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

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Date:   Mon, 20 Jan 2020 20:12:23 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Sam Ravnborg <sam@...nborg.org>
Cc:     Paul Cercueil <paul@...pouillou.net>,
        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 Sam,

On Mon, Jan 20, 2020 at 7:48 PM Sam Ravnborg <sam@...nborg.org> wrote:
> On Mon, Jan 20, 2020 at 11:10:37AM +0100, Geert Uytterhoeven wrote:
> > 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 :-)

The more you know, the more you realize what you don't know ;-)

> It is on my TODO list to make a mipi-dbi driver that in the future
> replaces the auxdisplay driver for hd44780.

Please note that hd44780 is a character controller.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds