicitly, matching the
->visible state variable? I haven't looked at that either...

Cheers,
Peter

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