linux-kernel - Re: [PATCH 6/7] drm/exynos/decon5433: signal vblank only on odd fields

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Message-id: <51bd59d2-e0ca-f265-c90f-d68725602b44@samsung.com>
Date:   Thu, 26 Jan 2017 09:22:27 +0100
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     Inki Dae <inki.dae@...sung.com>, dri-devel@...ts.freedesktop.org,
        Krzysztof Kozlowski <krzk@...nel.org>,
        linux-samsung-soc@...r.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-kernel@...r.kernel.org,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH 6/7] drm/exynos/decon5433: signal vblank only on odd fields

On 25.01.2017 15:06, Ville Syrjälä wrote:
> On Mon, Jan 23, 2017 at 10:15:16AM +0100, Andrzej Hajda wrote:
>> On 20.01.2017 14:55, Ville Syrjälä wrote:
>>> On Fri, Jan 20, 2017 at 07:52:24AM +0100, Andrzej Hajda wrote:
>>>> In case of interlace mode irq is generated for odd and even fields, but
>>>> vblank should be signaled only for the last emitted field.
>>> I'm pretty sure most drivers signal it for both fields. At least i915
>>> does.
>> The question is which behavior is correct? I have not found any clear
>> statement in the documentation, or drm core code.
> That's very typical for us unfortunately.
>
> I would say what we should do what i915 does. It allows more flexibility
> in how you use the hardware. Eg. then you can actually scan out
> interlaced material to an interlaced display and not mess up the fields,
> and you can also do 3:2 pulldown type of stuff. Or you can even just
> stuff progressive frames down the pipe at field rate.
>
> One problem with interlaced stuff is that we don't have any field
> indication in the events, nor do we have a way to flip on a specific
> field. I tried to specify the latter for the SETPLANE ioctl way
> back when, but it didn't end up being implemented and now we would
> need something different for atomic.
>
>> I have guessed that since vblank event is used to signal end of scan-out
>> of buffer it should be called after scan-out of whole buffer - in case
>> of interlaced mode after scan-out of 2nd field.
> Each field has a proper vertical blanking interval, so you'd just end up
> totally wasting one of them.

The problem in this particular case is that hardware does not allow to
change buffers between fields, or more precisely it updates its internal
registers after 2nd field - ie after reading full frame.
I am still investigating the issue, but it is possible this limitation
cannot be overcome.

Regards
Andrzej

>
>> Maybe my assumption is wrong, in such case this patch should be dropped
>> and mixer driver also should be fixed, but before doing that it would be
>> good to know for sure how it should be handled correctly.
>>
>> Regards
>> Andrzej