Re: [PATCH] drm/vc4: plane: Expand the lower bits using the LSB

From: Maxime Ripard <maxime.ripard@bootlin.com>
To: Eric Anholt <eric@anholt.net>
Cc: dri-devel@lists.freedesktop.org
Subject: Re: [PATCH] drm/vc4: plane: Expand the lower bits using the LSB
Date: Mon, 14 May 2018 15:20:46 +0200	[thread overview]
Message-ID: <20180514132046.x3k65splrawenniv@flea> (raw)
In-Reply-To: <87604gtuk7.fsf@anholt.net>

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Hi Eric,

On Tue, Apr 24, 2018 at 09:53:28AM -0700, Eric Anholt wrote:
> Maxime Ripard <maxime.ripard@bootlin.com> writes:
> 
> > The vc4 HVS uses an internal RGB888 representation of the frames, and will
> > by default expand formats using a lower depth using zeros.
> >
> > This causes an issue when we try to use other compositing software such as
> > pixman that seems to be filling the missing bits using the format least
> > significant bit value. As such, this prevents us from checking the display
> > output in a reliable way.
> 
> I don't think this is the right expansion function, though?  My
> understanding of proper unorm expansion, and what pixman's
> unorm_to_unorm() does, is that you replicate the value in the missing
> bits until you've filled them all (so SCALER_CTL0_EXPAND_REPEAT)

This was based on experiment, so I might have missed that case and you
are probably right. I'll test with SCALER_CTL0_EXPAND_REPEAT and let
you know :)

Thanks!
Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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