linux-kernel - Re: [PATCH v12 2/9] drm/ingenic: Add support for JZ4780 and HDMI output

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Date:   Wed, 02 Feb 2022 18:04:20 +0100
From:   Paul Boddie <paul@...die.org.uk>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     "H. Nikolaus Schaller" <hns@...delico.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Harry Wentland <harry.wentland@....com>,
        Sam Ravnborg <sam@...nborg.org>,
        Maxime Ripard <maxime@...no.tech>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        devicetree@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, letux-kernel@...nphoenux.org,
        Jonas Karlman <jonas@...boo.se>,
        dri-devel@...ts.freedesktop.org,
        Ezequiel Garcia <ezequiel@...labora.com>
Subject: Re: [PATCH v12 2/9] drm/ingenic: Add support for JZ4780 and HDMI output

On Wednesday, 2 February 2022 13:41:21 CET Paul Cercueil wrote:
> Le mer., févr. 2 2022 at 13:33:15 +0100, H. Nikolaus Schaller> 
<hns@...delico.com> a écrit :
> >>  Am 02.02.2022 um 13:28 schrieb Paul Cercueil <paul@...pouillou.net>:
> >>  
> >>  Note that there are other things not working with your current
> >> 
> >> implementation, for instance you cannot set the X/Y start position
> >> of the f1 plane, which means it's only really usable for fullscreen
> >> desktop/windows.
> > 
> > Is setting x/y possible for the other SoC?
> 
> Yes. They support different x/y positions, sizes, and pixel format for
> both f0, f1 and IPU planes.

One thing worth noting about the JZ4780 is that a lot of the registers that 
might otherwise be used for the above purposes appear to be read-only, at 
least for the different fields concerned.

For example, those affecting ingenic_drm_plane_config:

Control Register (LCDCTRL) - specifically the BPP0 field
OSD Control Register (LCDOSDCTRL)
Foreground 0 XY Position Register (LCDXYP0)
Foreground 1 XY Position Register (LCDXYP1)
Foreground 0 Size Register (LCDSIZE0)
Foreground 1 Size Register (LCDSIZE1)

These require changes to the extended descriptor members instead, and I am 
fairly sure I mentioned the implications for pixel depth configuration 
previously. So, as far as I can tell, we would need to update the descriptors, 
not the registers, to support the operations mentioned above.

As for the f0 plane "not working", I am not aware of any limitation around 
using only f0 (assuming it corresponds to what the manual calls fg0) or only 
f1 (again, assuming fg1 in the manual) or both. My assumption was that for 
this particular driver, f0 was reserved for some kind of overlay and that f1 
was to be used for the normal non-overlay display for products where the OSD 
peripheral is provided.

From the definition of struct ingenic_drm:

        /*
         * f1 (aka. foreground1) is our primary plane, on top of which
         * f0 (aka. foreground0) can be overlayed. Z-order is fixed in
         * hardware and cannot be changed.
         */

So, as I understood it, the driver would configure f1 in the case of the 
JZ4780 for basic display support. Configuring f0 as an overlay should be 
entirely possible, but I imagine that it needs to change the descriptors, not 
the registers, to have a chance of actually working.

I hope this is somewhat useful information. I honestly don't know if, say, the 
JZ4770 has a similar arrangement with regard to configuration via descriptors, 
as opposed to registers, but I think it is an important distinction between 
devices in this particular family that needs to be accommodated in the driver, 
and we obviously want to determine how this might best be achieved.

Paul