linux-kernel - Re: [PATCH v1 2/5] pardata: new bus for parallel data access

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Date:   Mon, 20 Jan 2020 19:48:04 +0100
From:   Sam Ravnborg <sam@...nborg.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Paul Cercueil <paul@...pouillou.net>
Cc:     Noralf T <noralf@...nnes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v1 2/5] pardata: new bus for parallel data access

Hi Geert.

On Mon, Jan 20, 2020 at 11:10:37AM +0100, Geert Uytterhoeven wrote:
> Hi Sam,
> 
> (stumbled on this accidentally)
> 
> On Thu, Aug 2, 2018 at 9:46 PM Sam Ravnborg <sam@...nborg.org> wrote:
> > The pardata supports implement a simple bus for devices
> > that are connected using a parallel bus driven by GPIOs.
> > The is often used in combination with simple displays
> > that is often seen in older embedded designs.
> > There is a demand for this support also in the linux
> > kernel for HW designs that uses these kind of displays.
> >
> > The pardata bus uses a platfrom_driver that when probed
> > creates devices for all child nodes in the DT,
> > which are then supposed to be handled by pardata_drivers.
> >
> > Signed-off-by: Sam Ravnborg <sam@...nborg.org>
> 
> > --- /dev/null
> > +++ b/Documentation/driver-api/pardata.rst
> > @@ -0,0 +1,60 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +=========================
> > +Parallel Data Bus/Drivers
> > +=========================
> > +
> > +Displays may be connected using a simple parallel bus.
> > +This is often seen in embedded systems with a simple MCU, but is also
> > +used in Linux based systems to a small extent.
> > +
> > +The bus looks like this:
> > +
> > +.. code-block:: none
> > +
> > +       ----+
> > +           |  DB0-DB7 or DB4-DB7      +----
> > +           ===/========================
> > +           |  E - enable              |  D
> > +           ----------------------------  I
> > +        C  |  Reset                   |  S
> > +        P  ----------------------------  P
> > +        U  |  Read/Write (one or two) |  L
> > +           ----------------------------  A
> > +           |  RS - instruction/data   |  Y
> > +           ----------------------------
> > +           |                          +----
> > +       ----+
> 
> Oh, cool!  Looks like this can be used by the hd44780 driver.
> 
>     Documentation/devicetree/bindings/auxdisplay/hit,hd44780.txt
>     drivers/auxdisplay/hd44780.c

This patchset was from a time when I knew next to nothing about DRM.
Now I am just confused on a different level :-)

It is on my TODO list to make a mipi-dbi driver that in the future
replaces the auxdisplay driver for hd44780.
But that TODO list have a growing tendency.
It seems that pretending to be co-maintainer on panel/ stuff
alone can take up most of my DRM time.

I hope Paul will contribute the i8080 support to drm_mipi_dbi,
at least he mentioned he planned to work on this.

	Sam