linux-kernel - Re: [PATCH v5 2/7] drm/ingenic: Add support for JZ4780 and HDMI output

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Date:   Wed, 19 Jan 2022 07:40:22 +0100
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Paul Boddie <paul@...die.org.uk>
Cc:     Paul Cercueil <paul@...pouillou.net>,
        Mark Rutland <mark.rutland@....com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Neil Armstrong <narmstrong@...libre.com>,
        David Airlie <airlied@...ux.ie>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-mips <linux-mips@...r.kernel.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Harry Wentland <harry.wentland@....com>,
        OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS 
        <devicetree@...r.kernel.org>, Kees Cook <keescook@...omium.org>,
        Jonas Karlman <jonas@...boo.se>,
        Mark Brown <broonie@...nel.org>,
        Maxime Ripard <maxime@...no.tech>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Liam Girdwood <lgirdwood@...il.com>,
        Robert Foss <robert.foss@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>
Subject: Re: [PATCH v5 2/7] drm/ingenic: Add support for JZ4780 and HDMI
 output

Hi Paul,

> Am 18.01.2022 um 23:59 schrieb Paul Boddie <paul@...die.org.uk>:
> 
> On Tuesday, 18 January 2022 17:58:58 CET Paul Cercueil wrote:
>> 
>> 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.
> 
> My concern was that something might be accessing the registers before the 
> clock had been enabled. It seems unlikely, given that the clock is enabled in 
> the bind function, and I would have thought that nothing would invoke the 
> different driver operations ("funcs") until bind has been called, nor should 
> anything called from within bind itself be accessing registers.
> 
>> 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?

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