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From:   Paul Boddie <paul@boddie.org.uk>
To:     "H. Nikolaus Schaller" <hns@goldelico.com>
Cc:     Paul Cercueil <paul@crapouillou.net>,
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        Discussions about the Letux Kernel 
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Subject: Re: [PATCH v5 2/7] drm/ingenic: Add support for JZ4780 and HDMI output
Date:   Wed, 19 Jan 2022 21:04:22 +0100
Message-ID: <4681844.iUxLBkU96p@jason>
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On Wednesday, 19 January 2022 07:40:22 CET H. Nikolaus Schaller wrote:
> Hi Paul,
> 
> > Am 18.01.2022 um 23:59 schrieb Paul Boddie <paul@boddie.org.uk>:
> > 
> > On Tuesday, 18 January 2022 17:58:58 CET Paul Cercueil wrote:
> >>
> >> Why is this JZ_LCD_OSDC_ALPHAEN bit needed now? I remember it working
> >> fine last time I tried, and now I indeed get a black screen unless this
> >> bit is set. The PM doesn't make it obvious that the bit is required,
> >> but that wouldn't be surprising.
> > 
> > It isn't actually needed. If the DMA descriptors are set up appropriately,
> > the OSD alpha bit seems to be set as a consequence. In my non-Linux
> > testing environment I don't even set any OSD registers explicitly, but
> > the OSD alpha and enable flags become set when the display is active.
> 
> Is it set by DMA descriptors or by explicit code?

The descriptors will cause it to be set when the peripheral is enabled, as far 
as I can tell.

> We did have an explicit setting of JZ_LCD_OSDC_ALPHAEN
> 
> https://www.spinics.net/lists/devicetree/msg438447.html
> 
> but that was postponed for further discussion. And now if we
> add it (from basic functionality) back, it is fine again.

It may be set in various versions of the Linux driver, but my observation was 
that in a non-Linux environment where nothing else is setting anything in the 
register concerned, initialising the descriptors seems to enable OSD and the 
OSD alpha enable bit.

Yesterday, I did consider what might be done to avoid the alpha bit being set, 
but I didn't immediately see anything in the descriptor fields that would 
offer such an alternative. The bit in question seems to be a global alpha 
enable setting, and so choosing per-pixel alpha would probably also result in 
it being set, although I didn't fire up the CI20 to check.

Paul