linux-kernel - Re: [PATCH v2] auxdisplay: hd44780: move cursor home after clear display command

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Date:   Tue, 25 Jul 2023 11:13:54 -0400
From:   Hugo Villeneuve <hugo@...ovil.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Hugo Villeneuve <hvilleneuve@...onoff.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] auxdisplay: hd44780: move cursor home after clear
 display command

On Mon, 24 Jul 2023 18:08:00 +0200
Geert Uytterhoeven <geert@...ux-m68k.org> wrote:

> Hi Hugo,
> 
> On Sat, Jul 22, 2023 at 8:18 PM Hugo Villeneuve <hugo@...ovil.com> wrote:
> > From: Hugo Villeneuve <hvilleneuve@...onoff.com>
> >
> > The DISPLAY_CLEAR command on the NewHaven NHD-0220DZW-AG5 display
> > does NOT change the DDRAM address to 00h (home position) like the
> > standard Hitachi HD44780 controller. As a consequence, the starting
> > position of the initial string LCD_INIT_TEXT is not guaranteed to be
> > at 0,0 depending on where the cursor was before the DISPLAY_CLEAR
> > command.
> >
> > Extract of DISPLAY_CLEAR command from datasheets of:
> >
> >     Hitachi HD44780:
> >         ... It then sets DDRAM address 0 into the address counter...
> >
> >     NewHaven NHD-0220DZW-AG5 datasheet:
> >         ... This instruction does not change the DDRAM Address
> >
> > Move the cursor home after sending DISPLAY_CLEAR command to support
> > non-standard LCDs.
> >
> > Signed-off-by: Hugo Villeneuve <hvilleneuve@...onoff.com>
> 
> Thanks for your patch!
> 
> > --- a/drivers/auxdisplay/hd44780_common.c
> > +++ b/drivers/auxdisplay/hd44780_common.c
> > @@ -82,7 +82,15 @@ int hd44780_common_clear_display(struct charlcd *lcd)
> >         hdc->write_cmd(hdc, LCD_CMD_DISPLAY_CLEAR);
> >         /* datasheet says to wait 1,64 milliseconds */
> >         long_sleep(2);
> > -       return 0;
> > +
> > +       /*
> > +        * The Hitachi HD44780 controller (and compatible ones) reset the DDRAM
> > +        * address when executing the DISPLAY_CLEAR command, thus the
> > +        * following call is not required. However, other controllers do not
> > +        * (e.g. NewHaven NHD-0220DZW-AG5), thus move the cursor to home
> > +        * unconditionally to support both.
> > +        */
> > +       return hd44780_common_home(lcd);
> 
> Sorry, I haven't tested your patch yet, as my HD44780 display is
> currently not connected to any system.
> 
> Your patch LGTM, so
> Reviewed-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> 
> Note that hd44780_common_home() calls hd44780_common_gotoxy().
> According to the documentation, both HD44780U and NHD-0220DZW-AG5
> support the Return Home command (0x02), so that could be a good
> alternative.

Hi Geert,
If this is desired, I think maybe it would be best to do it in a
separate patch. Maybe there is a good reason why hd44780_common_home()
calls hd44780_common_gotoxy() instead of using Return Home command?

Hugo.


> 
> >  }
> >  EXPORT_SYMBOL_GPL(hd44780_common_clear_display);
> 
> 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
>