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Subject: Re: [PATCH v5 2/7] drm/ingenic: Add support for JZ4780 and HDMI
 output
From:   "H. Nikolaus Schaller" <hns@goldelico.com>
In-Reply-To: <13356060.GkHXLIg068@jason>
Date:   Wed, 19 Jan 2022 07:40:22 +0100
Cc:     Paul Cercueil <paul@crapouillou.net>,
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        OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS 
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        Discussions about the Letux Kernel 
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To:     Paul Boddie <paul@boddie.org.uk>
Precedence: bulk

Hi Paul,

> Am 18.01.2022 um 23:59 schrieb Paul Boddie <paul@boddie.org.uk>:
>=20
> On Tuesday, 18 January 2022 17:58:58 CET Paul Cercueil wrote:
>>=20
>> Not at all. If the clock is disabled, the LCD controller is disabled,
>> so all the registers read zero, this makes sense. You can only read =
the
>> registers when the clock is enabled. On some SoCs, reading disabled
>> registers can even cause a complete lockup.
>=20
> My concern was that something might be accessing the registers before =
the=20
> clock had been enabled. It seems unlikely, given that the clock is =
enabled in=20
> the bind function, and I would have thought that nothing would invoke =
the=20
> different driver operations ("funcs") until bind has been called, nor =
should=20
> anything called from within bind itself be accessing registers.
>=20
>> 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.
>=20
> It isn't actually needed. If the DMA descriptors are set up =
appropriately, the=20
> OSD alpha bit seems to be set as a consequence. In my non-Linux =
testing=20
> environment I don't even set any OSD registers explicitly, but the OSD =
alpha=20
> and enable flags become set when the display is active.

Is it set by DMA descriptors or by explicit code?

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.

BR,
Nikolaus=